Explain thermal dissociation and thermal decomposition reactions with examples. What is the difference between them ?

A reversible decomposition reaction brought about only by heat is called thermal dissociation reaction.

For example, on application of heat, ammonium chloride decomposes into Ammonia and HCl:

NH₄Cl ⇌ NH₃ + HCl

A chemical reaction in which a compound decomposes to give two new elements / a new compound & an element / two new compounds on application of heat without any recombination on cooling is called a Thermal decomposition reaction.

For example, calcium carbonate decomposes to carbon dioxide and calcium oxide when heated to 1000°C

CaCO₃ (s) $\xrightarrow{\Delta}$ CaO (s) + CO₂ (g)

Difference between the two is that thermal dissociation is a reversible decomposition reaction whereas, thermal decomposition is an irreversible reaction.