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Enthalpy Change of Neutralization

The standard enthalpy change of neutralization is the enthalpy change when solutions of an acid and an alkali react together under standard conditions to produce 1 mole of water. Notice that enthalpy change of neutralization is always measured per mole of water formed. Enthalpy changes of neutralization are always negative - heat is released when an acid and and alkali react. For reactions involving strong acids and alkalis, the values are always very closely similar, with values between -57 and -58 kJ mol-1. That varies slightly depending on the acid-alkali combination (and also on what source you look it up in!).

Why do strong acids reacting with strong alkalis give closely similar values?

We make the assumption that strong acids and strong alkalis are fully ionized in solution, and that the ions behave independently of each other. For example, dilute hydrochloric acid contains hydrogen ions and chloride ions in solution. Sodium hydroxide solution consists of sodium ions and hydroxide ions in solution. The equation for any strong acid being neutralized by a strong alkali is essentially just a reaction between hydrogen ions and hydroxide ions to make water. The other ions present (sodium and chloride, for example) are just spectator ions, taking no part in the reaction.

The full equation for the reaction between hydrochloric acid and sodium hydroxide solution is:

\[ NaOH(aq) + HCl(aq) \rightarrow NaCl(aq) + H_2O (l)\]

but what is actually happening is:

\[ OH^-(aq) + H^+(aq) \rightarrow H_2O (l)\]

If the reaction is the same in each case of a strong acid and a strong alkali, it is not surprising that the enthalpy change is similar.

In a weak acid, such as acetic acid, at ordinary concentrations, something like 99% of the acid is not actually ionized. That means that the enthalpy change of neutralization will include other enthalpy terms involved in ionizing the acid as well as the reaction between the hydrogen ions and hydroxide ions. And in a weak alkali like ammonia solution, the ammonia is also present mainly as ammonia molecules in solution. Again, there will be other enthalpy changes involved apart from the simple formation of water from hydrogen ions and hydroxide ions. For reactions involving acetic acid or ammonia, the measured enthalpy change of neutralization is a few kJ less exothermic than with strong acids and bases.

For example, one source which gives the enthalpy change of neutralization of sodium hydroxide solution with HCl as -57.9 kJ mol-1:

\[ NaOH_{(aq)} + HCl_{(aq)} \rightarrow Na^+_{(aq)} + Cl^-_{(aq)} + H_2O\]

the enthalpy change of neutralization for sodium hydroxide solution being neutralized by acetic acid is -56.1 kJ mol-1 :

\[ NaOH_{(aq)} + CH_3COOH_{(aq)} \rightarrow Na^+_{(aq)} + CH_3COO^-_{(aq)} + H_2O\]

For very weak acids, like hydrogen cyanide solution, the enthalpy change of neutralization may be much less. A different source gives the value for hydrogen cyanide solution being neutralized by potassium hydroxide solution as -11.7 kJ mol-1, for example.

\[ NaOH_{(aq)} + HCN_{(aq)} \rightarrow Na^+_{(aq)} + CN^-_{(aq)} + H_2O\]

Note: Neutralizing Strong acids vs. Weak Acids

The experimentally measured enthalpy change of neutralization is a few kJ less exothermic than with strong acids and bases.

Contributors

Jim Clark (Chemguide.co.uk)