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Answer:
The Plantium and palladium are used for electrolysis of aqueous solution due to the chemically inert in nature and good adsorptive capacity for hydrogen.
Because Gases like hydrogen are absorbed on the surface of platinum and palladium (adsorption) and then passes into the bulk of the adsorbent (platinum and palladium).
In Haber's process, hydrogen is obtained by reacting methane with steam in presence of NiO as catalyst. The process is known as steam reforming. Why is it necessary to remove CO when ammonia is obtained by Haber’s process?
The variation in the amount of gas adsorbed by the adsorbent with pressure at constant temperature can be expressed by means of a curve termed as adsorption isotherm.
A plot of the amount of gas adsorbed x / m against the pressure at constant temperature is called an adsorption isotherm.Sponsor Area
Answer:
Colloids possess high surface area per unit mass and are therefore called good adsorbents.
The catalytic process in which the reactants and the catalyst are in different phases is known as heterogeneous.
Inversion of cane sugar: The invertase enzyme converts cane sugar into glucose and fructose
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Answer:
In butter, the
(i) dispersed phase is water.
(ii) dispersion medium is solid fat and proteins.
Freundlich adsorption isotherm: Freundlich gave an
empirical relationship between the quantity of gas adsorbed by unit mass of solid adsorbent and pressure at a particular temperature. The relationship can be expressed by the following equation
Freundlich adsorption isotherm: Freundlich gave an empirical relationship between the quantity of gas adsorbed by a unit mass of solid adsorbent and pressure at a particular temperature. The
relationship can be expressed by the following equation:
where x is the mass of the gas adsorbed on mass m of the adsorbent at pressure P, k and n are constants which depend on the nature of the adsorbent and the gas at a particular
temperature. The relationship is generally represented in the form of a curve where the mass of the gas adsorbed per gram of the adsorbent is plotted against pressure.
At low pressures: x/m varies linearly with p
At high pressures : x/m is independent of p
At intermediate pressures: The variation of x/m vs p can be expressed as
comparing the above-given equation with the equation of a straight line
y = mx + c
we know that, if we plot log p vs log x/m, we would get a straight line with slope equal to 1/n and intercept log k
Since adsorption is always an exothermic process, therefore, increase in temperature should decrease the amount adsorbed.
Common: The dispersed phase which is a solid.
Difference: In aqua-sols the dispersion medium is water while in aerosols it is air.
Lyophilic : Reversible, because on evaporating the liquid, the residue left will passes into colloidal state on addition of liquid.
Lyophobic : Irreversible, because on evaporating the liquid, the residue left cannot be converted into solution on mere addition of liquid.
Which sol is more stable-lyophilic or lyophobic?
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Explain the following observations:
(a) Cottrell’s smoke precipitator is fitted at the mouth of the chimney used in factories.
(b) Physical adosption is multi layered, while chemisorption non-tayered.
Answer:
(a) Smoke contains positively charged colloidal particles which are separated by Cottrell’s smoke precipitator.
(b) Physical adsorption is multilayered because forces between adsorbate and adsorbent molecules are of Vander Waal's type. Chemisorption is non-layers because forces of attraction are of chemical bond type.
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Absorption |
Adsorption |
|
1. It is the phenomenon in which the particles of gas or liquid get uniformly distributed throughout the body of the solid. 2. The concentration is the same through the material. Therefore, it is a bulk phenomenon. 3. Absorption occurs at uniform rate. |
1. It is the phenomenon of higher concentra-tion of particles of gas or liquid on the surface than in the bulk of the solid. 2. The concentration on the surface of the adsorbent is different from that in the bulk. Therefore, it is a surface phenomenon. 3. Adsorption is rapidly in the beginning and its rate slowly increases. |
Example of adsorption: NH3, adsorbed by charcoal, H2O adsorbed by silica, ink adsorbed by chalk.
Example of absorption: NH3 absorbed by water, water absorbed by anhydrous CaCl2, water absorbed by a sponge.
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Physisorption |
Chemisorption |
|
1. The forces of attraction between adsorbent and adsorbate are of Van der Waals type (weak forces). 2. This predominates at low temperatures. 3. Almost all gases show this type of adsorption at low temperatures. 4. The heat of adsorption is low and has a value of about 40 kJ per mole or less. 5. This type of adsorption attains equilibrium very rapidly on changing the temperature and pressure of the system. 6. This is reversible in nature. 7. The activation energy involved in this adsorption is small and is often less than 5 kJ. It is for this reason, this is even attained at low temperatures. 8. Adsorption in this case is often multilayer. |
1. The forces of attraction between adsorbent and absorbate are of a chemical nature (strong forces). 2. This usually occurs at high temperatures. 3. It is highly specific in nature. 4. The heat of adsorption is high and has a value of the order of 80 to 420 kJ per mole. 5. This type of adsorption is relatively slower. 6. This is usually irreversible in nature. For example, O2 adsorbed on charcoal, when adsorbed also releases CO and CO2. 7. The activation energy involved in this adsorption is high. It is for this reason, it is attained only at high temperatures. 8. Adsorption in this case in monolayer. |
Answer:
The factors which influence the adsorption of a gas on a solid are:
(1) The nature of a gas adsorbate and nature of solid adsorbent.
(2) Surface area of adsorbent.
(3) Pressure of gas.
(4) Temperature.
(5) Activation of adsorbent.
and n depend upon the nature of gas and the solid.At low pressure, the extent of adsorption is directly proportional to pressure (raised to power one).
At high pressure, the extent of adsorption is independent of pressure (raised to power zero).
Therefore at an intermediate value of pressure, adsorption is directly proportional to pressure raised to power 1/n.Here n is a variable whose value is greater than one.
Using constant of proportionality, k, also known as adsorption constant we get
The above equation is known as Freundlich adsorption equation.
As per Freundlich adsorption equation
The equation above equation is comparable with comparable with the equation of a straight line,
y = m x + c where m represents the slope of the line and c represents intercept on y-axis.
Plotting a graph between log(x/m) and log p, we will get a straight line with the value of slope equal to 1/n and log k as y-axis intercept.
Answer:
Activation of adsorbent implies increasing its adsorbing power. It is achieved by the following ways:
(i) Activation of the adsorbent is carried out by mechanical rubbing or by subjecting it to some chemical reaction.
(ii) Some adsorbents are activated by strong heating in contact with super-heated steam. For example, charcoal which is commonly employed in adsorption of gases, is activated by subjecting it to the action of superheated steam. This will remove the molecules of gases like CH4, C2H6 (hydrocarbons) that are occupying these pores.
(iii) To increase the adsorbing power, the adsorbent is sub divided into small fine pieces. By doing so, the surface area increases and so is the adsorbing power.
Answer:
Adsorption plays an important role in heterogeneous catalysis. Adsorption is surface phenomenon in which the substances (reactants) get concentrated only on the surfaces of solid adsorbents and do not penetrate into it.This increases concentration of reactants and there is a great probability of occurrence of reaction. The catalyst adsorbs reactants and forms a intermediate compound known as activated complex. Following are various ways in which catalytic adsorption increases the reaction rate:
(i) The concentration of reactants on surface of solid adsorbent increases and it becomes easier for molecules to attack each other for effective collisions, forming products.
(ii) A particular part of reactant molecule may come in contact with other molecules which otherwise is difficult.
(iii) Some adsorbed molecules dissociate into atoms or free radicals which are very reactive and thus increase the reaction rate.
(iv) Heat of adsorption released provides activation energy for formation of activated complex and then increase the rate of reaction.
Answer:
Adsorption is always exothermic, this statement can be explained by two.
i) Adsorption leads to a decrease in the residual forces on the surface of the adsorption.
This cause a decrease in the surface energy of the adsorption therefore adsorption is always exothermic.
ii) ΔH of adsorption is always negtive.
When a gas is adsorbed on a soild surface its movement is restricted leading to decrease in the entropy of the gas i.e. ΔS
Adsorption is accompanised by decrease in entropy, i.e., ΔS = – ve. If it is spontaneous then ΔG should be negative.
ΔG = ΔH – TΔS
Since ΔS is – ve, ΔG will be – ve only if
ΔH = – ve, i.e., if process is exothermic.
Answer:
A dispersed phase or dispersion medium in colloidal solution may be a solid, liquid or gas. Based on physical states, 8 types of colloidal systems are possible (gas in gas is not possible as gases are always miscible in all proportions forming homogeneous mixure). All other takes of combinations of gases, liquids and solids may exist as colloidal solutions. Examples are given in the following table:
|
Internal phase of Dispersed phase |
External phase or Dispersion medium |
Colloidal name |
Example |
|
Solid |
Solid |
Solid sols |
Alloys, Ruby glass, Gems or precious stones, marbles, optical and vision glasses. |
|
Liquid |
sols |
Muddy water, gold sol, protein, starch, agar, gelatin in water, paints, pigments in water. |
|
|
Gas |
Aerosols (or solids |
Smoke, particulate clouds. |
|
|
Liquid |
Solids |
Gels |
Cheese, jems, jellies, plants, fruits, vegetables |
|
Liquid |
Emulsions |
Butter, milk, cosmetic products, e.g., shampoo, creams, emulsified oils, polish and medicines. |
|
|
Gas |
Aerosols (or liquids) |
Fog, clouds, mist. |
|
|
Gas |
Solid |
Solid foams |
Pumicestone, styrene foam, foamed rubber, porous pot. thermocole rubber pillows and mattresses. |
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Liquid |
Foams and froths |
Lather, soap seeds, air bubble. |
|
|
Gas |
Homogeneous system |
Do not exist as colloids. |
Answer:
Effect of pressure on adsorption. The extent of adsorption of a gas per unit mass of adsorbent (x / m) increases with increase of pressure at a constant temperature.
