Q18 / 160 · AP EAMCET 2009 · Chemistry

Assertion (A) K, Rb and Cs form superoxides. Reason (R) The stability of the superoxides increases from ' $\mathrm{K}$ ' to ' $\mathrm{Cs}$ ' due to decrease in lattice energy. The correct answer is

  • A
    Both (A) and (R) are true and (R) is the correct explanation of (A)
  • B
    Both (A) and (R) are true but (R) is not the correct explanation of (A)
  • Correct answer
    C
    (A) is true but (R) is not true
  • D
    (A) is not true but (R) is true

Solution

Superoxides are the species having an $\mathrm{O}-\mathrm{O}$ bond and $\mathrm{O}$ in an oxidation state of $-\frac{1}{2}$ (superoxide ion is $\mathrm{O}_2^{-}$). Usually these are formed by active metals such as $\mathrm{KO}_2, \mathrm{RbO}_2$ and $\mathrm{CsO}_2$. For the salts of larger anions (like $\mathrm{O}_2^{-}$), lattice energy increases in a group. Since, lattice energy is the driving force for the formation of an ionic compound and its stability, the stability of the superoxides from ' $\mathrm{K}$ ' to 'Cs' also increases.

Hence, the correct answer is option: 'C'.

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