Thermochemistry

Thermochemistry is the study of the heat energy which is associated with chemical reactions and/or phase
changes such as melting and boiling. A reaction may release or absorb energy, and a phase change may do
the same. Thermochemistry focuses on the energy exchange between a system and its surroundings in the
form of heat. 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, while 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, and free energy.

Thermochemistry is one part of the broader field of chemical

thermodynamics, which deals with the exchange of all forms
of energy between system and surroundings, including not
only heat but also various forms of work, as well the exchange
of matter. When all forms of energy are considered, the
concepts of exothermic and endothermic reactions are
generalized to exergonic reactions and endergonic reactions.

History
Thermochemistry rests on two generalizations. Stated in
modern terms, they are as follows:[1]

1. Lavoisier and Laplace's law (1780): The energy

change accompanying any transformation is equal
and opposite to energy change accompanying the
reverse process.[2]
2. Hess' law of constant heat summation (1840): The
energy change accompanying any transformation is
the same whether the process occurs in one step or
many.[3]

These statements preceded the first law of thermodynamics The world's first ice-calorimeter, used in
(1845) and helped in its formulation. the winter of 1782–83, by Antoine
Lavoisier and Pierre-Simon Laplace, to
Thermochemistry also involves the measurement of the latent determine the heat evolved in various
heat of phase transitions. Joseph Black had already introduced chemical changes; calculations which
the concept of latent heat in 1761, based on the observation were based on Joseph Black's prior
that heating ice at its melting point did not raise the temperature discovery of latent heat. These
but instead caused some ice to melt.[4] experiments mark the foundation of
thermochemistry.
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.[5][6]

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 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 surroundings or
environment. A system may be:

a (completely) isolated system which can exchange neither energy nor matter with the
surroundings, such as an insulated bomb calorimeter
a thermally isolated system which can exchange mechanical work but not heat or matter,
such as an insulated closed piston or balloon
a mechanically isolated system which can exchange heat but not mechanical work or matter,
such as an uninsulated bomb calorimeter
a closed system which can exchange energy but not matter, such as an uninsulated closed
piston or balloon
an open system which it can exchange both matter and energy with the surroundings, such
as 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 (same-temperature) process occurs when temperature of the system remains
constant. An isobaric (same-pressure) process occurs when the pressure of the system remains constant. A
process is adiabatic when no heat exchange occurs.

See also
Science portal

Calorimetry
Chemical kinetics
Cryochemistry
Differential scanning calorimetry
Isodesmic reaction
Important publications in thermochemistry
Photoelectron photoion coincidence spectroscopy
Principle of maximum work
Reaction Calorimeter
 Thermodynamic databases for pure substances
Thermodynamics
Thomsen-Berthelot principle
Julius Thomsen

References
1. Perrot, Pierre (1998). A to Z of Thermodynamics. Oxford University Press. ISBN 0-19-
856552-6.
2. See page 290 of Outlines of Theoretical Chemistry (https://archive.org/details/bub_gb_En83
AAAAMAAJ_2/page/n308) by Frederick Hutton Getman (1918)
3. Petrucci, Ralph H.; Harwood, William S.; Herring, F. Geoffrey (2002). General Chemistry
(8th ed.). Prentice Hall. pp. 241–3. ISBN 0-13-014329-4.
4. Chisholm, Hugh, ed. (1911). "Black, Joseph" (https://en.wikisource.org/wiki/1911_Encyclo
p%C3%A6dia_Britannica/Black,_Joseph). Encyclopædia Britannica. Vol. 4 (11th ed.).
Cambridge University Press.
5. Laidler K.J. and Meiser J.H., "Physical Chemistry" (Benjamin/Cummings 1982), p.62
6. Atkins P. and de Paula J., "Atkins' Physical Chemistry" (8th edn, W.H. Freeman 2006), p.56

External links
Walker, James (1911). "Thermochemistry" (https://en.wikisource.org/wiki/1911_Encyclop%
C3%A6dia_Britannica/Thermochemistry). Encyclopædia Britannica. Vol. 26 (11th ed.).
pp. 804–808.

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