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

Dec 19, 2016

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:

$C u S {O}_{4} \left(s\right) + \text{excess water}$ ${H}_{2} O \left(l\right) \rightarrow C {u}^{2 +} \left(a q\right) + S {O}_{4}^{2 -} \left(a q\right)$

And this is clearly a chemical reaction, right down to the visible colour change observed. $C {u}^{2 +} \left(a q\right)$ is a coordination complex, that is probably ${\left[C u {\left(O {H}_{2}\right)}_{6}\right]}^{2 +}$.

On the other hand, when we take the blue hydrate, $C u S {O}_{4} \cdot 5 {H}_{2} O$ and place it in water, strong ionic bonds between the sulfate and copper ions must be broken for dissolution.