Calorimetry is a way to determine the amount of heat flow that occurs in a process. In thermodynamics, we divide up the universe into two parts: the system and the surroundings. A calorimeter is a well insulated device that surrounds the system. Since it is insulated, heat doesn't flow from the calorimeter to the outside world. Thus, any heat that flows out of the reaction will be absorbed by the calorimeter and can be measured by the change in temperature of the calorimeter. In the same way, if the reaction is endothermic, the heat flow into the system will come from the calorimeter, thus cooling the calorimeter.

Mathematically, this can be written as q_system = -q_cal. (You will often see q_reaction used for q_system.) Since q_cal = C_cal*DT, you can use the change in temperature of the calorimeter to determine q for the calorimeter, and thus q for the system. It is necessary to calibrate the calorimeter to determine C_cal, the heat capacity of the calorimeter.

Examples:

A reaction occurs in a calorimeter. If the heat capacity C_cal of the calorimeter is 1.64 kJ/^oC and the reaction causes the temperature of the calorimeter to rise from 25.0 to 29.6 ^oC, what is q_cal? What is q_system?

Solution: Use the two relations q_cal = C_cal*DT and q_cal = -q_reaction

C_cal = 1.64 kJ/^oC
DT = (T_f-T_i) = (29.6-25.0) = 4.6^oC

q_cal = 1.64 kJ/^oC * 4.6^oC = 7.5 kJ.
q_reaction = -q_cal = -7.5 kJ.

Information provided by: http://learn.chem.vt.edu