Chemistry Part Ii Chapter 9 Hydrogen

A molecule of hydrogen is formed by the combination of two atoms of hydrogen with one electron each present in the 1s orbital. hydrogen (1s¹) has one electron less than the stable inert gas configuration (He;1s²) and therefore it shares its single electron with an electron of another hydrogen atom to form a stable diatomic molecule. Thus two electrons are present in the hydrogen molecule and both will be accommodated in the molecular orbital of lowest energy. The bond order of H₂ is +1. The positive value of bond order indicates that the H₂ (diatomic) molecules are stable.

It can also be prepared by the reaction of zinc with aqueous alkali.
Zn + 2NaOH → Na₂ZnO₂ + H₂

Protium

Ionic hydrides liberate H^- ions which act as a Bronsted base. They combine with water to produce H₂ gas.

The ionic hydrides can be used to remove last traces of water from organic compounds.

Dihydrogen is not preferred for filling of balloons and airships because dihydrogen is flammable in nature.

When molecular hydrogen is passed through tungsten electric are (2000 - 3000°C), at low pressure, it dissociates to form atoms of hydrogen known as atomic hydrogen.
                    

Atomic hydrogen is used for welding purposes in the form of atomic hydrogen torch. 

Lithium aluminium hydride (LiAlH₄) and sodium borohydride (NaBH₄) used as reducing agents in the laboratory.

Carbon monoxide and hydrogen (CO, H₂)

It is of bent shape. 

Drinking of hard water is harmful. If the concentration ca²⁺ and Mg²⁺ of ions is present in an excess amount. 

Hard water is not suitable for washing purposes since a lot of soap is wasted in precipitating out Ca²⁺ and Mg²⁺ ions present in hard water. 

The hardness of water depends on the concentration of Ca²⁺ and Mg²⁺ion. Also, sodium ions do not precipitate soap. Hence, sodium bicarbonate does not make hard water.

Sodium hexa-metaphosphate:
Na₂[Na₄(PO₃)₆] is called calgon.

Yes, Distilled water does not contain any cations and anions and hence it can be called deionized water.

It is obtained by repeated electrolysis of ordinary and water containing 3% NaOH.

Due to lower bond dissociation energy of protium bonds in H – O – H than deuterium bonds in D – O – D, electrolysis of H₂O occurs much faster than that of D₂O.

Advantage of using hydrogen as fuel over gasoline:
i) The high heat of combustion
ii) No pollutants like SO₂, NO₂, CO₂ etc.

By the action of sulphuric acid  on hydrated barium peroxide BaO₂.8H₂O

BaO₂.8H₂O + H₂SO₄ → BaSO₄ ↓+ H₂O₂ + 8H₂O
It must be noted that anhydrous barium peroxide does not react readily with sulphuric acid (because a coating of insoluble barium sulphate is formed on its surface which stops the further action of the acid). Therefore, hydrated barium peroxide, BaO₂.8H₂O must be used.

Saline hydrides react violently with water producing dihydrogen gas.

NaH(s)+ H₂O (aq) → NaOH (aq) + H₂ (g)

(i) 2-Ethyl anthraquinone is reduced to 2-ethyl anthraquinol by H₂ in the presence of palladium.
(ii) This 2-ethyl anthraquinol is oxidised by air to 2-ethyl anthraquinone again. Then H₂O₇ (20% solution) forms.

The decomposition can be accelerated in the presence of:
(i) MnO₂, carbon or metals like Pt.
(ii) light.

Hydrogen peroxide oxidises black coloured PbS to PbSO₄ and hence is used to restore the colour of old paintings. 

