Enthalpy change

Nearly all chemical reactions are accompanied by the loss or gain of heat energy. If a reaction is accompanied by a large increase in volume (when it produces some gas) it has to use up some energy in pushing back the weight of the atmosphere. This energy would not be needed if the reaction were done at constant volume (in a sealed container), and so under these conditions the reaction would release more heat energy. If we are measuring how much heat is given out by a reaction it is, therefore, important that we know whether the reaction has taken place at constant pressure or at constant volume.

It is more usual for a reaction to take place at constant pressure (like in a test tube) so we define the important term of enthalpy:

The enthalpy change of a reaction is the heat it exchanges with the surroundings at constant pressure.

Enthalpy is given the symbol H, and enthalpy change has the symbol DH. D is the Greek letter delta, and DH is pronounced delta H. It is sometimes useful to think of enthalpy, H, as the "heat content" of the substance.

There are many exothermic reactions - a burning match is one of them

If a reaction gives out heat it is said to be exothermic and DH is negative. This often confuses students because the temperature of the reaction mixture rises (is positive). As chemists, we are much more interested in the chemicals rather than the surroundings. Whilst the surroundings have increased in energy, the chemicals must have lost energy, so DH is negative.

If a reaction takes in heat and gets colder it is said to be endothermic and DH is positive. This causes even more confusion! Surely, if it takes in heat it will get hotter. The confusion here is that when the heat energy is taken in, it is converted to potential (chemical) energy in the chemical bonds. The heat is lost and the temperature drops.

Don't forget to put in the plus or minus sign - you will lose marks in your assessment if you don't put them in! As the exact enthalpy change depends on the temperature and pressure the reaction is carried out at, it is important to have standard conditions. The standard conditions usually used are 298 K (room temperature) and 1 atmosphere pressure. Standard conditions are indicated using a circle with a line through it at the top right of the DH symbol. At other conditions there will be a different value for DH, but if the conditions are only slightly changed the difference will be small and is usually ignored.

The standard enthalpy change for a reaction, , refers to the amounts shown in the equation, at a pressure of 1 atmosphere, at a temperature of 298K, with the substances in the physical states normal under these conditions. Solutions must have a concentration of 1 mol dm^-3.