(i) For a lower range of pressure (x / m) is directly proportional to the applied pressure. Larger the pressure more is the amount of gas adsorbed, lower the pressure small is the amount of a gas adsorbed. That is
(as a constant temperature)
(ii) For a high pressure range the extent of adsorption of a gas per unit mass of the adsorbent (x / m) is independent of the applied pressure. That is
(iii) For a moderate pressure range the value of x/m is proportional to a Fractional power of pressure. That is
(where T is constant)
where 1 / n is a fraction. Its value may be between 0 and 1. Fig. (a) and (b) show the variations of the extent of adsorption of a gas on an adsorbent as predicted by Frundlich equations (i) and (ii) respectively.
...(i)
and ...(ii)
Effect of temperature on adsorption: The amount of a gas adsorbed per unit mass of a solid surface (x / m) decreases with increase of temperature in case of physical adsorption. However, in case of chemical adsorption as the temperature increases x / m increases, attains a maximum value then decreases.
Answer:
On the basis of attraction between dispersed phase and dispersion medium colloids may be classified into lyophillic sol and lyophobic sol.
If dispersed phase (solid) tends to attract (i.e., like or love) dispersion medium (liquid), the resultant sol is termed as lyophilic sol. Examples are: Sol of gum, gelatine, starch etc. are lyophilic sols.
On the other hand if dispersed phase tends to repel (i.e., dislike or hatred) dispersion medium, the resultant sol is termed as lyophobic sol. Examples are: Sols of metals, metal hydroxides, metal sulphides etc. are lyophobic sols.
Hydrophobic sols easily coagulated as in this dispersion medium has no apparent affinity or interaction with the dispersed phase.
Answer:
Multimolecular colloids:
(i) Consist of aggregates of atoms or molecules with diameter less than 10–9 m.
(ii) The atom or molecules are held together by weak Vander Waals forces.
Macromolecular colloids:
(i) Are themselves large. Many behave as macromolecular colloids because of their large molecular masses.
(ii) Since the molecules are flexible, they can take on various shapes.
Associated colloids:
(i) Behave as true solution in smaller concentrations and colloids at higher concentrations.
(ii) Sodium stearate behaves as strong electrolyte in dilute solutions and shows the properties of sodium and stearate ions form aggregates and behave as colloids. The aggregates formed are called micelles which revert to individual ions on dilution.
Example of multi-molecular colloid: Gold sol, Sulphur sol.
Example of macro-molecular colloid:Cellulose, Starch.
Answer:
Enzymes are biological catalysts which increases the rate of cellular reactions.
Mechanism of enzyme catalysis : Enzymes are proteins (globular proteins) and have large molecular masses ranging from 12000 to 40,000. Thus, these are much bigger than the molecules which they catalyse. The substances which are catalysed are known as substrates. The mechanism of the enzyme catalysed reaction is completed in the following steps:
Step-1. Binding enzyme to substrate (reactant) to form a complex.
Step-2. Product formation in the complex:
Step-3. Release of the product from the enzyme complex.
Actually there are a number of cavities present on the surface of the molecules of enzymes. These cavities have specific shapes and contain in their active groups such as – NH2, – COOH, – SH, – OH etc. These function as active sites on the surface of enzyme. The molecules of the reactant (substrate) which have complementary shapes fit into these cavities in the same manner as a key fits into a lock. This results in an activated complex which breaks to give the product and releases the enzyme catalyst.
Answer:
(i) Based on physical states of components. Depending upon whether the dispersed phase and the dispersion medium are solids, liquids or gases, eight types of colloidal system are possible. The examples of the various type of colloids along with their typical names are given following:
|
Dispersed |
Dispersion |
Type of |
Examples |
|
phase |
medium |
colloid |
|
|
Solid Solid Solid Liquid Liquid Liquid Gas Gases |
Solid Liquid Gas Solid Liquid Gas Solid Liquid |
Solid Sol Sol Aerosol Gel Emulsion Aerosol Solid sol Foam |
Some coloured glasses and gem stones. Paints, cell fluids Smoke, dust Cheese, butter, jellies. Milk, hair cream Fog, mist, cloud, insecticide sprays. Pumice stone, form rubber Froth, whipped cream,soap-lather. |
(ii) Nature of dispersion medium. Depending upon the type of the particles of the dispersed pahse, colloids are classified as multimolecular, macromolecular and associated colloids.
Multimolecular colloids. The colloids in which the colloidal particles consists of aggregate of atoms or small molecules with diameters of less than 1 mm and called multimolecular colloids e.g., sulphur sol.
Macro molecular colloids. Macro molecules have large molecular masses. These on dissolution in a suitable solvent form a solution in which the size of the macromolecules may be in the colloidal range. Such systems are called macromolecular colloids. Example: proteins, starch and cellulose.
Associated colloids. There are some substances which at low concentrations behave as normal, strong electrolytes but at higher concentrations exhibit colloidal behaviour due to the formation of aggregated particles. The aggregated particles thus formed are called micelles. These are also known as associated colloids.
(iii) Interaction between dispersed phase and dispersion medium. Depending upon the nature of interaction between the dispersed phase and the dispersion medium, colloidal sols are divided into two categories, namely, lyophilic and lyophobic.
Lyophilic colloids. The colloidal solution in which the particles of the dispersed phase have a greater affinity for the dispersion medium are called lyophilic colloids. The common example of lyophilic colloids are glue, gelatin, starch, protein, rubber, etc.
Lyophobic colloids. The colloidal solutions in which the particles of the dispersed phase have no affinity or love rather have hatred for the dispersion medium are called lyophobic colloids.
Explain what is observed
(i) when a beam of light is passed through a colloidal sol.
(ii) an electrolyte, NaCl is added to hydrated ferric oxide sol.
(iii) electric current is passed through a colloidal sol?
(i) When a beam of light is passed through a colloidal solution, the path of light inside the colloidal solution is illuminated and becomes visible. The path of light inside a colloidal solution becomes visible because colloidal particles are big enough to scatter the light falling on them in all the directions. The scattering of light by colloidal particles is called Tyndall effect.
(ii) When NaCl is added to ferric oxide sol it dissociate to give Na+ and Cl- ions. Particles of ferric oxide sol are positively charge.
thus they get coagulated in the presece of negatively charge Cl- ion.
(iii)The colloidal particles are charged and carry either positive or negtive charged particles in colloidal solution. When an electric curent is passed through a colloidal, solution, then depending on the nature of their charge, the colloidal particles move either towards the anode or towards the cathode, lose their charges and get deposited on the electrode. The migration of colloidal particles towards the oppositely charged electrodes under the influence of an electric field is called elecrophoresis.
Answer:
The colloidal solution in which both the dispersed phase and desperssion medium are liquid is called an emulsion.The emulsion droplets are somewhat larger than the particles of sol but smaller than coarse suspension.
There are two types of emulsions:
(i) Oil-in-water emulsion. In this type of emulsion the dispersed phase is oil and the dispersion medium is water. Milk is an example of this type of emulsion in which liquid fat is dispesed in water.
(ii) Water-in-oil emulsion. In this type of emulsion the dispersed phase is water while the dispersion medium is oil. Butter is an example of this type of emulsion in which water is dispersed in oil.
Answer:
The technique of breaking the emulsion and then causing the separations of its components is called de-emulsification. Emulsion can be de-emulsified by separating or destroying the emulsifier. Boiling, centrifusion, freezing and addition of electrolytes cause de-emulsification.
Examples: (i) Centrifugation separation cream (fat) from milk which is an emulsion.
(ii) When water is the dispersion medium, the presence of a dehydrating agent can result in the de-emulsification.
Answer:
Heterognous catalysis :
The catalytic process in which the reactant and the catalyst are in different phase is known as heterogenous catalyst.
The examples of heterogeneous system are given below:
1. Manufacture of NH3 from N2 and H2 by Haber’s process, using iron as catalyst.
2. Oxidation of NH3 with O2 using Pt as catalyst in Ostwald process.
3. Oxidation of CO in automobile exhaust by platinum catalyst.
4. Hydrogenation of oils to form vegetable ghee using finely divided nickel as catalyst
Answer:
The catalytic reaction that depends upon the pore structure of the catalytic and te size of the reactant and the sizes of the reactant and product molecule is called shap-selective catalysis.
Zeolites are microporous complex silicates in which some of silicon atoms are replaced by Al3+ ions. These are sodium aluminum silicates or calcium aluminum silicates. Their general formula is Mx/m [(AlO2)x (SiO2)y] m H2O where, n is valency of metal cation, Mn+ These are found in nature as such and are also synthesised in laboratory.
The characteristic features are:
(i) These are shape-selective catalyst or can act as molecular sieves.
(ii) These have honey-comb like structure.
(iii) These have specific pore-size lying between 260 to 740 pm.
Sponsor Area
Answer:
The colloidal solution in whichboth the dispersed and dispersion medium are liquid is called an emulsion.
Uses of emulsification:
(i) Removal of grease, oils and dirt from the skin, garments and utensils is due to the role of soap as an emulsifier during washing with water.