Isotopes: Isotopes are the atoms of the same element having the same number of electrons and protons but a different number of neutrons. They have the same atomic number but different mas-numbers.
Isotopes of the same element have the same chemical properties because of the presence of an equal number of electrons in their extra nuclear parts. Chemical properties of an element are dependent only on the number of electrons and their distribution in various shells around the nucleus. Isotopes of the same element differ in their physical properties because these isotopes differ in the number of neutrons which cause a difference in the mass numbers.
Isotopes of hydrogen:
Three isotopes of hydrogen are known. They are:
(i) Protium or ordinary hydrogen or light hydrogen  It is the most abundant isotope of hydrogen. Its atomic number is 1 and mass number is 1. Its nucleus consists of one proton and has one electron in the 1s orbital. It is a non-radioactive isotope of hydrogen.
(ii) Deuterium or heavy hydrogen 
It is present in heavy water (D₂O) and can be recovered from it by fractional electrolysis. Its atomic number is 1 and mass number is 2. Its nucleus consists of one proton and one neutron and has one electron in the 1s orbital. It is a non-radioactive isotope of hydrogen.
(iii) Tritium  It is the rarest isotope of hydrogen  Its atomic number is 1 and mass number is 3. Its nucleus consists of one proton and two neutrons and has one electron in the 1s orbital. Tritium is the only radioactive isotope of hydrogen.

Name of isotopes of hydrogen:

The electronic configuration of all the isotopes of hydrogen is same, therefore, chemical properties of all the isotopes are identical. However, they differ from each other in their rates of reaction and equilibrium constants for reversible reactions e.g. protium reacts with chlorine 13.4 times as fast as deuterium does.
Due to the difference in the masses of the isotopes of hydrogen, they have different physical properties. Property differences arising from differences in mass are called isotopic effects.
Properties: The difference in the masses of protium and deuterium produce appreciable changes in many of their physical properties like melting point and boiling point, latent heat of fusion, evaporation, sublimation etc. As deuterium is heavier than hydrogen, so the rates of a reaction involving deuterium are slower than reactions involving hydrogen. For the same reason, D₂ has a higher boiling point than that of H₂.

(i) Completely pure zinc metal is not used for the preparation of hydrogen because it is slowly attacked by the acid. The presence of impurities increases the rate of the reaction by forming electrochemical couples.
(ii) Concentrated H₂SO₄ cannot be used for the preparation of hydrogen gas as it results in the formation of SO₂ gas instead of H₂ from zinc.

These days, dihydrogen is produced commercially from hydrocarbons by reaction with steam in the presence of a catalyst and at high temperature. 
    
CO is converted to CO₂ by passing the gases and steam over an iron oxide or cobalt oxide catalyst at 673K resulting in the production of more H₂.


This is called water-gas shift reaction. A mixture of CO₂ and H₂ is passed through water under pressure. Carbon dioxide dissolves in water leaving behind hydrogen gas.

It is manufactured by the electrolysis of water containing a small amount of acid or alkali(electrolyte) using nickel plated iron as anode and iron as a cathode. The two electrodes are separated from each other by an asbestos diaphragm which prevents mixing of H₂ and O₂. On passing an electric current, water is decomposed into hydrogen and oxygen. Dihydrogen is collected at the cathode while oxygen is collected at the anode.
        
The role of electrolyte (acid) is to make water conducting. 

Lane's process consists of two stages:
(i) Oxidation stage: Super -heated steam is passed over iron filings, at 1023-1073 K when dihydrogen is formed.
               
(ii) Reduction stage: Iron is regenerated by reducing magnetic oxide with water gas (CO + H₂). This reaction is called vivification.

Thus, by passing steam and water gas alternatively over iron, dihydrogen gas can be manufactured from a small quantity of iron. 

The H-H bond dissociation enthalpy is the highest for a single bond between two atoms of any element.This influences the chemical behaviour of dihydrogen. It is inert at room temperature with many metals and non-metals. However, at high temperature or in the presence of the catalyst, it combines with metals and non-metals to form hydrides.

Since its orbital is incomplete with 1s electronic configuration, it combines with all the almost all elements.It takes part in reaction by
i) Loss of the only electron to give H+
ii)The gain of an electron to form H-.
iii)Sharing electron to form a single covalent bond.

