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?

Copyright © 1997-2010 by Fred Senese
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Last Revised 08/17/15.URL: http://antoine.frostburg.edu/chem/senese/101/thermo/faq/print-always-add-acid.shtml