(ii) Many pharmaceutical preparations like cod liver oil and Halibit liver oil are emulsions. The emulsifying agents used for this purpose are agar and gumacacia. The emulsified oil is readily acted upon by the digestive juices.
(iii) Many lotions, creams and ointments are emulsions of O / W type or W / O type.
(iv) The disinfectants like phenyl and lysol yield
emulsions of O/W type when poured in water.
(v) Emulsions are of physiological importance also. Emulsification facilitates the digestion of fats in the digestive tract by bringing the fats, water and digestive enzymes into contact with other.
Answer:
Micelle formation is done by substance such as soaps and detergent when dissolved in water. The molecules of such substance contain a hydrophobic and hydrophillic part. When present in water these substance arrange themseleves in spherical structure in such a manner that their hydrophobic parts are present towards the centre, while hydrophilic parts are pointing towards outside.
This is known as micelle formation.
A particle of colloidal size formed due to aggregation of several units of soap (surfactant) molecules in a dispersion medium is called micelle. A micelle usually consists of 50 to 100 molecules of the surfactants.
Examples of Miceller system: (i) Sodium stearate + water
[CH3(CH3)16 COO– Na + H2O]
(ii) Sodium dodecyl sulphate + water
[CH3(CH2)11 OSO3– Na+ + H2O]
(iii) Cetylrimethyl ammonium bromide + water
Answer:
(i) Alcosol: A colloidal solution having alcohol as the dispersion medium and soild substance as the dispersed phase is called an alcosol.
for example:Colloidal sol of cellulose nitrate in ethyl alcohol is an alcosol.
(ii) Aerosol. If the dispersion medium is gas the sol is called aerosol. Examples smoke. Dust.
(iii) Hydrosol. When the dispersion medium is water, the sol is called hydrosol. Examples-paints, cell fluids.
Answer:
emulsion is a mixture of two liquids that would ordinarily not mix together, like oil and vinegar. Or in other word When liquid is dispersed in liquid this type of colloidal solution is called emulsion.
There are two types of emulsions.
(i) Oil dispersed in water (O/W type) and
(ii) Water dispersed in oil (W/O type).
Applications: (i) Soap emulsifies grease and takes it away in water along with the dirt.
(ii) Digestion of fats in the intestines is aided by emulsification.
Explain the following term:
(i) Electro-dialysis
(ii) Phases of a colloidal solution.
Answer:
(i) It is a process in which colloidal solution is purified under influence of electric-field. The colloidal solution containing ionic impurities is put in a bag of parchment paper under electric field so that ions pass through parchment paper but colloidal solution does not.
(ii) Colloidal solution is heterogeneous which consists of two phases, the dispersed phase and dispersion medium.
(a) Dispersed phase. It is the component which is present in small amount and consists of particles (1 to 100 nm). It may be solid, liquid or gas.
(b) Dispersion medium. The medium in which colloidal particles are dispersed is called dispersion medium. It may also be solid, liquid or gas.
Answer:
The catalytic process in which the reactants and the catalyst are in different phases is known as heterogeneous catalysis.
Adsorption theory of heterogenous catalysis explained in two ways.
(i) The reactants in gaseous state or in solutions, are adsorbed on the surface of the solid catalyst. The increase in concentration of the reactants on the surface increases the rate of reaction. Adsorption being an exothermic process, the heat of adsorption is utilised in enhancing the rate of the reaction.
(ii) The catalytic activity is localised on the surface of the catalyst. The mechanism involves five steps:
(i) Diffusion of reactants to the surface of the catalyst.
(ii) Adsorption of reactant molecules on the surface of the catalyst.
(iii) Occurrence of chemical reaction on the catalyst’s surface through formation of an intermediate.
(iv) Desorption of reaction products from the catalyst surface, and thereby, making the surface available again for more reaction to occur.
(v) Diffusion of reaction products away from the catalyst’s surface.
The surface of the catalyst unlike the inner part of the bulk, has free valencies which provide the seat for chemical forces of attraction. When a gas comes in contact with such a surface, its molecules are held up there due to loose chemical combination. If different molecules are adsorbed side by side, they may react with each other resulting in the formation of new molecules. Thus, formed molecules may evaporate leaving the surface for the fresh reactant molecules.
(i)
(ii)
Heterogeneous catalysts are V2O5 and platinum respectively. Reacting gases get adsorbed on heterogeneous solid catalyst and probability of collision increases which result in increase in number of effective collisions, resulting an increase in rate of reaction.
Products formed get desorbed from the catalyst and get ‘absorbed and react, leading to increase in rate of reaction.
Explain the following terms with an example of each:
(i) Emulsification,
(ii) Chemisorption.
Answer:
(i) Emulsification. The process of making an emulsion in presence of emulsifying agent is called emulsification e.g., fats are emulsified by bile juice in our body and help in metabolism.
(ii) Chemisorption. When the force of attraction between adsorbent and adsorbate is similar to that of chemical bond it is called chemisorption e.g., N2 and H2 get adsorbed on ‘Fe’ in manufacture of NH3 by Haber’s Process.
(i) What is the charge of AgI colloidal particles in the two test tube (A) and (B).
(ii) Give reason for the origin of change.
Answer:
When silver nitrate solution is added to potassium iodide solution, negatively charged sol of AgI is formed this due to selective adsorption of I- ion from the dispersion medium.
AgI + I- ---> [AgI]I-
On other hand when KI is added to AgNO3 solution a positively charged sol of AgI is formed this is due to slective adsorption of Ag+ ion present in the dispersion medium.
AgI + Ag+ ---> [AgI]Ag+.
(a) What is gel? Give an example.
(b) How is gold sol prepared by reduction method ?
Answer:
(a) Gel is a colloidal solution of liquid in a solid, i.e., liquid is dispersed phase and solid is the dispersion medium. A gel is a semi-solid which melts on heating. Examples of gels are: Thin jellies, gum arobic, processed cheese, silicic acid.
(b) It is prepared by reduction of gold chloride (AuCl3) solution with stannous chloride
2AuCl3 + 2SnCl → 3SnCl4 + 2Au
(Violet gold sol.)
Answer:
(i) How are the values of k and n determined experimentally?
(ii) Explain why the value of n should be equal to or greater than one?
Freundlich isotherm explains the behaviour
of adsorption in an approximate manner. The
factor 1 n can have values between 0 and 1
(probable range 0.1 to 0.5). Thus, above equation
When 1/n = 0, x/ m= constant, the adsorption is independent of pressure.
When 1/n = 1, x/m= k p, i.e. x/m∝ p, the adsorption varies directly with pressure.
Explain the following observations:
(a) Ferric hydroxide sol gets coagulated on addition of sodium chloride solution.
(b) Cottrell’s smoke precipitator is fitted at the mouth of the chimney used in factories.
(c) Physical adsorption is multi layered, while chemisorption non layered.
Answer:
(a) When sodium chloride is added to ferric hydroxide sol, the sol gets coagulated. This is because the Cl– ions from NaCl neutralize the +ve charge on Fe(OH)3 sol.
Fe2O3.xH2O/Fe3+
(b) Smoke coming out of the chimenys contain a lot of unburnt carbon particles smoke is the colloidal solution of the solid particles such as carbon arsenic dust particles etc.
These particles are injurious to health and have to be precipitated since carbon particlesare charged particles and cannot be precipitated, thus metal plate of oppositely charged particles is used for their precipitation.
This is done by the cottrell smoke precipitator which neutralizes the charge on the carbon.
(c) Physical adsorption is multilayered because forces between adsorbate and adsorbent molecule are of Van der Waal’s type.
Chemisorption is non-layers because forces of attraction are of chemical bond type.
|
Homogeneous catalysis |
Heterogeneous catalysis |
|
1. When the reactants and the catalyst are in the same phase (i.e., liquid or gas), the process is said to be homogeneous catalysis. Oxidation of sulphur dioxide into sulphur trioxide with dioxygen 2. The reaction is assumed to proceed through the formation of an intermediate between the catalyst and one of the reactants. |
1. The catalytic process in which the reactant and the catalyst are in different phase is known as heterogenous. Oxidation of sulphur dioxide into sulphur trioxide in the 2. The reaction proceeds through the adsorption of one or more reactants over the surface of the catalyst. |
(i) temperature increases at constant pressure.
(ii) pressure increases at constant temperature.
Answer:
(i) The extent of absorption (x/m) decreases with the increase in temperature.
(ii) The extent of absorption (x/m) increases with the increase in pressure.
Answer:
The catalytic activity is localised on the surface of the catalyst. The mechanism involves five steps:
(i) Diffusion of reactants to the surface of the catalyst.
(ii) Adsorption of reactant molecules on the surface of the catalyst.
(iii) Occurrence of chemical reaction on the catalyst’s surface through formation of an intermediate.
(iv) Desorption of reaction products from the catalyst surface, and thereby, making the surface available again for more reaction to occur.
(v) Diffusion of reaction products away from the catalyst’s surface. The surface of the catalyst unlike the inner part of the bulk, has free valencies which provide the seat for chemical forces of attraction. When a gas comes in contact with such a surface, its molecules are held up there due to loose chemical combination. If different molecules are adsorbed side by side, they may react with each other resulting in the formation of new molecules. Thus, formed molecules may evaporate leaving the surface for the fresh reactant molecules.
