Question

Difference between thermal dissociation reaction and thrrmal decomposition reaction

Answer 1

Thermal dissociation is a simple, single-step reaction where a molecule splits into two products. Usually, only one bond breaks, and the reaction is reversible. Examples of thermal dissociation are: 
CH3Br --> CH3 + Br 
CH3CH3 --> CH3 + CH3 

Thermal decomposition implies a multi-step reaction with a complicated mixture of products. The process is not reversible. An example of thermal decomposition is the formation of crud on the bottom of the pan when something is overcooked on the stove, or in the bottom of the flask when a reaction is overheated. 

Another example: dead bodies decompose, they don't dissociate

Answer 2

Thermal decomposition:-

Reaction in which heat is used  for the breaking of a chemical substances in two or more parts.

CaCO3+heat --> CO2+CaO

 

Thermal dissociation:-

A compound is breaks reversibly into two parts in a single step

NH4Cl+heat --> NH3+HCl