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. Scottish physician and scientist Joseph Black, who was the first to recognize the distinction between heat and temperature, is said to be the founder of calorimetry.cite book|author= Laider, Keith, J.|title=The World of Physical Chemistry|publisher=Oxford University Press|year=1993|id=ISBN 0-19-855919-4]

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 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.

Types

Calorimetry is performed using one of two methods: constant volume or constant pressure, or constant mass.

Constant-mass

"Constant-mass calorimetry" is a calorimetry performed at a constant mass and specific heat.

:$q\; =\; mc\; Delta\; T\; ,$

where

:"q" is energy, or heat,:"m" is mass, :"c" is specific heat,:"ΔT" is change in temperature.

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 (the heat capacity at constant volume is assumed to be constant):

:$q\; =\; C\_V\; Delta\; T\; =\; Delta\; U\; ,$

where

:"ΔU" is change in internal energy,:"ΔT" is change in temperature and:"C_V" is the heat capacity at constant volume.

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":

: $q\; =\; Delta\; H\; =\; H\_mathrm\{final\}\; -\; H\_mathrm\{initial\}\; ,$

This formula is a simplified representative of Hess's Law.

ee also

* Accelerating rate calorimetry
* Differential scanning calorimetry
* Isothermal titration calorimetry
* Respirometry
* Thermodynamic databases for pure substances
* Thermochemistry

References