Answer:
Purification of colloidal solutions:
Dialysis: It is the process of separating colloidal particles from those of crystalloids by diffusing the mixture through a parchment or animal membrane. Colloidal particles do not pass through while those of crystalloids do. The impure sol is filled into a cellophane bag, which is suspended in a vessel containing distilled water. The crystalloid particles pass through while colloidal particles do not.
Ultra filtration: It is the process of purifying a colloidal particle by treatment with a filter paper treated with colloidion which reduces the pore size of the filter paper. The sol is poured over the ultrafilter which permits the electrolytic solution to pass but retains the colloidal particles.
Answer:
Colloids including emulsions have great number of uses in day to day life, such as
(i) Sewage disposal Sewage water contains charged colloidal particles of dirt, rubbish etc. which are removed by electrophoresis.
(ii) Rubber planting. The negatively charged rubber particles from rubber sol are deposited on handles of tools.
(iii) Cottrell smoke precipitator. Smoke contains positively charged colloidal particles which are separated by Cottrell smoke precipitation.
(iv) In preparing nano-material Nano material are used as catalysts and are prepared by using reverse micelles.
(v) Building roads Emulsified asphalt in water is used in making roads.
(vi) In disinfectants Dettol, lysol etc. disinfectants forms emulsions of oil in water and are then used.
(vii) In medicine Medicines and pharmaceutical preparation in the form of emulsions are more easily assimilated.
(viii) In metallurgical operations Froth floatation process involves treatment of pulverised ore in emulsion of pure oil.
Answer:
Colloids and colloidal state: In the light of present day researches, Thomas Graham's classification of substances into colloids and crystalloids has been found to be wrong. Graham thought of colloids as substances. Now-a-days the term is applied not so much to the substance but rather to a state in which that substance exists. The same substance can behave as a crystalloid in one solvent and a colloid in another.
Origin of charge on colloidal particles:
According to the modern theory of electrical double layer (proposed by Helmholtz) the charge on colloidal particles is due to the preferential adsorption of common ion on their surface and consequently an electrical double layer is formed, in which adsorbed ions form one layer; while the remaining oppositely charged ions form the second diffused layer. Examples:
(i) When freshly precipitated ferric hydroxide is peptized with dilute FeCI3, a +ve sol is formed, due to the adsorption of Fe3+ ions of FeCl3 on the surface of Fe(OH)3 colloid particles, while the negative CI– ions form the diffused layer. This may be shown as:
(ii) As2S3 sol prepared by passing H2S gas through As2O3 solution., is – ve due to preferential adsorption of common ions S2– on its surface, while the H+ ions forming the diffused layer. This may be shown as:
A.
Starch in water behaves as a macro molecular colloid.B.
Physical adsorption is a spontaneous process.C.
The amount of gas adsorbed on charcoal increases with increase in temperature and pressure both.D.
Inversion of cane sugar in the presence of dilute sulphuric acid is an example of homogeneous catalysis.E.
A catalyst can initiate the reaction.C.
dialysisA.
liquid dispersed in gasB.
Fat dispersed in waterC.
Solid dispersed in gasB.
A layer of medium of dispersion on their particlesC.
The colloidal particles are solvatedD.
Arsenic sulphide solutionD.
Urea solutionC.
Impact of molecules of the dispersion medium on the colloidal particlesC.
Scattering of blue light by water molecules.Answer:
Adsorption involves attracting molecules of adsorbate on surface of the adsorbent. Due to this, energy is released and thus heat of adsorption is negative i.e. adsorption is always exothermic.
Answer:
In sol dispersed medium is liquid and dispersed phase is solid. for example paints, cell fluid
In emulsion both dispersed phase and dispersion phase are in liquid foam. for example milk,butter.
Answer:
This can be explained as the higher the critical temperture the more easily gas is liquified that is larger the vander waal's force of attraction and hence more readily it will be adsorbed.
NH3 gas has higher critical temperature than N2
thus NH3 adsorbed more readily on the surface of charcoal.
Answer:
The brownian movement has been explained to be due to the unblanced bombardment of the particles by the molecules of the dispersion medium.
When colloidal solutions are viewed under
a powerful ultramicroscope, the colloidal particles appear to be in a state of continuous zig-zag motion all over the field of view. This motion was first observed by the British botanist, Robert Brown, and is known as Brownian movement.
This motion isindependent of the nature of the colloid but depends on the size of the particels and viscosity of the solution samller the size and lesser the viscosity faster is the motion.
Answer:
A colloidal sol of sulphur can be obtained by passing hydrogen sulphide into solution of sulphur dioxide in water or through a solution of an oxidising agent.
SO2 + 2H2S --------> 3S + 2H2O
Answer:
when we pass a beam of light through the colloidal solution tyndall effect shows.
tyndall effect is due to the fact that colloidal particles scatter light in all direction in space.
Answer:
Ultra- filtration is the physical removal of particles and microbiological cotaminates from an aqueous solution using a membrane filter with pore size less than 0.1 micron . It does not remove dissolved ion and small molecules.
membrane may be supplied as flat sheet sprialwound or hollow fibre modules.
Answer:
Peptization may be defined as the process of converting a precipitate into colloidal soln by shaking it with dispersion medium in the presence of a small amount aof electrolyte.
The electrolyte is used for this purpose called peptizing agent.
This method is applied generally to convert a freshly prepared precipitate into a colloidal sol.
Answer:
The variation in the amount of gas adsorbed by the adsorbent with pressure at constant temperature can be expressed by means of a curve termed as adsorption isotherm.
Freundlich gave an empirical relationship between the quantity of gas adsorbed by unit mass of solid adsorbent and pressure at a particular temperature. The relationship can be expressed by the following equation:
x/m = k.p1/n (n > 1)
where x is the mass of the gas adsorbed on mass m of the adsorbent at pressure P, k and n are constants which depend on the nature of the adsorbent and the gas at a particular
temperature.
Answer:
3H2S + 2HNO3 -----> 3S + 2NO + 4H2O
Due to the formation of colloidal sulphur the colourless solution turned to turbid.
Answer:
Emulsifying Agents are the substances added to an emulsion to prevent the coalescence of the globules of the dispersed phase. They are also known as emulgents or emulsifiers. They act by reducing the interfacial tension between the two phases and forming a stable interfacial film. The choice of selection of emulsifying agent plays a very important role in the formulation of a stable emulsion.
Substances that are soluble in both fat and water and enable fat to be uniformly dispersed in water as an emulsion.
Answer:
Particles of dispersed phase have great affinity for the dispersion medium.
They are self stabilised because of strong attractive forces operating and the dispersion medium. for example gums, gelatin etc.
Answer:
Sorption is the process in which adsorption and absorption take place simultaneously, example dyeing of cotton fibres by azo dyes.
Answer:
When freshly precipitated Fe(OH)3 is shaken with little amount of dilute solution of FeCl3(electrolyte).
Peptization process (ions of the electrolyte are adsorbed by the precipitate particles) takes place by converting the Fe(OH)3 precipitate into colloidal solution positively charged solution of Fe(OH)3.
Answer:
The process of dialysis is quite slow it can be made faster by applying an electric field.
Answer:
An emulsion may be defined as a colloidal dispersion of two immiscible or partially immiscible liquids, in which one liquid acts as the dispersion medium and the other as the dispersed phase.
Depending on the nature of the dispersed phase, emulsions are broadly classified into two types
i) Oil in water type emulsion.
Oil acts as the dispersed phase water acts as a dispersion medium. e.g. milk.
ii) Water in oil type emulsion
water acts the dispersed phase and oil acts as the dispersion medium e.g. butter.
Emulsion are generally unstable and separate in two layer on standing.
Thus, to stabilise an emulsion small amount of certain other substance, called emulsifier or emulsifying agent are added.
Protein are the principal emulsifying agent for oil in water types of emulsion.
Answer:
The process of Adsorption is an Exothermic Reaction. Thus according to Le-chatlier’s Principle, the magnitude of adsorption should increase with decrease in temperature. Infact it is found to be so in case of physical adsorption because vanderwaal’s forces are strong at low temperatures.
However, the chemisorption first increases with rise in temperature and then starts decreasing. The initial increase shows that like chemical reactions, chemisorption also needs activation energy.
If a plot is drawn between amount of gas adsorbed (x/m) and temperature at constant equilibrium pressure, then curve obtained for physical adsorption shows there is a regular decrease in adsorption with temperature rise. While for chemisorption it first increases and then shows regular decrease. Such curves are known as Adsorption Isobars.
Answer:
When alum (Al2(SO4)3 is added into water. It forms a gel like precipitate such as Al(OH)3 . The alum precipitate then forms small bundles that trap suspended particles as they fall through water.
This process is known as floculation.
After floculation the particles will be large enough to settle and can be removed.
Answer:
In gel Dispersing phase is liquid and Dispersing medium is solid. for example cheese, butter.
In emulsion both dispersing phase asnd dispersing medium are in liquid . for example milk.
Answer:
The catalytic process in which the reactants and the catalyst are in different phases is known as heterogeneous catalysis.
examples of heterogeneous catalysis are given below:
Oxidation of sulphur dioxide into sulphur trioxide in the
presence of Pt.
2SO2 (g) 2SO3 (g)
The reactant is in gaseous state while the catalyst is in the solid state.
(a) How can a colloidal solution and true solution of the same colour be distinguished from each other ?
(b) List four applications of adsorption.
Explain the following observations:
(a) Lyophilic colloids are more stable than lyophobic colloid.
(b) Coagulation takes place when sodium chloride solution is added to a colloidal solution of ferric hydroxide.
