what is the difference between thermal dissociation and thermal decomposition?

Any process of decomposition by heat is called thermal decomposition. A thermal decomposition may be reversible or irreversible reaction. 

 

 

Reversible thermal decomposition reactions, i.e. in which the prodcuts of decomposition can re-unite to form the original substance when the agent (heat) which brings about the decomposition is withdrawn are called thermal dissociation.

Thus, thermal dissociation is a special case of thermal decomposition.