<< Main Revision Index

Energy index:



What is the difference between enthalpy and energy?

If you are a secondary school student none.

If you do A-level or degree level chemistry perhaps you will find a small difference, of about 1 or 2% in some special cases.
So don't get confused with this new concept, because it is very much something that you already know, and that you here about all the time, which is energy.

Both enthalpy and energy are measured in joules, which already suggests that  they are about the same thing.

Formally there is a difference, as the formula for enthalpy (H) is:

H = E + PV,

where E=energy, P=pressure and V=volume.

So unless the reaction is performed at a very high pressure or there is a significant change in volume , the term PV will be negligible and the enthalpy will equal the energy.

Why don't you challenge your science teacher and ask him/her to explain the difference between these 2 concepts? It would be instructive and interesting to see how he/her approaches this question.

What is enthalpy?

It is the energy involved in the formation/breaking of chemical bonds in a particular reaction.

If a reaction has a large enthalpy, it may provide us with useful energy, usually in the form of heat, like in the case of burning fuels, for instance.In this case we are dealing with the enthalpy of combustion.

Other types of reactions also have an enthalpy associated to it. It may be an oxidation, an hydrogenation , solvation. There is also an enthalpy of formation: energy transferred when making a new substance out of the pure substances.

Types of calculations involving enthalpy>>

How to calculate enthalpy >>




© Ricardo Esplugas. All images in this site can be bought in an enlarged version. Please contact me on