(c) Sky appears blue in colour.
Answer:
a) Particles of dispersed phase have great affinity for the dispersion medium. They are self stabilised because of strong attractive forces operating between the suspended particles and the dispersion medium. e.g. gums gelatin.
b) When freshly precipitate Fe(OH)3 is shaken with little amount of dilute solution of FeCl3 (electrolyte)
Peptization process takes place by converting the Fe(OH)3 precipitate into colloidal solution positively charged solution of Fe(OH)3.
c) Transmitted light is made up of a spectrum of colors the longest wavelength of light are on red end of spectrum and the shortest wavelength areon the blue/ violet end of the spectrum.
When transmitted light such as sunlight enters ours atmosphere it colloide with dust and water droplet and scattered violet/blue light.
But because our eyes are more sensitive to blue color thus we see blue color of sky.
(a) In which of the following does adsorption take place and why?
(i) Silica gel placed in the atmosphere saturated with water.
(ii) Anhydrous CaCl2 placed in the atmosphere saturated with water.
(b) How does BF3 act as a catalyst in industrial process?
(c) Give an example of shape selective catalysis.
Answer:
When soap molecule are present in water the molecules arrange themselves in the form of culster in such a manner that their hydrophobic ends are away from the water molecules and their hydrophillic or ionic end are towards the water molecules.
This is known as micelle formation and the cluster that is formed is called as micelle.
micelle system behaves as a true solution in low concentration and as colloidal solution as high concentration.
Answer:
i)The ions carrying charge opposite to that of sol particles are effective in bringing about the coagulation sol.
ii)coagulation power of the fourth power of the valency of the ions causing coagulation.
(a) Of NH3 and CO2 which will be adsorbed more readily on the surface of charcoal and why?
(b) How are multimolecular colloids different from macro-molecular colloids?
(c) State the principle of electro-dialysis.
Answer:
The variation in the amount of gas adsorbed by the adsorbent with pressure at constant temperature can be expressed by means of a curve termed as adsorption isotherm.
Freundlich adsorption isotherm: Freundlich gave an empirical relationship between the quantity of gas adsorbed by unit mass of solid adsorbent and pressure at a particular temperature. The
relationship can be expressed by the following equation:
x/m = k.p1/n (n > 1)
where x is the mass of the gas adsorbed on mass m of the adsorbent at pressure P, k and n are constants which depend on the nature of the adsorbent and the gas at a particular
temperature.
LANGMUIR ADSORPTION:
It tells about the number of active site of the surface undergoing adsorption and pressure.
Where,
θ= the number of sites of the surface which are covered with gaseous molecule,
P= pressure
K =is the equilibrium constant for distribution of adsorbate between the surface and the gas phase .
The basic limitation of Langmuir adsorption equation is that it is valid at low pressure only.
At lower pressure, KP is so small, that factor (1+KP) in denominator can almost be ignored. So Langmuir equation reduces to
At high pressure KP is so large, that factor (1+KP) in denominator is nearly equal to KP. So Langmuir equation reduces to
(a) Which will absorb more gas, a lump of charcoal or its powder and why?
(b) Describe the preparation of the following colloidal solutions. Name the method used in each case (i) silver sol, (ii) sulphur sol.
Answer:
The accumulation of molecular species at the surface rather than in the bulk of a solid or liquid is termed adsorption. The molecular species or substance, which concentrates or accumulates at the surface is termed adsorbate and the material on the surface of which the adsorption takes place is called adsorbent.
There are mainly two types of adsorption of gases on solids:
i)physisorption
ii)chemisorption
The extent of adsorption increases with the increase of surface area of the adsorbent. Thus, finelydivided lamp of charcoal powder and porous substances having large surface areas are good adsorbents.
b)
Silver sol can be prepared by the double decomposition reaction.
In this dilute solution of silver nitrate react with potassium chloride to give silver chloride solution.
AgNO3 + KCl -----> AgCl + KNO3
sulphur sol may be prepared by oxidation of hydrogen sulphide by bromine.
H2S +Br2---> S + 2HBr
Explain What is observed when:
(a) A beam of light is passed through a colloidal solution of As2S3
(b) An electrolyte (NaCl) is added to ferric hydroxide sol.
(c) An electric current is passed through a colloidal solution.
a) When beam of light passed through a colloidal solution of As2S3. the path of the light become visible beacuse of the tyndall effect.
b) When freshly precipitated Fe(OH)3 is shaken with little amount of dilute solution of NaCl (electrolyte).
Peptization process take place by converting the Fe(OH)3 into colloidal solution.
As ferric hydroxide Fe(OH)3 is positively charged sol it gets coagulated by Cl- ion released by NaCl
c)The movement of colloidal particles under an applied electric potential is called electrophoresis. Positively charged particles move towards the cathode while negatively charged particles move towards the anode.
When the reactants and the catalyst are in the same phase (i.e., liquid or gas), the process is said to be homogeneous catalysis. The following are some of the examples of homogeneous catalysis:
(i) Oxidation of sulphur dioxide into sulphur trioxide with dioxygen in the presence of oxides of nitrogen as the catalyst in the lead chamber process.
2SO2(g) + O2(g) 2SO3(g)
The reactants, sulphur dioxide and oxygen, and the catalyst, nitric oxide, are all in the same phase.
(ii) Hydrolysis of methyl acetate is catalysed by H+ ions furnished by hydrochloric acid.
CH3COOCH3(l) + H2O(l) CH3COOH(aq) + CH3OH(aq)
The catalytic process in which the reactants and the catalyst are in different phases is known as heterogeneous catalysis. Some of the examples of heterogeneous catalysis are given below:
(i) Oxidation of sulphur dioxide into sulphur trioxide in the presence of Pt.
2SO2 2SO3
The reactant is in gaseous state while the catalyst is in the solid state.
(ii) Combination between dinitrogen and dihydrogen to form ammonia in the presence of finely divided iron in Haber’s process.
N2(g) +3H2(g) 2NH3(g)
The reactants are in gaseous state while the catalyst is in the solid state.
Illustrate with example:
(i) Lyophilic and Lyophobic sols.
(ii) Multimolecular and macromolecular colloids.
(iii) Homogeneous and Heterogeneous catalysis.
|
property |
Lyophilic sols (suspensoid) |
Lyophobic sols (Emulsoid) |
|
Surface tension |
Lower than that of the medium |
Same as that of the medium |
|
Viscosity |
Much higher than that of the medium |
Same as that of the medium |
|
Reversibility |
Reversible |
Irreversible |
|
Stability |
More stable |
Less stable |
|
Visibility |
Particles can’t be detected even under ultramicroscope |
Particles can be detected under ultramicroscope. |
The extent of adsorption of gases on a solid surface depend on following factor:
Surface area of adsorbent: The extent of adsorption increases with the increase of surface area of the adsorbent. Thus, finely divided metals and porous substances having large surface areas are good adsorbents.
It depends on the nature of gas. More easily liquefiable gases are adsorbed readily.
Low temperature is favourable for adsorption. It decreases with increase of temperature
| MULTIMOLECULAR COLLOIDS | MACROMOLECULAR COLLOIDS | ACCOCIATED COLLOIDS |
| They are formed by the aggregate of a large number of atoms or molecules which generally have diameters less than 1nm, e.g. Sols of gold, sulpher etc. | They are molecules of large size e.g. polymers like rubber, nylon, starch. | They are formed by of aggregation of a large number of ions in concentrated solution e.g. soap sol. |
| Their molecular masses are not very high. | They have high molecular masses. | Their molecular masses are generally high. |
| Their atoms or molecules are held together by weak vander Waal’s forces. | Due to long chain, the vander Waal’s forces holding them are comparatively stronger. | Higher is the concentration; greater are the vander Waal’s forces. |
| They usually have lyophilic character. | They usually have lyophobic character. | Their molecules contain both lyophobic and lyophilic characters |
Enzymes are complex nitrogenous organic compounds which are produced by living plants and animals. They are actually protein molecules of high molecular mass and form colloidal solutions inwater.
There are a number of cavities present on the surface of colloidal particles of enzymes. These cavities are of characteristic shape and possess active groups such as -NH2 , -COOH, -SH, -OH, etc. These are actually the active centres on the surface of enzyme particles. The molecules of the reactant (substrate), which have complementary shape, fit into these cavities just like a key fits into a lock. On account of the presence of active groups, an activated complex is formed which then decomposes to yield the products.
Thus, the enzyme-catalysed reactions may be considered to proceed in two steps.
Step 1: Binding of enzyme to substrate to form an activated complex.
E + S → ES≠
Step 2: Decomposition of the activated complex to form product.
ES≠ → E + P
The process of Adsorption is an Exothermic Reaction. Thus according to Le-chatlier’s Principle, the magnitude of adsorption should increase with decrease in temperature. Infact it is found to be so in case of physical adsorption because vanderwaal’s forces are strong at low temperatures. However, the chemisorption first increases with rise in temperature and then starts decreasing. The initial increase shows that like chemical reactions, chemisorption also needs activation energy.
If a plot is drawn between amount of gas adsorbed (x/m) and temperature at constant equilibrium pressure, then curve obtained for physical adsorption shows there is a regular decrease in adsorption with temperature rise. While for chemisorption it first increases and then shows regular decrease. Such curves are known as Adsorption Isobars.
