Assertion: Nitrogen and hydrogen combine together to form ammonia and heat.
Reason: It is an exothermic reaction.
Both assertion and reason are true and the reason is the correct explanation of assertion.
In the reaction, bonds between two atoms of nitrogen molecule and atoms of hydrogen molecules are broken and new bonds for ammonia are formed. During this process, heat energy is liberated to the surroundings. Hence this reaction is exothermic.
Hence, both the statements are correct, but reason cannot explain the assertion.