The change in heat of a system can be measured with a calorimeter. Essentially, this device consists of a loosely covered insulating container with a thermometer. The container is insulated in order to obtain an accurate temperature reading. The container is loosely covered so that the constant pressure of the atmosphere can be maintained. The reaction proceeds inside the container, and the temperature change can be read off of the thermometer. The change in temperature of the system and the heat applied are then related by
where C is the heat capacity of the calorimeter. Heat capacity has units of energy per degree Kelvin. Equation (1) gives the heat absorbed by one mole in the system. For the total amount of heat absorbed, the right side of the equation must be increased by a factor equal to the number of moles absorbing the heat.
The use of heat capacity presents a problem in that it may depend on quantities as varied as the amount of substance present and the shape of the container (i.e., it is an extensive property). A more common quantity is the specific heat of a substance, given by