The quantity of the electrolyte which is required to coagulate a definite amount of a colloidal solution depends upon the valency of the ion having a charge opposite to that of the colloidal particles. This observation of Hardy and Schulze are known as Hardy Schulze law.
Dialysis:It is a process of removing a dissolved substance from a colloidal solution by means of diffusion through a suitable membrane.
Since particles ions or smaller molecules) in a true solution can pass through animal membrane (bladder) or parchment paper or cellophane sheet but not the colloidal particles, the membrane can be used for dialysis.
A bag of suitable membrane containing the colloidal solution is suspended in a vessel through which fresh water is continuously flowing. The molecules and ions diffuse through membrane into the outer water and pure colloidal solution is left behind.
The accumulation of molecular species
at the surface rather than in the bulk of a solid or liquid is termed adsorption. The molecular species or substance, which concentrates or accumulates at the surface is termed adsorbate and the material on the surface of which the adsorption takes place is called adsorbent.
|
ADSORPTION |
ABSORPTION |
| It is surface phenomenon i.e. it occurs only at the surface of the adsorbent. | It is bulk phenomenon i.e. it occurs throughout the body of the material. |
| In this phenomenon, the concentration on the surface of adsorbent is different from that in the bulk. | In this phenomenon, the concentration is same throughout the material. |
| Its rate is high in the beginning and then decreases till equilibrium is attained. | Its rate remains same throughout the process. |
The variation in the amount of gas adsorbed by the adsorbent with pressure at constant temperature can be expressed by means of a curve termed as adsorption isotherm.
Most of the adsorptions are exothermic reactions, Hence adsorption generally depends on temperature. Exothermic reactions are mostly spontaneous at low temperature. Hence the extent of adsorption decreases with the increase of temperature at constant pressure. A plot of the extent of adsorption versus temperature at constant pressure is known as adsorption Isobar.
Freundlich adsorption isotherm: Freundlich gave an empirical relationship between the quantity of gas adsorbed by a unit mass of solid adsorbent and pressure at a particular temperature. The
the relationship can be expressed by the following equation:
Freundlich Isotherm: The relationship between x/m and pressure of the gas at constant temperature is called adsorption isotherm and given as by 
and n depend upon the nature of gas and the solid.
x/m first increase with the increase in pressure at low pressure but becomes independent of pressure at high pressure.
Thus three cases arise from the graph
At low pressure, the extent of adsorption is directly proportional to pressure (raised to power one).
At high pressure, the extent of adsorption is independent of pressure (raised to power zero).
Therefore at an intermediate value of pressure, adsorption is directly proportional to pressure raised to power 1/n.Here n is a variable whose value is greater than one.
Using constant of proportionality, k, also known as adsorption constant we get
The above equation is known as Freundlich adsorption equation.
As per Freundlich adsorption equation
Taking logarithm on both sides, we get
Here x is the weight of the gas absorbed by m mass of the adsorbent at a pressure p, k and n are constant (at a particular temperature) and for a particular adsorbate-adsorbent pair.
The equation above equation is comparable with comparable with the equation of a straight line,
y = m x + c where m represents the slope of the line and c represents intercept on y-axis.
Plotting a graph between log(x/m) and log p, we will get a straight line with the value of slope equal to 1/n and log k as y-axis intercept.
log(x/m) vs. log p graph.
Colloidal Solution is a heterogeneous mixture in which particle size of substance is intermediate of true solution and suspension i.e. between 1-1000 nm. Smoke from a fire is example of colloidal system in which tiny particles of solid float in air.
|
Property |
True Solution |
Colloidal Solutions |
|
Size of the particles |
1nm– 1000nm |
1000nm |
|
Nature |
Homogeneous |
Heterogeneous |
|
Filterability |
Particles of true Solution diffuse rapidly through filter paper as well as parchment paper. |
Colloidal particles pass through filter paper but not through parchment paper |
|
VisibilityParticles |
True Solution are not visible to naked eye. |
Colloidal particles are not seen to naked eye but can be studied through ultra microscope |
|
Tyndall effect |
Solution does not show Tyndall effect. |
Colloids shows Tyndall effect.
|
|
Brownian movement |
True solution does not show Brownian movement. |
Brownian movement shown in colloidal solution |
Since the colloidal solutions consist of dispersed phase and dispersion medium, the components of these phases have certain specific characteristics of their own.
a) CLASSIFICATION BASED ON PHYSICAL STATE OF DISPERSED PHASE AND DISPERSION MEDIUM
Based on the physical state of dispersed phase and dispersion medium, there are eight types of colloidal solutions. Either these phases may be solid, liquid or gas. Since gases gives homogeneous mixture in all proportions, hence colloidal solution can not be obtained in which dispersed phase and dispersion medium are gases.
|
S.No.
|
Dispersed Phase
|
Dispersion Medium
|
Name of Colloidal Solution
|
Examples
|
|
1.
|
Gas
|
Liquid
|
Foam
|
Soap lather, whipped cream, soda water, froth on beer.
|
|
2.
|
Gas
|
Solid
|
Solid foam
|
Cake, Bread, Pumice stone lava.
|
|
3.
|
Liquid
|
Gas
|
Aerosol
|
Mist, fog, Clouds.
|
|
4.
|
Liquid
|
Liquid
|
Emulsion
|
Milk, cream, butter, oil in water
|
|
5.
|
Liquid
|
Solid
|
Gel.
|
Curd, Cheese, Jellies, boot polish.
|
|
6.
|
Solid
|
Gas
|
Solid Aerosol
|
Smoke, dust.
|
|
7.
|
Solid
|
Liquid
|
Sols
|
Paints, gold sol, inks.
|
|
8.
|
Solid
|
Solid
|
Solid Sols
|
Alloys, coloured glass, gem stones.
|
b)
(i) Explain the meaning of the statement. ‘Adsorption is a surface phenomenon.’
(ii) State two features of chemical adsorption which are not found in physical adsorption.
(iii) Explain the terms with suitable examples: (i) Gel, (ii) Aerosol and (iii) Hydrosol.
i)The accumulation of molecular species at the surface rather than in the bulk of a solid or liquid is termed adsorption.
Adsorption is essentially a surface phenomenon. Solids, particularly in finely divided state, have large surface area and therefore, charcoal, silica gel, alumina gel, clay, colloids, metals in finely divided state, etc. act as good adsorbents.
ii)
|
Physisorption |
Chemisorption |
|
It arises because of van der |
It is caused by chemical bond |
|
It is not specific in nature. |
It is highly specific in nature. |
|
It is reversible in nature |
It is irreversible. |
ii) Aersol: A colloidal solution having a gas as the dispersion medium and a solid as dispersed phase is called an aerosol. For example fog.
iii) Hydrodol: A colloidal solution having water as the dispersion medium and a solid as the dispersed phase is called a hydrosol. For example starch sol, gold sol.
Out of BaCl2 and KCl, which one is more effective in causing coagulation of a negatively charged colloidal Sol? Give reason.
According to the Hardy–Schulze rule, the greater the valency of a flocculating ion, the greater will its coagulating power. In Between Ba2+ (from BaCl2) and K+ (from KCl), Ba2+ has greater valency 2 whereas KCl has valency 1. Therefore, BaCl2 will be more effective in causing the coagulation of a negatively charged colloidal sol.
Give one example each of 'oil in water' and 'water in oil' emulsion.
|
Type of emulsion |
Example |
|
Oil in water |
Milk, vanishing cream |
|
Water in oil |
Butter, cold cream, cod liver oil |
In reference to Freundlich adsorption isotherm, write the expression for adsorption of gases on solids in the form of an equation.
Freundlich adsorption isotherm for adsorption of gases on solids:
where, x is the mass of the adsorbate, m is the mass of the absorbent and p is the pressure of the gas and n is a constant which is greater than 1.
Write an important characteristic of lyophilic sols.
Lyophilic sols are sols that are solvent-attracting. An important characteristic of these sols is that if the dispersion medium is separated from the dispersion phase by any method, the sol can be reconstituted by simply remixing the two again. That is why these sols are also known as reversible sols
Based on the type of particles of dispersed phase, give one example each of associated colloid and multimolecular colloid.
Example of associated colloid: Soap solution
Example of multimolecular colloid: Gold sol, sulphur sol
Write the dispersed phase and dispersion medium of the following colloidal systems:
(i) Smoke (ii) Milk
Dispersion medium means the medium in which the substance in form of colloidal particles and dispersed phase means the substance distributed in the dispersion medium in form of colloidal particles.
(i) Dispersed phase in smoke: Solid and dispersion medium in smoke: Gas
(ii) Dispersed phase in milk: Liquid Fat and dispersion medium in milk: Water
What are lyophilic and lyophobic colloids? Which of these sols can be easily coagulated on the addition of small amounts of electrolytes?
Lyophilic colloids: It is made up of two words; ‘Lyo’ meaning liquid and ‘Phillic’ meaning loving, so those colloids which are attracted by the liquid (solvent), are called as lyophilic colloids. These are also called reversible sols. These are quite stable and cannot be easily coagulated. Example are Sols of organic substances like gelatin, gum, starch and proteins
Lyophobic colloids: It is made up of two words; ‘Lyo’ meaning liquid and ‘Phobic’ meaning repelling, so those colloids which are repelled by the liquid ( solvent), are called as lyophobic colloids. These are also called irreversible sols and these are unstable and can be easily coagulated due to lack of protecting layer around charged colloidal particles, they easily form a cluster. Hence, they got easily coagulated on the addition of small amount of electrolyte. Examples are Sols of inorganic substances like Arsenic (As2S3), Iron (Fe(OH)3) and Platinum.
