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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