On combustion of $\mathrm{Li}, \mathrm{Na}$ and K in excess of air, the major oxides formed, respectively, are:

(B) $\mathrm{Li}_{2} \mathrm{O}, \mathrm{Na}_{2} \mathrm{O}_{2}$ and $\mathrm{KO}_{2}$

(A) $\mathrm{Li}_{2} \mathrm{O}, \mathrm{Na}_{2} \mathrm{O}_{2}$ and $\mathrm{K}_{2} \mathrm{O}$

(C) $\mathrm{Li}_{2} \mathrm{O}, \mathrm{Na}_{2} \mathrm{O}$ and $\mathrm{K}_{2} \mathrm{O}_{2}$

(D) $\mathrm{Li}_{2} \mathrm{O}, \mathrm{Na}_{2} \mathrm{O}_{2}$ and $\mathrm{K}_{2} \mathrm{O}_{2}$

JEE Main · 2020 · 04 Sep 2020 · Shift 1 · s-Block Elements

On combustion of

Li, Na

and K in excess of air, the major oxides formed, respectively, are:

Answer: (B)

How an alkali metal burns in excess air depends on the size of its cation stabilising the oxide. Small lithium forms the normal oxide,

4 Li + O X_{2} 2 Li X_{2} O

; sodium forms the peroxide,

2 Na + O X_{2} Na X_{2} O X_{2}

; and the larger potassium forms the superoxide,

K + O X_{2} KO X_{2}

. The major products are thus

Li X_{2} O

Na X_{2} O X_{2}

and

KO X_{2}

JEE Main · 2020 · 04 Sep 2020 · Shift 1 · Single correct option