Write the differences between physisorption and chemisorption with respect to the following:
(i) Specificity
(ii) Temperature dependence
(iii) Reversibility and
(iv) Enthalpy change
Chemisorption is a kind of adsorption which involves a chemical reaction between the surface and the adsorbate.
Physiosorption, also called physical adsorption, is a process in which the electronic structure of the atom or molecule is barely perturbed upon adsorption.
|
S. No. |
Point of difference |
Physiosorption |
Chemisorption |
|
(i) |
Specificity |
This is not specific in nature. |
This is highly specific in nature. |
|
(ii) |
Temperature dependence |
Low temperature is favourable for physisorption. It decreases with increase in temperature. |
High temperature is favourable for chemisorption. It increases with the increase in temperature. |
|
(iii) |
Reversibility |
This is reversible in nature. |
This is irreversible in nature. |
|
(iv) |
Enthalpy change |
Enthalpy of adsorption is low. |
Enthalpy of adsorption is high. |
What is meant by coagulation of a colloidal solution? Describe briefly any three methods by which coagulation of lyophobic sols can be carried out.
Coagulation is a process of aggregating together the colloidal particles so as to change them into large sized particles which ultimately settle as a precipitate. It can also be called as precipitation.
Coagulation is generally brought about by the addition of electrolyte.
When an electrolyte is added to the colloidal solution, the particles of the sol take up the ion which is oppositely charged and thus get neutralised.
Following are the three methods by which coagulation of lyophobic sols can be carried out.
(i) Electrophoresis: In this process, the colloidal particles move towards oppositely charged electrodes and get discharged resulting in coagulation.
(ii) Mixing of two oppositely charged sols: When equal proportions of oppositely charged sols are mixed, they neutralise each other resulting in coagulation.
(iii) Dialysis: By this method, electrolytes present in sol are removed completely and colloid becomes unstably resulting in coagulation.
Define each of the following terms:
(i) Micelles
(ii)Peptization
(iii)Desorption
(i) A micelle is an aggregate of surfactant molecules dispersed in a liquid. A micelle in aqueous solution forms an aggregate such that the hydrophilic 'head' regions are in contact with the surrounding solvent and the hydrophobic tail regions are in the center of micelle.
(ii) Peptization is the process of conversion of a precipitate into a colloidal sol by shaking it with the dispersion medium in the presence of an electrolyte. The electrolyte used in this reaction is known as a peptizing agent.
(iii) Desorption is the process of removing an adsorbed substance from the surface through which it was adsorbed.
Write the dispersed phase and dispersion medium of butter.
In colloidal dispersion, one substance is dispersed as very fine particles in another substance called dispersion medium. In the case of dust, solid particles are dispersed in air as the dispersion medium.
Dispersed phase: Liquid
Dispersion medium: Solid
Define the following terms:
(i) Brownian movement
(ii) Peptization
(iii) Multimolecular colloids
(i) The continuous random zigzag motion of colloidal particles in the colloidal solution, when observed under an ultramicroscope, is called Brownian movement/motion.
(ii) Peptization is defined as the process of conversion of a precipitate into a colloidal solution by shaking it with the dispersion medium in presence of small amount of an electrolyte.
(iii) Multimolecular colloids are species having a size in the colloidal range (diameter < 1 nm) that are formed by the aggregation of a large number of atoms or small molecules of substances when dissolved in the dispersion medium.
Of physisorption or chemisorption, which has a higher enthalpy of adsorption?
Chemisorption has a higher enthalpy of adsorption as compared to physisorption.
Define the following terms giving an example of each:
(i) Associated colloids
(ii) Lyophilic sol
(iii) Adsorption
(i) Associated colloids: Associated colloids are those colloids which behave as normal strong electrolytes at low concentrations but exhibit colloidal properties at higher concentrations due to the formation of aggregated particles thus formed are micelles. For e.g. grease droplet surrounded by stearate ions.
(ii) Lyophilic sol: Lyophilic means liquid-loving colloidal sols directly formed by mixing substances like gum, gelatine etc. with a suitable liquid (the dispersion medium) are called lyophilic sols. If the dispersion medium is separated from dispersed phase (say by evaporation), the sol can be reconstituted by simply remising with the dispersion medium that is why these sols are also known as reversible sols.
(iii) Adsorption: The accumulation of molecular species at the surface rather than in the bulk of a solid or liquid is known as adsorption. The molecular species which accumulates on the surface is termed as adsorbate and the material on the surface of which the adsorption takes place is called adsorbent.
For the e.g. aqueous solution of raw sugar, when passed over beds of animal charcoal, becomes colourless as the colouring substances are adsorbed by the charcoal.
Define ‘peptization’.
Peptization can be defined as a process of converting a precipitate into colloidal sol by shaking it with dispersion medium in the presence of small amount of electrolyte.The electrolyte used in this process is known as a peptizing agent.
Write three distinct features of chemisorptions which are not found in physisorptions.
Chemisorption is a kind of adsorption which involves a chemical reaction between the surface and the adsorbate.
Physisorption, also called physical adsorption, is a process in which the electronic structure of the atom or molecule is barely perturbed upon adsorption.
|
S. No. |
Point of difference |
Physiosorption |
Chemisorption |
|
(i) |
Specificity |
This is not specific in nature. |
This is highly specific in nature. |
|
(ii) |
Temperature dependence |
Low temperature is favourable for physiosorption. It decreases with increase in temperature. |
High temperature is favourable for chemisorption. It increases with the increase in temperature. |
|
(iii) |
Reversibility |
This is reversible in nature. |
This is irreversible in nature. |
|
(iv) |
Enthalpy change |
Enthalpy of adsorption is low. |
Enthalpy of adsorption is high. |
Write the main reason for the stability of colloidal sols.
There are two main reasons for the stability of colloidal sols:
Solvation: Colloidal particles are covered by a sheath of liquid in which they are extensively solvated, thereby providing stability.
Electrostatic stabilisation: Presence of equal and similar charges on the colloidal particles prevents coagulation of the colloidal sol.
(i) Differentiate between adsorbtion and absorption.
(ii)Out of MgCl2 and AlCl3, which one is more effective in causing coagulation of negatively charged sol and why?
(iii)Out of sulphur sol and proteins, which one form multimolecular colloids?
(i) Adsorption is a surface phenomenon that causes the accumulation of molecules of a substance at the surface of a solid or liquid rather than in the bulk. In adsorption, the substance gets concentrated at the surface only. It does not penetrate through the surface to the bulk of the solid or liquid. For example, when a chalk stick is dipped into an ink solution, only its surface becomes coloured.
On the other hand, the process of absorption is a bulk phenomenon. In absorption, the absorbed substance gets uniformly distributed throughout the bulk of the solid or liquid. For example, when a sponge is dipped in water the whole sponge gets wets.
(ii)According to the Schulze-Hardy rule, the effectiveness of the salt causing flocculation depends on the charge on the ion of opposite sign to the charge on the sol particles. The greater the magnitude of the opposite charge, the higher the ability of a salt to coagulate the sol.
Thus, trivalent salt AlCl3 is more effective in causing the coagulation of a negatively charged sol than divalent salt MgCl2.
(iii)Out of sulphur sol and proteins, sulphur sol forms multimolecular colloids. Sulphur sol consists of particles containing a thousand or more S8 molecules. On the other hand, proteins form macromolecular colloids.
What is the reason fo the stability of colloidal sols ?
In colloidal sols, the colloidal particles carry a particular charge and the dispersion medium carries an equal but opposite charge. Due to the presence of these similar and equal charges, the particles of the colloid sol repel one another and cannot combine together to form larger particles which keep them dispersed in the medium and the colloidal remains stable.
Define the following terms:
i)Lyophilic colloid
ii) Zeta potential
iii) Associated colloids
i) Lyophilic colloids are liquid loving colloids (Lyo means solvent and philic means loving). When these colloids are mixed with the suitable liquid, high force of attraction exists between colloidal particles and liquid. This result in the formation of very stable solution called lyophilic sol. These sols are formed by substances like gums, starch and proteins.
ii) Zeta Potential : zeta potential is the potential difference between the dispersion medium and the stationary layer of fluid attached to the dispersed particle.
iii) Association colloids are microheterogeneous systems in which the particles of the colloidal dispersed phase (micelles) are formed by the agglomeration of molecules or ions of the substance dissolved in the dispersion medium.
Classify colloids where the dispersion medium is water. State their characteristics and write an example of each of these classes.
OR
Explain what is observed when
(i) An electric current is passed through a sol
(ii) A beam of light is passed through a sol
(iii) An electrolyte (say NaCl) is added to ferric hydroxide solColloids containing dispersion medium as water can be classified as follows:
|
Dispersed phase |
Dispersion medium |
Type of colloid |
Example |
|
Solid |
Liquid |
Sol |
Paints, cell fluids |
|
Liquid |
Liquid |
Emulsion |
Milk, hair cream |
|
Gas |
Liquid |
Foam |
Froth, soap lather |
OR
(i) The colloidal particles are charged and carry either a positive or a negative charge. The dispersion medium carries an equal and opposite charge. This makes the entire system neutral. Under the influence of an electric current, the colloidal particles move towards the oppositely charged electrode. When they come in contact with the electrode, they lose their charge and coagulate.