(i) With chlorine. H₂ reduces chlorine into chloride (Cl^-) ion and itself get oxidised to H⁺ ion by chlorine to form HCl. An electron pair is shared between H and Cl leading to the formation of a covalent molecule.
H₂ +Cl₂ --> 2HCl
(ii) With sodium. H₂ is reduced by sodium to form NaH. An electron is transferred from Na to H leading to the formation of Na⁺H^- (ionic compound).
(iii) With copper (II) oxide. H₂ reduces copper (II) oxide to copper and itself gets oxidised to H₂O, which is a covalent molecule.
CuO +H₂ ---> Cu +H₂O

Hydrogenation means the addition of hydrogen across double and triple bonds to form saturated compounds.
Hydrogenation is used for the conversion of polyunsaturated oils into edible fats. When dihydrogen under pressure is bubbled through vegetable oils (contain many C = C bonds) in the presence of finely divided nickel at about 400K, they are converted into solid fats. The product formed is called vanaspati ghee (hydrogenated vegetable oil).

The above reaction is known as hardening of oils. 

A fuel cell is a device which converts the energy produced during the combustion of a fuel directly into electrical energy. Dihydrogen gas is used in H₂ – O₂ fuel cells for generating electrical energy. It should be noted that it has many advantages, for example, it does not produce any pollution and releases more amount of energy per unit mass of fuel as compared to gasoline and other fuels. 

Dihydrogen has higher bond dissociation energy (436 kJ mol^-1). Therefore, it does not react easily with other elements or substances at room temperature. 

The binary compounds of hydrogen with metals and non-metals are called hydrides. The hydrides are of three types:
(i) Ionic or salt like hydrides: These hydrides are formed by the highly electropositive elements of Group 1 and Group 2 except Be and Mg. They are formed by the transfer of electrons from the metals to the hydrogen atom. As such they are ionic compounds (Li⁺H^-) and thus behave like salts. For example, LiH, NaH, CaH₂.
(ii) Molecular or covalent hydrides: These are formed by elements having higher electronegativity than hydrogen i.e. by the p-block elements. These are formed by sharing of electrons between the element and hydrogen atom. As such they are covalent or molecular compounds. For example, B₂H₆, NH₃, H₂O, H₂S etc.
(iii) Metallic or interstitial hydrides: The d-block and f-block elements combine with hydrogen to form non-stoichiometric interstitial hydrides. Hydrogen is in the atomic state which occupies interstitial holes in close-packed metal structures. The composition of an interstitial hydride changes with temperature and pressure. These hydrides give out hydrogen easily and hence act as strong reducing agents.

Electron deficient compounds. Hydrides of group 13 (i.e. BH₃, AlH₃etc.) have lesser electrons to form normal covalent bonds and hence are called electron deficient hydrides. To make up this deficiency, these hydrides generally exist in polymeric forms such as B₂H₆, B₄H₁₀, (AlH₃)_n etc. They act as Lewis acids i.e. electron acceptors.
(ii) Electron-precise compounds. Electron precise compounds have the required number of electrons to write their conventional Lewis structures. All elements of group 14 form such compounds (i.e. CH₄, SiH₄, GeH₄, SnH₄, PbH₄), which are tetrahedral in geometry. They do not act as Lewis acids or Lewis bases.
(iii) Electron rich compounds. Electron rich hydrides have excess electrons which are present as lone pairs. Elements of group 15, 16, 17 form such compounds (NH₃, PH₃, H₂O, H₂S, HF, HCl etc.). They all behave as Lewis bases i.e. electron donors.

