In reversible reactions, the reaction in one direction will be exothermic and the reaction in the other direction will be endothermic.
The decomposition of ammonium chloride is a reversible reaction:
ammonium chlorideammonia + hydrogen chloride
ammonia + hydrogen chloride
Ammonium chloride decomposes when it is heated, so the forward reaction is endothermic - energy must be transferred from the surroundings for it to happen. The backward reaction is exothermic - energy is transferred to the surroundings when it happens.
Copper sulfate
The reaction between anhydrous copper sulfate and water is reversible:
hydrated copper sulfate (blue)anhydrous copper sulfate (white) + water
Water is driven off from hydrated copper sulfate when it is heated, so the forward reaction is endothermic - energy must be transferred from the surroundings for it to happen. The backward reaction is exothermic - energy is transferred to the surroundings when it happens. This is easily observed. When water is added to anhydrous copper sulfate, enough heat is released to make the water bubble and boil.
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