(ii) When a beam of light is passed through a colloidal solution, a scattering of light is observed. This is known as Tyndall effect. This scattering of light illuminates the path of the beam in the colloidal solution.
(iii) When NaCl is added to hydrated ferric hydroxide sol, it dissociates to give Na+ and Cl- ions. Particles of ferric hydroxide sol are positively charged. This neutralises the colloidal particles which then unite to form bigger particles that are consequently precipitated. Thus, they get coagulated in the presence of negatively charged Cl- ions.
What are emulsions? What are their different types? Give one example of each type.
Emulsions are the liquid−liquid colloidal systems, obtained on mixing two immiscible or partially miscible liquids.
There are two types of emulsions.
(1) Oil dispersed in the water type (O/W) : Here water is the dispersion medium, e.g. milk.
(2) Water dispersed in oil type (W/O) : Here oil is the dispersion medium, e.g. butter.
Write one similarity between Physisorption and Chemisorption?
(i) Physisorption and Chemisorption both are a surface phenomenon.
(ii) Physisorption and Chemisorption both increase with increase in surface area
Write one difference in each of the following :
Lyophobic sol and Lyophilic sol
Lyophilic sol: These are also called reversible sols. These are quite stable and cannot be easily coagulated. Example are Sols of organic substances like gelatin, gum, starch and proteins
Lyophobic sol: These are also called irreversible sols and these are unstable and can be easily coagulated due to lack of protecting layer around charged colloidal particles, they easily form a cluster. Hence, they got easily coagulated on the addition of small amount of electrolyte. Examples are Sols of inorganic substances like Arsenic (As2S3), Iron (Fe(OH)3 and platinum.
Write one difference in each of the following :
Homogeneous catalysis and Heterogeneous catalysis
Homogeneous catalysis: When the reactants and the catalyst are in same phase (i.e., Liquid or gas), the process is said to be homogeneous catalysis. For example, Oxidation of sulphur dioxide into sulphur trioxide with dioxygen in presence of oxides of nitrogen as the catalyst in the process.
2SO2(g) + O2(g)→ NO(g) +2SO3(g)
Heterogeneous catalysis: The catalytic process in which the reactant and the catalyst are in different phase is known as heterogeneous.For example Oxidation of sulphur dioxide into sulphur trioxide in the presence of Pt.
2SO2(g) + pt(s)→ 2SO3 (g)
CO(g) and H2 (g) react to give different products in the presence of different catalysts. Which ability of the catalyst is shown by these reactions?
The characteristic shown by the catalyst is known as catalytic selectivity, i.e, the catalyst determines the products formed. For example,
What happens when
A freshly prepared precipitate of Fe(OH)3 is shaken with a small amount of FeCl3 solution?
When freshly precipitated Fe(OH)3 is shaken with the little amount of dilute solution of FeCl3(electrolyte).
Peptization process (ions of the electrolyte are adsorbed by the precipitate particles) takes place by converting the Fe(OH)3 precipitate into a colloidal solution positively charged solution of Fe(OH)3.
What happens when
Persistent dialysis of a colloidal solution is carried out?
On persistent dialysis, the electrolyte present in the sol is removed completely, the colloid becomes unstable and coagulates.
What happens when
An emulsion is centrifuged?
When an emulsion is centrifuged, demulsification takes place, i.e. the emulsion is broken into constituent liquids.
For a linear plot of log (x/m) versus log p in a Freundlich adsorption isotherm, which of the following statements is correct? (k and n are constants)
1/n appears as the intercept.
Only 1/n appears as the slope.
log (1/n) appears as the intercept.
Both k and 1/n appear in the slope term
B.
Only 1/n appears as the slope.
According to Freundlich adsorption isotherm,
On taking logarithm both sides, we get
c= intercept = log k, m =slope = 1/n and x = log p
The coagulating power of electrolytes having ions Na+, Al3+ and Ba2+ for arsenic sulphide sol increases in the order:
Al3+<Ba2+<Na+
Na+<Ba2+<Al3+
Ba2+< Na2+<Al3+
Al3+< Na+<Ba2+
B.
Na+<Ba2+<Al3+
As2S3 is an anionic sol (negative sol) hence coagulation will depend upon coagulating power of cation, which is directly proportional to the valency of cation (Hardy-Schulze rule).
According to Freundlich adsorption isotherm, which of the following is correct?
All of the above are correct for a different range of pressure
D.
All of the above are correct for a different range of pressure
Which of the following statements is incorrect regarding physissorptions?
It occurs because of vander Waal’s forces.
More easily liquefiable gases are adsorbed readily.
Under high pressure, it results into multimolecular layer on the adsorbent surface.
Enthalpy of adsorption ( ∆Hadsorption) is low and positive
D.
Enthalpy of adsorption ( ∆Hadsorption) is low and positive
Enthalpy of adsorption in physisorption is negative
∆G = ∆H - T∆S
As the entropy decreases (∆S = –ve) the ∆H must be negative having a high magnitude. Therefore, the spontaneous adsorption will have negative enthalpy change.
Gold numbers of protective colloids A, B, C and D are 0.50, 0.01, 0.10 and 0.005, respectively. The correct order of their protective powers is
D < A < C < B
C < B < D < A
A < C < B < D
B < D < A < C
C.
A < C < B < D
Higher the gold number lesser will be the protective power of colloid.
Fog is a colloidal solution of
Gas in liquid
Solid in gas
Gas in gas
Liquid in gas
D.
Liquid in gas
Fog is a colloidal solution of liquid in a gas in which liquid is the dispersed phase whereas gas is the dispersion medium.
Which property of colloidal solution is independent of charge on the colloidal particles?
Coagulation
Electrophoresis
Electroosmosis
Tyndall effect
D.
Tyndall effect
Coagulation is generally brought about by the addition of electrolytes. When an electrolyte is added to a colloidal solution, the particles of the sol take up the ions which are oppositely charged. As a result, their charge gets neutralised.
Electrophoresis: The movement of colloidal particles under an applied electric potential is called electrophoresis. Electroosmosis may be defined as a phenomenon in which the molecules of the dispersion medium are allowed to more under the influence of an electric field whereas colloidal particles are not allowed to more.
Tyndall effect is the scattering of light by sol particles, which cannot be affected by change on them
Which property of colloids is not dependent on the charge on colloidal particles?
Coagulation
Electrophoresis
Electro-osmosis
Tyndall effect
D.
Tyndall effect
Tyndall effect is an optical phenomenon which occurs due to the scattering of light by the colloidal particles. Hene, it is independent of charge.
The ease of adsorption of the hydrated alkali metal ions on an ion exchange resins follows the order
Li+ > K+ >Na+> Rb+
Rb+ < K+ < Na+ < Li+
K+ >Na+> Rb+ > Li+
Na+ < Li+ < K+ < Rb+
B.
Rb+ < K+ < Na+ < Li+
Ease of adsorption of the hydrated alkali metal ions on an ion-exchange resin decreases as the size of alkali metal ion increases.
since, the order of the size of alkali metal ions
Li+ < Na+ < K+ < Rb+
Thus, the ease of adsorption follows the order
Rb+ < K+ < Na+ < Li+
The protecting power of lyophilic colloidal sol is expressed in terms of
Coagulation value
Gold number
Critical micelle concentration
Oxidation number
B.
Gold number
Lyophobic sols are unstable, so they re-stabilised by adding some lyophilic colloids which protect them from precipitation. Thus, lyophilic colloids are called protecting colloids. Their protecting power is expressed in terms of gold number.
[Lesser the gold number, higher si the protecting power,]
The Langmuir adsorption isotherm is deduced using the assumption:
the adsorption takes place in multilayers
The adsorption sites are equivalent in their ability to adsorb the particles
The adsorbed molecules interact with each other
The adsorbed molecules interact with each other
A.
the adsorption takes place in multilayers
The main points of Langmuir theroy of adsorption are as:
i) Adsorption takes place on the surface of the solid only till the whole of the surface is completely covered with a unimolecular layer of the adsorbed gas.
ii) Adsorption consist fo two opposing processes (a) condensation, and (b) evaporation.
iii) The rate of condensation depend upon the uncovered surface of the adsorbent available for condensation.
A plot of log x/m versus log p for the adsorption of a gas on a solid gives a straight line with slope equal to:
-log k
n
1/n
log k
C.
1/n
The empirical relation x/m = kp1/n put forward by Freundlich is known as Freundlich adsorption isotherm. Taking logarithm
If the following curve is plotted
On which of the following properties does the coagulating power of an ion depend?
The magnitude of the charge on the ion alone
Size of the ion alone
The sign of the charge on the ion alone
Both magnitude and sign of the charge on the ion
D.
Both magnitude and sign of the charge on the ion
According to Hardy Schulze rule, coagulating power of an ion depends on both magnitude and sign of the charge on the ion.
Given van der Waals constant for NH3, H2, O2 and CO2 are respectively 4.17, 0.244, 1.36 and 3.59, which one of the following gases is most easily liquefied?
NH3
H2
CO2
O2
A.
NH3
Vander Waal constant 'a', signifies intermolecular forces of attraction.
Higher is the value of 'a', easier will be the liquefaction of gas.
Which of the following is not applicable to the phenomenon of adsorption?
ΔH> 0
ΔG<0
ΔS<0
ΔH<0
D.
ΔH<0
Due to adsorption attractive forces increases, thus ΔH<0 i.e. (-) ve.
Sponsor Area
Sponsor Area