‘Non-stoichiometric’ are those hydrides in which the ratio of the metal to hydrogen is fractional. This fractional ratio is not fixed but varies with temperature and pressure. Many of the d-block elements and f-block elements (lanthanoids and actinoids) react with hydrogen at elevated temperature to give metallic or non-stoichiometric hydrides. For example LaH_2.87, TiH_1.8.,Zr H_1.9, VH_1.6 etc. Since in these hydrides, hydrogen is present in the interstices (holes or voids) existing in between the atoms, therefore these hydrides are also called interstitial hydrides.

Alkali metals do not form non-stoichiometric hydrides and form only stoichiometric hydrides. Alkali metals hydrides are ionic in which H^- ions are present in holes existing in between the atoms in the lattice. Since hydride ion (H⁺) is formed by complete transfer of lone electron of alkali metal to hydrogen, the, therefore, ratio of metal to hydrogen is always fixed. As a result, these alkali metals do not form non-stoichiometric hydrides. 

(i) The element having Z = 15 is a non-metal (i.e. P), therefore it forms covalent hydride (i.e. PH₃).
(ii) The element having Z = 19 is an alkali metal (i.e. K), therefore it forms ionic or saline hydride (i.e. K⁺ H^-).
(iii) The  element having Z = 23 is a transition metal (i.e. V) of group 5, therefore it forms an interstitial or metallic hydride (i.e. V H_1.6).
(iv) The element having Z =44 is a transition metal (i.e. Ru) of group 8, therefore it does not form any hydride.
Behaviour towards the water. Only ionic hydrides react with water evolving H₂ gas.

Organic compounds containing traces of water can be dried by placing these organic compounds in a desiccator containing saline hydrides (i.e. NaH, CaH₂ etc.) at the bottom for overnight. Saline hydrides absorb water and thus traces of water can be easily removed from the organic compounds.

Electrical conductance. The order of increasing electrical conductance is
BeH₂ < CaH₂ < TiH₂.
Reason: BeH₂ being a covalent hydride, therefore it does not conduct electricity. On the other hand, CaH₂ conducts electricity in the fused state and TiH₂ conducts electricity at room temperature. Hence order of increasing electrical conductance is
BeH₂<CaH₂<TiH₂

Reducing property. The order of increasing reducing character is:
H₂O < MgH₂ < NaH
Reason: NaH being an ionic hydride, therefore it is a powerful reducing agent. On the other hand, both MgH₂ and H₂O are covalent hydrides, but the bond dissociation energy of H₂O is more than that of MgH₂. Hence the reducing character increases in the order:
H₂O < MgH₂ < NaH

(i) When hydrogen is burnt in oxygen, heat is liberated. This heat is used in cutting as well as welding of metals. The apparatus in which heat is produced is called a oxy-hydrogen torch.
(ii) When dihydrogen is passed through vegetable oil at 8-10 atmospheric pressure and a temperature of 470 K.,vanaspati ghee (solid fat) is produced. The process is called hydrogenation or hardening of oils.

(iii) Dihydrogen combines with dinitrogen at 770 K and a pressure of 400-600 atmospheres in the presence of finely divided iron containing molybdenum, ammonia is produced. This is called Haber’s process for the manufacture of ammonia.

Atomic hydrogen is formed by passing dihydrogen gas through electric are struck between two hydrogen two tungsten electrodes, when dihydrogen molecules dissociate into H-atoms. 

The life period of atomic hydrogen is 0.3 sec. Consequently, it returns to molecular form (H₂) liberating a tremendous amount of energy which is used for cutting and welding purposes in the form of atomic hydrogen. 

It is used:
(i) in filling toy balloons and airships.
(ii) in the hydrogenation of oils to get vanaspati ghee.
(iii) in the manufacture of ammonia, methyl alcohol, water gas and fertilisers.
(iv) in oxy-hydrogen torch for welding purposes
(v) as a reducing agent in laboratory and industry.

Hydrogen is required for the synthesis of a number of industrial products and thus, affects the economy of a country. For example:
(i) Synthesis of ammonia: A mixture ofN₂ and H₂ in the ratio of 1:3 compressed to 200 atmospheres and heated to 673 K in the presence of a catalyst is used for the manufacture of ammonia in Haber's process.

