Standard Enthalpy of Neutralisation

[carlasilvia]

Explain why the enthalpy of neutralisation differs when using solid reactants to using aqueous reactants.

[r1ckworthy]

Hey!

Saw your question on the chemistry questions thread, and thought about it for a long time. I asked my teacher about this, and this is what she told me.
Remember that the change is enthalpy is the change in energy a system undergoes over a period of time. Neutralisation reactions are exothermic, so they release energy as the reaction proceeds.

Let's imagine we have hydrochloric acid solution reacting with either solid sodium hydroxide or aqueous sodium hydroxide.
When we add aqueous sodium hydroxide to the solution, the neutralisation will act as normal, and will release energy.

When we add solid sodium hydroxide, there are two reactions going on. One is the neutralisation reaction, and the other is the solid NaOH reacting with water as well, since the original solution is a reaction between HCl and water. This will also give off heat.

So this heat coupled with the heat released of the neutralisation reaction will result in the enthalpy to be greater, as there are two reactions going on.

Hopefully that made sense. If not, reply back and point out what part you did not get.