Why does mixing a strong acid with water release so much heat?

Breaking a chemical bond requires energy (just as stretching a spring until it breaks requires energy). Forming a chemical bond will release energy. So in a reaction that releases heat (called an exothermic reaction), there must be net bond formation. If you look at the equation for dissolving a strong acid like HCl in water,

HCl(concentrated)

H₂O

H⁺ (aq) + Cl^-(aq)

you might think at first that this would have to be a heat-absorbing (endothermic) process, because it looks like the bond between H and Cl is broken. But there is another reaction hiding here. The hydrogen ion reacts with water to form a complex of the form H₃O•(H₂O)⁺_n (where the n can vary a bit). Whenever we write H⁺(aq), we actually mean H₃O•(H₂O)⁺_n. (It's much easier to write H⁺(aq)!) Because the hydrogen ion is so tiny, a large amount of charge is concentrated in a very small area, and the polar water molecules are strongly attracted to it. This 'hydration' of the hydrogen ion involves the formation of a covalent bond to one of the waters and a large number of strong hydrogen bonds, so it's a strongly exothermic process. This causes the mixing of a strong acid with water to be strongly exothermic overall.

Author: Fred Senese senese@antoine.frostburg.edu

General Chemistry Online! Why does mixing a strong acid with water release so much heat?