(ii) Hydrogenation of oils: Hydrogen is used in the hydrogenation of edible oils (unsaturated in nature) to form solid fats for the production of vegetable ghee.

(iii) Manufacture of methyl alcohol: Water gas enriched with H2 is compressed to 200 atmospheres and is then passed over catalyst mixture of ZnO and Cr₂O₃ at 573K to form methyl alcohol.
   
(iv) Oxy-hydrogen flame. When hydrogen is burnt in oxygen heat is liberated. This heat is used in cutting as well as welding of metals. The apparatus in which heat is produced is called an oxy-hydrogen torch. Atomic hydrogen torch used for the same purpose employs atomic hydrogen.
(v) Synthetic petrol: Hydrogen is used as a constituent of synthetic petrol. It is formed by, passing hydrogen into a paste of powdered coal in crude oil by using a suitable catalyst.

Using H₂ as a fuel has two major advantages:
(i) Heat of combustion of H₂ is by far the largest, i.e. 115 megajoules per kilogram (MJ/kg):
(ii) Exhaust is free from pollutants.
The most effective means of storing hydrogen is in the liquid form. The liquid H₂ is widely used as a rocket fuel since it gives higher thrusts than most of the other fuels. The range of supersonic aircraft could be increased if the aircraft used liquid H₂ as a fuel. Further hypersonic aircraft would also become possible if liquid H₂ is used as a fuel.

Thus, in the liquid state, H₂O exists as an associated liquid.
We know that in ice, each oxygen atom is surrounded by four hydrogen atoms in such a way that two hydrogen atoms are linked to an oxygen atom by covalent bonds whereas other two hydrogen atoms are linked by hydrogen bonds. In ice (solid state), a water molecule is associated with four other water molecules through hydrogen bonding in a tetrahedral manner. This gives rise to open cage-like structure which prevents the close packing of molecules (lower density). When ice absorbs heat and melts to form water, hydrogen bonds break and close packing of water molecules take place. Due to this close packing, the density of water is higher than that of ice and ice floats over water.

Electronegativity of N, O F increases as N < O < F.
Therefore strength of hydrogen bond also increases as
N - H ... N < O - H ... O < F - H ... F
In other words, the magnitude of the +ve charge on hydrogen and -ve charge on F is highest and hence electrostatic attraction or hydrogen bonding is strongest in H – F.

(i) Water gas is formed

(ii) Perchloric acid is formed
                   
(iii) Bismuth oxychloride is formed
           
(iv) Silicon dioxide is formed. 
                  

With strongly active non-metals such as fluorine or chlorine, water acts as weak reductant and thus releases dioxygen. 
          

(i) Fresh tap water has a specific taste due to the presence of dissolved salts in it while distilled water has a flat taste.

ii) Tap water contains chloride ions due to the presence of dissolved chlorine. Hence tap water gives the white precipitate with silver nitrate solution. Distilled water does not give a white precipitate with silver nitrate solution indicating the absence of chloride ions.

Air dissolved in water has the following advantages:
(i) Oxygen present in water is very useful for giving life to fish and other sea animals in the water.
(ii) Carbondioxide dissolved in water is very useful for carrying out photosynthesis in plants.
(iii) Carbon dioxide dissolved in water reacts with calcium carbonate present in marble rocks to produce calcium bicarbonate which is soluble.

Marine organisms fulfil their requirement of calcium carbonate from calcium bicarbonate.

Water is a universal or ideal solvent because it has high dielectric constant (79.39C²/Nm²) and high dipole moment (μ = 1 . 84 D). Due to these properties, water dissolves most of the inorganic compounds and many covalent compounds. The solubility of ionic compounds in water is due to ion-dipole interaction or solvation of ions. Covalent compounds (such as alcohols, amines, urea, glucose sugar etc.) dissolve in water because such compounds form intermolecular hydrogen bonds with water.
Water can also hydrolyse many oxides, hydrides, carbides, nitrides, phosphides and other salts. In such reactions, H⁺ and OH^- ions of water interact with anions and cations respectively to form acid or base or both.

