T,
where C is the heat capacity of the calorimeter and its contents and T
is the change in temperature. The key measurement in calorimetry then is
the measurement of temperature. Calorimetry
The key to measuring the enthalpy of reaction (or non chemical process such as solution or vaporization etc) is in measuring quantities of heat. The measurement of a quantity of heat is called calorimetry and the apparatus with which the measurement is made is called the calorimeter. The name, no doubt comes from the non SI unit of energy, the calorie. The name should now probably be the jouleometer, but that is not likely to happen. The underlying principle of calorimetry is the law of conservation of energy, which says that whatever heat is lost by a system must be gained by the surroundings.
We can state this formally as follows qprocess + qcalorimeter
= 0, where qprocess is the heat lost or gained by the
reaction or non chemical process such as solution or vaporization, and qcalorimeter
is the heat gained or lost by the calorimeter. qcalorimeter is
obtained from qcalorimeter = CT,
where C is the heat capacity of the calorimeter and its contents and T
is the change in temperature. The key measurement in calorimetry then is
the measurement of temperature.
A calorimeter can be as simple as a pair of styroform-cups with a thermometer. The heat of the reaction is calculated from the temperature change caused by the reaction, and since this is a constant-pressure process the heat measured can directly related to the enthalpy change.
For reactions involving gases, such as combustion reactions, we must use a bomb calorimeter. This is a calorimeter which confines the reactants and products to a constant volume. This device uses a container with strong steel walls to withstand explosively exothermic reactions. A small sample is placed in a metal cup in the bomb, the bomb is assembled and filled with pure oxygen at a pressure of about 30 atm. The bomb is then placed in an insulated container filled with a known amount of water. The reaction is started by passing an electric current through a fuse wire. The heat of the reaction is determined from the temperature rise in the water that surrounds the bomb.