Question

On heating to $400^\circ C - 500^\circ C$, relatively unstable hydrides and carbonates decompose. Which of the following will decompose when heated to $400^\circ C - 500^\circ C$?
I. $LiH$
II. $NaH$
III. $L{i_2}C{O_3}$
IV. $N{a_2}C{O_3}$
A)II, III
B) I, II, III
C) I, III
D) III, IV

Answer 1

Thermal decomposition, or thermolysis, is a chemical decomposition caused by heat. The decomposition temperature of a substance is the temperature at which the substance chemically decomposes. The compounds of the metal that are at the bottom of the reactivity series generally decompose easily at high temperatures.

Complete answer:
$LiH$ and $NaH$ are the hydrides of group one while $L{i_2}C{O_3}$ and $N{a_2}C{O_3}$ are the carbonates of group one.
We know that thermal stability of hydrides decreases while moving down the group. It is highest in lithium hydride and lowest in caesium hydride and follows the trend: $LiH > NaH > KH > RbH > CsH$ .
Thermal stability decreases because as we move down the group, the size of atoms increases due to addition of new shells in orbitals. This increase in size results in increased bond length between alkali metal and hydrogen atom. Hence, the decomposition of hydride becomes easy.
Hence, sodium hydride will decompose more easily than lithium hydride when heated to $400^\circ C - 500^\circ C$.
In case of carbonates of group one, thermal stability increases down the group. The general trend for carbonates is:
$L{i_2}C{O_3} < N{a_2}C{O_3} < {K_2}C{O_3} < R{b_2}C{O_3}$
This happens because the charge density is very low and decreases down the group. Due to this, the thermal stability keeps on increasing down the group.
Hence, lithium carbonate is least stable and will decompose easily when heated to $400^\circ C - 500^\circ C$.
Therefore, $NaH$ and $L{i_2}C{O_3}$ are the compounds which will decompose when heated to $400^\circ C - 500^\circ C$.
Therefore, the correct option is (A).

Note:
The reactions in thermal decomposition are usually endothermic as heat is required to break chemical bonds in the compound undergoing decomposition. If decomposition is sufficiently exothermic, a positive feedback loop is created producing thermal runaway and possibly an explosion or other chemical reaction.