(i) The high heat of vaporisation and heat capacity of water are responsible for moderation of the climate and body temperature of living beings.
(ii) High specific heat capacity of water enables it to absorb heat of various biochemical and physiological reactions, going on inside the body, with the minimum rise of temperature.
(iii) Water has a high dipole moment (μ = 1.84D). So, water is an ideal medium for the dissolution of a wide variety of compounds.
(iv) Water is an ideal solvent for transportation of minerals and other nutrients for plant and animal metabolism.
(v) Water is an essential component for the photosynthesis in plants which releases O₂ in the atmosphere.

Water combines with many salts during crystallisation to form hydrates. For example, CuSO₄.5H₂O; FeSO₄.7H₂O ; Na₂SO₄, 10H₂CO etc.

This water in combination with ionic salts is called water of crystallisation and such crystals are called hydrated salts or simply hydrates. Water may be bonded to an anhydrous salt in three ways:
(i) Water molecules are coordinated to the central metal ion in complex ions such as [Li(H₂O)₆]⁺Cl^-.
(ii) Water may be bonded by hydrogen bonds in certain oxygen-containing anions. For example, in CuSO₄.5H₂O four water molecules are coordinated to a central Cu²⁺ ion while the fifth water molecule is linked to sulphate group through hydrogen bonding.
(iii) Water molecules may occupy voids in the crystal lattice. For example, BaCl₂.2H₂O.

Only one molecule of water, which is outside the bracket (coordinate sphere), is hydrogen bonded. The other four molecules of water are coordinated.

Hard water: A sample of water which does not produce lather with soap readily is called hard water. The hardness of water is due to the presence of bicarbonates, chlorides and sulphates of calcium and magnesium dissolved in water, e.g. sea water, river water, lake water and well water.
Soft water. A sample of water which produces lather with soap readily is called soft water. This type of water either does not contain or contains negligible amounts of dissolved salts in it. For example, rain water distilled water and demineralised water.

The hardness of water is due to the presence of bicarbonates, chlorides and sulphates of calcium and magnesium dissolved in water. It is of two types:
(i) Temporary hardness: It is due to the presence of bicarbonates of calcium, magnesium and iron dissolved in water, it is described as temporary hardness because it is easily removed by simply boiling the water. This type of hardness is also known as carbonate hardness.
(ii) Permanent hardness: It is due to the presence of chlorides and sulphates of calcium, magnesium and iron in the water. This type of hardness is called permanent hardness since it cannot be removed simply by boiling the water. It is also called non-carbonate hardness.


Excess of lime must be avoided because a solution of slaked lime shows hardness again due to absorption of CO₂ from the air. 

Soap is sodium or potassium salt of certain long chain fatty acids such as stearic acid, palmitic acid, oleic acid etc. When hard water is treated with soap solution, Ca²⁺and Mg²⁺ ions present in hard water react with the anions of fatty acids present in the soap to form a scum (curdy white precipitate).

Soap will not produce a lather with hard water until all the calcium and magnesium ions have been precipitated as stearates. Hard water thus wastes soap.

The hardness of water is of two types:
i) Temporary hardness
ii) permanent hardness

Temporary hardness:
Temporary hardness is due to the presence of magnesium and calcium hydrogen-carbonates. It can be removed by;
Boiling: During boiling, the soluble Mg(HCO₃)₂ is converted into insoluble Mg(OH)₂ and Ca(HCO₃)₂ is changed to insoluble CaCO₃. It is because of high solubility product of Mg(OH)₂ as compared to that of MgCO₃, that Mg(OH)₂ is precipitated. These precipitates can be removed by filtration. Filtrate thus obtained will be soft water.