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Thermochemistry

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From Wikipedia, the free encyclopedia

Branch of thermodynamics

For other uses, seechemical thermodynamics.

Thermochemistry is the study of the heat energy which is associated withchemical reactions and/or phase changes such asmelting andboiling. 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 itssurroundings in the form of heat. Thermochemistry is useful in predicting reactant and product quantities throughout the course of a given reaction. In combination withentropy determinations, it is also used to predict whether a reaction is spontaneous or non-spontaneous, favorable or unfavorable.

Endothermic reactions absorb heat, whileexothermic 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 asheat capacity,heat of combustion,heat of formation,enthalpy,entropy, andfree energy.

The world's firstice-calorimeter, used in the winter of 1782–83, byAntoine Lavoisier andPierre-Simon Laplace, to determine theheat evolved in variouschemical changes; calculations which were based onJoseph Black's prior discovery oflatent heat. These experiments mark the foundation ofthermochemistry.

Thermochemistry is one part of the broader field ofchemical thermodynamics, which deals with the exchange of all forms of energy between system and surroundings, including not only heat but also various forms ofwork, as well the exchange of matter. When all forms of energy are considered, the concepts of exothermic and endothermic reactions are generalized toexergonic reactions andendergonic reactions.

History

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

Lavoisier andLaplace's law (1780): The energy change accompanying any transformation is equal and opposite to energy change accompanying the reverse process.^[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 thefirst law of thermodynamics (1845) and helped in its formulation.

Thermochemistry also involves the measurement of thelatent heat ofphase transitions.Joseph Black had already introduced the concept of latent heat in 1761, based on the observation that heating ice at itsmelting point did not raise the temperature but instead caused some ice to melt.^[4]

Gustav Kirchhoff showed in 1858 that the variation of the heat of reaction is given by the difference inheat capacity between products and reactants: dΔH / dT = ΔC_p. Integration of this equation permits the evaluation of the heat of reaction at one temperature from measurements at another temperature.^[5]^[6]

Calorimetry

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The measurement of heat changes is performed usingcalorimetry, usually an enclosed chamber within which the change to be examined occurs. The temperature of the chamber is monitored either using athermometer orthermocouple, 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 thedifferential scanning calorimeter.

Systems

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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 insulatedbomb calorimeter
athermally isolated system which can exchange mechanical work but not heat or matter, such as an insulated closedpiston orballoon
amechanically isolated system which can exchange heat but not mechanical work or matter, such as anuninsulatedbomb calorimeter
aclosed system which can exchange energy but not matter, such as anuninsulated closed piston or balloon
anopen system which can exchange both matter and energy with the surroundings, such as a pot of boiling water

Processes

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A system undergoes a process when one or more of its properties changes. A process relates to the change of state. Anisothermal (same-temperature) process occurs when temperature of the system remains constant. Anisobaric (same-pressure) process occurs when the pressure of the system remains constant. A process isadiabatic when no heat exchange occurs.

References

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^Perrot, Pierre (1998).A to Z of Thermodynamics. Oxford University Press.ISBN 0-19-856552-6.
^See page 290 ofOutlines of Theoretical Chemistry by Frederick Hutton Getman (1918)
^Petrucci, Ralph H.; Harwood, William S.; Herring, F. Geoffrey (2002).General Chemistry (8th ed.). Prentice Hall. pp. 241–3.ISBN 0-13-014329-4.
^Chisholm, Hugh, ed. (1911)."Black, Joseph" .Encyclopædia Britannica. Vol. 4 (11th ed.). Cambridge University Press.
^Laidler K.J. and Meiser J.H., "Physical Chemistry" (Benjamin/Cummings 1982), p.62
^Atkins P. and de Paula J., "Atkins' Physical Chemistry" (8th edn, W.H. Freeman 2006), p.56

External links

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Walker, James (1911)."Thermochemistry" .Encyclopædia Britannica. Vol. 26 (11th ed.). pp. 804–808.

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