Calorimetry

Calorimetry is the science of measuring the heat of chemical reactions or physical changes. Calorimetry involves the use of a calorimeter. The word calorimetry is derived from the Latin word calor, meaning heat.

Indirect calorimetry calculates heat that living organisms produce from their production of carbon dioxide and nitrogen waste (frequently ammonia in aquatic organisms, or urea in terrestrial ones), OR from their consumption of oxygen. Lavoisier noted in 1780 that heat production can be predicted from oxygen consumption this way, using multiple regression. The Dynamic Energy Budget theory explains why this is procedure is correct. Of course, heat generated by living organisms may also be measured by direct calorimetry, in which the entire organism is placed inside the calorimeter for the measurement.

Temperature and Internal Energy
If an object or system is isolated from the rest of the universe, its temperature must stay constant. If energy enters or leaves, the temperature must change. Energy moving from one place to another is called heat and calorimetry uses measurement of temperature change, along with heat capacity, to track the movement of heat.

Constant-volume
Constant-volume calorimetry is calorimetry performed at a constant volume. This involves the use of a constant-volume calorimeter.

No work is performed in constant-volume calorimetry, so the heat measured equals the change in internal energy of the system. The equation for constant-volume calorimetry is:


 * $$q = C_V \Delta t = \Delta U \,$$

where


 * ΔU = change in internal energy

Since in constant-volume calorimetry the pressure is not kept constant, the heat measured does not represent the enthalpy change.

Constant-pressure
Constant-pressure calorimetry is calorimetry performed at a constant pressure. This involves the use of a constant-pressure calorimeter.

The heat measured equals the change in internal energy of the system minus the work performed:


 * $$ q = \Delta U - w \,$$

Since in constant-pressure calorimetry, pressure is kept constant, the heat measured represents the enthalpy change:


 * $$q = \Delta H = H_\mathrm{final} - H_\mathrm{initial} \,$$