Calorimetry
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Calorimetry is the science of measuring the heat of chemical reactions or physical changes. Calorimetry involves the use of a calorimeter.
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Temperature and Internal Energy
At all temperatures above absolute zero, atoms possess varying amounts of kinetic energy of vibration. As neighboring atoms collide with each other, this energy is passed back and forth. Although the energy of individual atoms may vary as a result of these energy-sharing collisions, a collection of atoms isolated from the outside world has a total amount of energy that doesn't change as it gets passed around from atom to atom. Conceptually, an average energy per atom can be calculated by dividing the total energy by the number of atoms.
Although we don't know the total energy of an object's atoms or perform that calculation directly, we can measure the effect of that average kinetic energy - it is that object's temperature. An increase in the average kinetic energy of the atoms in the object show up as an increase in its temperature and vise versa.
If an object 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 the measurement of temperature changes 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:
- <math>q = C \Delta t = \Delta U \,<math>
Since in constant-volume calorimetry 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:
- <math> q = \Delta U - w \,<math>
Since in constant-pressure calorimetry, pressure is kept constant, the heat measured represents the enthalpy change:
- <math>q = \Delta H = H_\mathrm{final} - H_\mathrm{initial} \,<math>
See also
External links
- More Information, History - from Parr with over 100 years of Calorimetry (http://www.parrinst.com?page_id=183)de:Kalorimetrie