Commenting on enthalpy change of neutralisation.

The definition of the enthalpy change of neutralisation is the heat evolved when one more of water is formed between an acid and an alkali. Essentially, it is the reaction between 1 mole of H⁺ and 1 mole of OH^- to give 1 mole of H₂O.

Typically, enthalpy change of neutralisation between a strong acid and a strong base is -57 kJ mol^-1. While, if it is between a weak acid and a strong base, or a strong acid and a weak base, the enthalpy change of neutralisation will be slightly less exothermic than -57 kJ mol^-1. This is largely because some amount of heat is taken in my the molecule to allow for complete acid (or base) dissociation.

1. Interesting question:

However, when we react 1 mole of H₂SO₄ with 2 moles of NaOH, do we get a more exothermic enthalpy change of neutralisation? The answer is NO.

This is because the reaction between sulfuric acid and sodium hydroxide produces 2 moles of water. This implies that the enthalpy change for this reaction is twice of that of enthalpy change of neutralisation; since the latter is defined to be per mole of water formed.

Therefore, when we obtain -114 kJ mol^-1 (which is more exothermic than -57 kJ mol^-1) for the reaction between 1 mole of sulfuric acid and 2 moles of sodium hydroxide, this value refers is the enthalpy change of reaction. Hence, when this number is divided by two because two moles of water is obtained, we will get enthalpy change of neutralisation.

2. Application - in terms of planning an experiment:

In conclusion, we often use calorimetry method to typical to determine the enthalpy change of neutralisation of an acid and a base. However, in using energetics in determining strength of acid, we are essentially trying to determine enthalpy change of reaction.

For example, when the reaction between sulfuric acid and sodium hydroxide is compare it with ethanoic acid and sodium hydroxide (and both acids are equimolar), it is important to ensure that the amount of hydroxide used is in excess. It is also good to ensure the volumes of hydroxide used are the same - this facilitates the comparison.

The former will produce an enthalpy change of reaction that is slightly more than twice of that of the latter; resulting in the former to have a temperature change that is twice as much as the latter.

^{-- -- -- -- --}

^{Article written by Kwok YL 2010.}

^{Disclaimer and remarks:}

• ^{If you would like to use this source, kindly drop me a note by leaving behind a comment with your name and institution. I am all for sharing as the materials on this blog is actually meant for the education purpose of my students.}
• ^{This material is entirely written by the author and my sincere thanks will be given to anyone who is kind, generous and gracious to point out any errors.}