Question

Why is the dissolution of anhydrous copper sulfate exothermic, and the dehydration of `CuSO_4*5H_2O` endothermic?

Answer 1

Well one reaction is bond making, and the other is bond breaking.

Explanation:

And, clearly, bond making reactions are exothermic.

For hydration of anhydrous copper sulfate:

`CuSO_4(s) + "excess water"` `H_2O(l) rarr Cu^(2+)(aq) + SO_4^(2-)(aq)`

And this is clearly a chemical reaction, right down to the visible colour change observed. `Cu^(2+)(aq)` is a coordination complex, that is probably `[Cu(OH_2)_6]^(2+)`.

On the other hand, when we take the blue hydrate, `CuSO_4*5H_2O` and place it in water, strong ionic bonds between the sulfate and copper ions must be broken for dissolution.