Question

Decomposition brought about by heat is known as thermal decomposition. What is the difference between thermal dissociation and thermal decomposition?

Answer 1

Hint: Heating any substance can bring about decomposition of that substance or can be converted into ions of constituent atoms of that substance. Thermal decomposition is a chemical decomposition taking place due to action of heat. It is also referred to as thermolysis. During thermal decomposition of compounds, compounds break down to form two or more products from one reactant. For example, calcium carbonate decomposes on heating to produce calcium oxide and carbon dioxide gas.

Complete answer:
> We know the meaning of thermal decomposition of a compound. Let us understand the meaning of thermal dissociation of a compound.
> Thermal dissociation of a compound is the breaking down of that compound due to action of heat into smaller compounds or elements, which then reunite to form the original compound in the absence of heat. Therefore, thermal dissociation is one of the types of thermal decomposition but it is reversible in nature.

Point of difference	Thermal dissociation	Thermal decomposition
Meaning	It is a chemical reaction that involves breaking down compounds into ions that are cation and anion by the action of heat but the reaction is reversible.	It is a chemical decomposition that takes place by the action of heat.
Number of steps involved	Thermal dissociation is a single step process.	Thermal decomposition is a multistep process.
Reversibility of the reaction	It is generally reversible in nature, as the products formed during the dissociation react again to produce reactant. This happens when the reaction mixture containing products is cooled.	It may be a reversible or irreversible reaction. Products may or may not react back on cooling to form reactants.

Additional information:
As we are aware, thermal decomposition of a compound will occur only when heat energy is supplied to it. This heat energy supplied will be used for breaking down the chemical bonds in that compound. Hence, to bring about thermal decomposition of a compound energy must be supplied from outside. Hence, thermal decomposition of a compound is an endothermic reaction.

Note:
> We have understood the difference between thermal decomposition and thermal dissociation. Let us understand it with the help of examples.
> When zinc carbonate is heated, it decomposes to form zinc oxide and carbon dioxide. This is an example of thermal decomposition reaction where the compound decomposes into two simple compounds, zinc oxide and carbon dioxide.
$ZnC{O_3} \to ZnO + C{O_2}$
> When ammonium chloride is heated, it dissociates to produce ammonia and hydrogen chloride. The chemical reaction for the same is shown below.
$[N{H_4}Cl \rightleftharpoons N{H_3} + HCl]$
> Here, on heating ammonium chloride, two compounds $[N{H_3}]$ and $[HCl]$ is formed. This reaction mixture containing $[N{H_3}]$ and $[HCl]$ on cooling react back to form $[N{H_4}Cl]$. Here, thermal decomposition occurs but it is categorised as the thermal dissociation due to the reversible nature of the chemical reaction.