The s-Block Elements

Question 201
CBSEENCH11007179

Explain why can alkali and alkaline earth metals not be obtained by chemical reduction methods?

Solution

In the process of chemical reduction, oxides of metals are reduced using a stronger reducing agent. alkali and alkaline earth metals are themselves strong reducing agent and hence cannot be extracted by reduction of their oxides and alkaline earth metals.
alkali metals being highly electron positive cannot be displaced from the aqueous solution of their salts by other metals.

Question 202
CBSEENCH11007180

Why are potassium and caesium, rather than lithium used in photoelectric cells?

Solution
Potassium and caesium are  used in photoelectric cells rather than lithium because potassium and caesium have much lower ionisation enthalpy than that of lithium. As a result, these two metals when subjected to light, emit electrons but lithium does not. Hence potassium and caesium, rather than lithium are used in photoelectric cells.
Question 203
CBSEENCH11007181

Compare the solubility and thermal stability of the following compounds of the alkali metals with those of the alkaline earth metals. (a) Nitrates (b) Carbonates (c) Sulphates. 

Solution

Carbonates of metal: Thermal stability
The carbonates of alkali metals except lithium carbonate are stable to heat. The carbonates of group-2 metals and that of lithium decompose on heating, forming an oxide and carbon dioxide . For example,
Li2CO3 +heat -> Li­2O +CO2
MgCO3 +Heat -> MgO +CO2
Na2CO3 +heat -> no effect.
The stabilities of carbonates of alkaline earth metals increase on moving down the group.For example, BeCO3 decompose at 373K, MgCO3 at 813K, CaCO3 at 1173K, SrCO3 at 1563K .
solubilities in water:
carbonates of alkali metals, except Li2CO3, are soluble in water. Their solubilities increase on moving down the group. Carbonates of group-2 metals are almost insoluble in water and their solubilities further decrease on moving down the group. In fact, these metals can precipitate from their salt solutions as carbonates.
Nitrates: Thermal stability
Nitrates of alkali metals,except LiNO3, decompose on strong heating forming nitrites and oxygen. for example,
2KNO3 -> 2KNO2 +O2
Nitrates of alkaline-earth metals and LiNO3 decompose on heating to form oxides, nitrogen to form oxides, nitrogen dioxide and oxygen.
2LiNO3 +Heat -> Li2O +2NO2 +O2
2Ca(NO3)2 +Heat -> 2CaO +4NO2 +O2
Thermal stabilities of nitrates of group-1 and group-2 metals increase on moving down the group from top to bottom.
solubility :
Nitrates of group -1 and group-2 metals are all soluble in water. The solubilities of these salts further increase on descending the group.

Sulphates: Thermal stability
The sulphates of group-1 and group-2 metals are all thermally stable.
solubility: 
sulphates of alkali metals are soluble in water. The sulphate of alkaline earth metals is less soluble. Their solubilities decrease on moving down the group from Be to Ba.

Question 204
CBSEENCH11007182

Starting with sodium chloride how would you proceed to prepare (i) sodium metal (ii) sodium hydroxide (iii) sodium peroxide (iv) sodium carbonate?

Solution

Na is prepared from NaCl by the following method:
bold Electrolysis bold colon bold space bold NaCl bold space bold rightwards arrow bold space bold Na to the power of bold plus bold space bold plus bold Cl to the power of bold minus
bold At bold space bold cathode bold colon bold space bold Na to the power of bold plus bold space bold plus bold e to the power of bold minus bold space bold rightwards arrow bold Na bold space
bold At bold space bold anode bold colon bold space bold Cl to the power of bold minus bold space bold rightwards arrow bold space bold Cl bold space bold plus bold e to the power of bold minus
bold space bold space bold space bold space bold space bold space bold space bold space bold space bold space bold space bold space bold space bold space bold Cl bold space bold plus bold Cl bold space bold rightwards arrow bold Cl subscript bold 2

2) Sodium hydroxide is prepared by carrying out the electrolysis of the aqueous solution of sodium chloride either in Nelson' cell or Castner Kellner cell.
Preparation of sodium peroxide: Sodium chloride is first converted in sodium by electrolytic reduction. The metals are then heated with an excess of oxygen at about 573K in an atmosphere free from moisture and carbon dioxide to form sodium peroxide.
bold space bold space bold 2 bold Na bold space bold plus bold O subscript bold 2 bold space bold rightwards arrow with bold 573 bold K bold space on top bold space stack bold Na subscript bold 2 bold O subscript bold 2 with bold sodium bold space bold peroxide below
space space

3) sodium carbonate is prepared from an aqueous solution of NaCl by Solvay process. In this process, CO2 is passed through NaCl solution saturated with ammonia when the following reaction occurs
bold space bold 2 bold NH subscript bold 3 bold space bold plus bold H subscript bold 2 bold O bold space bold plus bold CO subscript bold 2 bold space bold rightwards arrow bold left parenthesis bold NH subscript bold 4 bold right parenthesis subscript bold 2 bold CO subscript bold 3
bold left parenthesis bold NH subscript bold 4 bold right parenthesis subscript bold 2 bold CO subscript bold 3 bold space bold plus bold H subscript bold 2 bold O bold space bold plus bold CO subscript bold 2 bold space bold rightwards arrow bold space bold 2 bold NH subscript bold 4 bold HCO subscript bold 3
bold NH subscript bold 4 bold HCO subscript bold 3 bold space bold plus bold NaCl bold space bold rightwards arrow bold NH subscript bold 4 bold Cl bold space bold plus bold NaHCO subscript bold 3
bold 2 bold NaHCO subscript bold 3 bold rightwards arrow with bold Heat bold space bold space on top bold space bold Na subscript bold 2 bold CO subscript bold 3 bold space bold plus bold CO subscript bold 2 bold space bold plus bold H subscript bold 2 bold O:

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