Why are acid base neutralization reactions exothermic? What does the strength of the acid have to do with it?

1 Answer
Jul 25, 2016

Acid base neutralization involves the formation of a salt and water. Such a process is inevitably exothermic.


Bond-breaking is endothermic, whereas bond formation is exothermic. Whether a given reaction is exothermic or endothermic depends on the balance between bond-breaking and bond-making.

In the generalized acid base reaction, as shown below, we form 2 substances, salt and water, whose formation should be intrinsically exothermic:

#"Acid" + "base" rarr "salt" + "water" + "energy"#

And for an obvious example...

#underbrace(HCl(aq) + NaOH(aq))_" acid + base" rarr underbrace(NaCl(aq) +H_2O(l))_" salt + water"#

The reaction is exothermic because we make strong #O-H# bonds, and thus the stronger the acid, the farther to the right this reaction should be driven. In practice, the water solvent exercises a moderating effect, and the energy released by the reaction of strong acids and strong bases should be similar.