The s-Block Elements

Question 165
CBSEENCH11007143

Explain why alkaline earth metals are poor reducing agents as compared to alkali metals.

Solution

The ionisation enthalpies of alkaline earth metals are higher and their electrode potentials are less negative than the corresponding alkali metals, therefore alkaline earth metals are weaker reducing agents than alkali metals.

Question 166
CBSEENCH11007144

Explain the trend of solubility of carbonate, sulphates and hydroxides of alkaline earth metals ?

Solution

The solubility of carbonates and sulphates of these metals decreases downward in the group. This is because the magnitude of the lattice enthalpy remains almost constant as the carbonate or sulphate is so big that small increase in the size of the cations from Be to Ba does not make any difference. However, the hydration enthalpy decreases from Be2+ to Ba2+ sufficiently with the increase in their size resulting in the decrease of solubility of their carbonates or sulphates.

The solubility of hydroxides of these metals in water increases downward in the group. This is due to the fact that the lattice enthalpy decreases down the group due to increase in the size of the cation of the alkaline earth metal. On the other hand, the hydration enthalpy of the cations of alkaline earth metals decreases as we go down the group. As a result ∆Hsolution (∆Hlattice – ∆Hhydration) becomes more negative and solubility increases.

Question 167
CBSEENCH11007145

Account for the following:
(i) Be(OH)2 is amphoteric while Mg(OH)is basic.

(ii) Be(OH)2 is insoluble but Ba(OH)2 is fairly soluble in water.


Solution

(i) This is because I.E. of Mg < I.E. of Be. So bond M – OH can break more easily in Mg(OH)2 than in Be(OH)2.
(ii) This is because with the increase in size (from Be to Ba), the lattice enthalpy decreases significantly but hydration enthalpy remains almost constant.

Question 168
CBSEENCH11007146

The hydroxides and carbonates of sodium and potassium are easily soluble in water while the corresponding salts of magnesium and calcium are sparingly soluble in water. Explain ?

Solution
This is due to the larger size of Na and K. as compared to that of Mg and Ca. As a result, the lattice energies of hydroxides and carbonates of sodium and potassium are much lower than most of the corresponding salts of magnesium and calcium. Consequently, the hydroxides and carbonates of sodium and potassium are easily soluble in water while the corresponding salts of magnesium and calcium are sparingly soluble in water.

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