Electrical compensation calorimetry

This is a more technical method than the plastic cup method which, in theory at least, should give better results. The experiment is done in two parts. A Dewar flask (a Thermos flask) is used to carry out the reaction. This will limit heat loss. However, any heat loss should be compensated for so should not lead to error. The reaction is carried out and the temperature change is measured. In the second part of the experiment a light bulb is used to heat the same reaction mixture by the same number of degrees as in the first part of the experiment. A joulemeter is used to measure the electrical energy supplied to the light bulb. This should be the same amount of energy as was absorbed or released by the chemical reaction. If we know how many moles of chemical were used we can calculate the enthalpy change in kilojoules per mole.

This technique has two main advantages. Any heat loss which occurs during the chemical reaction should occur in the electrical part of the experiment and be measured. Also no calculation has to be done to work out the number of joules, it is simply read off the joulemeter.

The electrical heating should be done over exactly the same temperature range as in the chemical part of the experiment and over the same time. If a longer time is spent during the electrical part of the experiment more heat will be lost and this will lead to an error. Similarly if the temperature rises in the first part by 10 degrees centigrade and we heat it electrically by another 10 degrees centigrade, more heat will be lost during the second phase as the apparatus is at a higher temperature. However, for the accuracy of our experimental work this is usually not too critical. As long as the time spent is similar and the temperature range similar, the results should be satisfactory. If the reaction is exothermic we would not usually wait until the temperature fell to its original value, but simply continue heating from where the chemical reaction finished off.