Question

Which of the following is a decomposition reaction?
[A] $A{{g}^{+}}(aq)+C{{l}^{-}}(aq)\to AgCl(s)$
[B] ${{C}_{6}}{{H}_{12}}(l)+8{{O}_{2}}(g)\to 5C{{O}_{2}}(g)+6{{H}_{2}}O(l)$
[C] $N{{H}_{3}}(l)+{{H}_{2}}O(l)\rightleftharpoons N{{H}_{4}}^{+}(aq)+O{{H}^{-}}(aq)$
[D] $H{{C}_{2}}{{H}_{3}}{{O}_{2}}(l)+O{{H}^{-}}(aq.)\to {{H}_{2}}O(l)+{{C}_{2}}{{H}_{3}}{{O}_{2}}^{-}(aq)$
[E] $CaC{{O}_{3}}(s)\to CaO(s)+C{{O}_{2}}(g)$

Answer 1

Hint: Decomposition reaction is a reaction where a single compound breaks and gives us two or more products. Here, the correct answer is a metal carbonate. The atomic number of the metal is 20 and upon decomposition we get its oxide which is also known as quick lime.

Complete step by step answer:
Before answering this question, let us discuss what a decomposition reaction is.
We know that chemical reactions are of different types and we can classify them according to their working mechanism like combination or decomposition etc.
Decomposition reaction- It is the type of chemical reaction in which a single compound breaks down into two or more elements. These reactions can be represented as-
     \[AB\to A+B\]

In decomposition reaction, as we can see from the above reaction that a bond is broken to give us two (or more) products form a single compound. These reactions are generally endothermic, as breaking of bond requires a high amount of energy.
A simple example of decomposition reaction is hydrolysis of water where a water molecule is broken down into hydrogen and oxygen gas.
 We can write the reaction as-
     \[2{{H}_{2}}O(l)\to 2H(g)+{{O}_{2}}(g)\]

There are some compounds (like metal carbonates) that readily decompose when exposed to high heat while others do not-
     \[MC{{O}_{3}}\xrightarrow{\Delta }MO+C{{O}_{2}}\]

The metal is denoted as ‘M’ in the above reaction where metal carbonates when heated decomposes into its oxide and releases carbon dioxide.

Now let us go through the options and find out which one is the decomposition reaction.
In the first option we have $A{{g}^{+}}(aq)+C{{l}^{-}}(aq)\to AgCl(s)$, as we can see here two different ions are combining here to give a single product thus this is not a decomposition reaction.
In the second option we have ${{C}_{6}}{{H}_{12}}(l)+8{{O}_{2}}(g)\to 5C{{O}_{2}}(g)+6{{H}_{2}}O(l)$, as we can see here that the products are formed from the combination of two different reactants thus this is not a decomposition reaction either.

Then we have $N{{H}_{3}}(l)+{{H}_{2}}O(l)\rightleftharpoons N{{H}_{4}}^{+}(aq)+O{{H}^{-}}(aq)$, we can see here that ammonia has dissolved in water and formed ions in the solution thus this is not a decomposition reaction either.

In the fourth option we have $H{{C}_{2}}{{H}_{3}}{{O}_{2}}(l)+O{{H}^{-}}(aq.)\to {{H}_{2}}O(l)+{{C}_{2}}{{H}_{3}}{{O}_{2}}^{-}(aq)$ where a proton transfer is taking place thus this is not a decomposition reaction.

And lastly we have $CaC{{O}_{3}}(s)\to CaO(s)+C{{O}_{2}}(g)$ where calcium carbonate which is a metal carbonate decomposes into calcium oxide and carbon dioxide gas is liberated. We have seen in the above discussion too that such a reaction is a decomposition reaction.
So $CaC{{O}_{3}}(s)\to CaO(s)+Ca{{O}_{2}}(g)$ is a decomposition reaction.
So, the correct answer is “Option E”.

Note: Decomposition reactions are generally disproportionation reactions. Here, the same compound oxidises and reduces to give us two different products. But, not all decomposition reactions are disproportionation reactions. For example, decomposition of carbonic acid does not involve reduction or oxidation.
     \[{{H}_{2}}C{{O}_{3}}\to {{H}_{2}}O+C{{O}_{2}}\]