sblock Elements 1 Question 12
8. A metal on combustion in excess air forms $X . X$ upon hydrolysis with water yields $\mathrm{H} _{2} \mathrm{O} _{2}$ and $\mathrm{O} _{2}$ along with another product. The metal is
(2019 Main, 12 Jan I)
(a) $\mathrm{Li}$
(b) $\mathrm{Mg}$
(c) $\mathrm{Rb}$
(d) $\mathrm{Na}$
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Answer:
Correct Answer: 8. (c)
Solution:
- Metal $(A)$ is rubidium $(\mathrm{Rb})$. In excess of air, it forms $\mathrm{RbO} _{2}(X)$. $X$ is a superoxide that have $\mathrm{O} _{2}^{-}$ion. It is due to the stabilisation of large anion by large cations through lattice energy effects. $\mathrm{RbO} _{2}(X)$ gets easily hydrolysed by water to form the hydroxide, $\mathrm{H} _{2} \mathrm{O} _{2}$ and $\mathrm{O} _{2}$.
The reaction involved are as follows:
$$ \begin{aligned} & \mathrm{Rb}+\mathrm{O} _{2} \longrightarrow \underset{(X)}{\mathrm{RbO} _{2}} \text { (superoxide) } \\ & \underset{(X)}{2 \mathrm{RbO} _{2}+2 \mathrm{H} _{2} \mathrm{O}} \longrightarrow 2 \mathrm{RbOH}+\mathrm{H} _{2} \mathrm{O} _{2}+\mathrm{O} _{2} \end{aligned} $$
