Thermochemistry is the study of the energy and heat associated with chemical reactions
and/or physical transformations. A reaction may release or absorb energy, and a phase
change may do the same, such as in melting and boiling. Thermochemistry focuses on these
energy changes, particularly on the system's energy exchange with its surroundings. Thermochemistry
is useful in predicting reactant and product quantities throughout the course of a given
reaction. In combination with entropy determinations, it is also used to predict whether a reaction
is spontaneous or non-spontaneous, favorable or unfavorable.
Endothermic reactions absorb heat. Exothermic reactions release heat. Thermochemistry coalesces
the concepts of thermodynamics with the concept of energy in the form of chemical bonds. The
subject commonly includes calculations of such quantities as heat capacity, heat of
combustion, heat of formation, enthalpy, entropy, free energy, and calories.
History Thermochemistry rests on two generalizations.
Stated in modern terms, they are as follows: Lavoisier and Laplace’s law: The energy
change accompanying any transformation is equal and opposite to energy change accompanying
the reverse process. Hess' law: The energy change accompanying
any transformation is the same whether the process occurs in one step or many.
These statements preceded the first law of thermodynamics and helped in its formulation.
Lavoisier, Laplace and Hess also investigated specific heat and latent heat, although it
was Joseph Black who made the most important contributions to the development of latent
energy changes. Gustav Kirchhoff showed in 1858 that the variation
of the heat of reaction is given by the difference in heat capacity between products and reactants:
dΔH / dT = ΔCp. Integration of this equation permits the evaluation of the heat of reaction
at one temperature from measurements at another temperature.
Calorimetry The measurement of heat changes is performed
using calorimetry, usually an enclosed chamber within which the change to be examined occurs.
The temperature of the chamber is monitored either using a thermometer or thermocouple,
and the temperature plotted against time to give a graph from which fundamental quantities
can be calculated. Modern calorimeters are frequently supplied with automatic devices
to provide a quick read-out of information, one example being the DSC or differential
scanning calorimeter. Systems
Several thermodynamic definitions are very useful in thermochemistry. A system is the
specific portion of the universe that is being studied. Everything outside the system is
considered the surrounding or environment. A system may be: an isolated system — when
it cannot exchange energy or matter with the surroundings, as with an insulated bomb calorimeter;
a closed system — when it can exchange energy but not matter with the surroundings, as with
a steam radiator; an open system — when it can exchange both matter and energy with
the surroundings, as with a pot of boiling water.
Processes A system undergoes a process when one or more
of its properties changes. A process relates to the change of state. An isothermal process
occurs when temperature of the system remains constant. An isobaric process occurs when
the pressure of the system remains constant. An adiabatic process occurs when no heat exchange
occurs. See also
Differential scanning calorimetry Important publications in thermochemistry
Isodesmic reaction Principle of maximum work
Reaction Calorimeter Thomsen-Berthelot principle
Julius Thomsen Thermodynamic databases for pure substances
Calorimetry Photoelectron photoion coincidence spectroscopy
Thermodynamics References
^ Perrot, Pierre. A to Z of Thermodynamics. Oxford University Press. ISBN 0-19-856552-6.
^ See page 290 of Outlines of Theoretical Chemistry by Frederick Hutton Getman
^ Laidler K.J. and Meiser J.H., "Physical Chemistry", p.62
^ Atkins P. and de Paula J., "Atkins' Physical Chemistry", p.56
External links Thermochemistry - Britannica