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

1 Answer
anor277
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:

#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.

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