Question

Why is a neutralization reaction exothermic?

Answer 1

In order to answer this question, one must understand what goes on during a neutralization reaction. In a neutralization raction, an acid and a base react to form salt and water.

Also, it's important to understand that during an exothermic reaction, bonds are being made and energy is released to the surroundings. This is what ultimately happens during a neutralization reaction that gives its exothermic character.

Take, for instance, the classic reaction between `HCl` (a strong acid) and `NaOH` (a strong base):

`HCl_((aq)) + NaOH_((aq)) -> NaCl_((aq)) + H_2O_((l))`

The complete ionic equation for this reaction is

`H_3O_((aq))^(+) + Cl_((aq))^(-) + Na_((aq))^(+) + OH_((aq))^(-) -> Na_((aq))^(+) + Cl_((aq))^(-) + 2H_2O_((l))`

The net ionic equation is determined by removing the spectator ions (the ions present on both the reactants, and the products' side):

`H_3O_((aq))^(+) + OH_((aq))^(-) -> 2H_2O_((l))`

SInce strong acids and strong bases are completely dissociated in solution, no formal bonds are being broken. The formation of two very strong covalent bonds between hydrogen and the hydroxide ion is responsible for the neutralization reaction's exothermic character.

Here's a related question that tackles this process:

http://socratic.org/questions/why-does-a-neutralization-reaction-occur