Editing Thermochemistry

{{Short description|Study of the heat energy associated with chemical reactions and/or physical transformations}}
{{Other uses|chemical thermodynamics}}
'''Thermochemistry''' is the study of the heat energy which is associated with [[chemical reaction]]s 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 process|Endothermic reactions]] absorb heat, while [[Exothermic process|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 [[Thermodynamic free energy|free energy]].
[[Image:Ice-calorimeter.jpg|250px|right|thumb|The world's first '''ice-calorimeter''', used in the winter of 1782–83, by [[Antoine Lavoisier]] and [[Pierre-Simon Laplace]], to determine the [[heat]] evolved in various [[chemical change]]s; calculations which were based on [[Joseph Black]]'s prior discovery of [[latent heat]].  These experiments mark the foundation of '''thermochemistry'''.]]

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 [[thermodynamic work|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 reaction]]s and [[endergonic reaction]]s.

==History==
Thermochemistry rests on two generalizations.  Stated in modern terms, they are as follows:<ref>{{cite book | author=Perrot, Pierre | title=A to Z of Thermodynamics | publisher=Oxford University Press | year=1998 | isbn=0-19-856552-6}}</ref>

#[[Antoine Lavoisier|Lavoisier]] and [[Pierre-Simon Laplace|Laplace's]] law (1780): The energy change accompanying any transformation is equal and opposite to energy change accompanying the reverse process.<ref>See page 290 of ''[https://archive.org/details/bub_gb_En83AAAAMAAJ_2/page/n308 <!-- pg=290 quote=Lavoisier and Laplace's law. --> Outlines of Theoretical Chemistry]'' by Frederick Hutton Getman (1918)</ref>
#[[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.<ref>{{cite book |last1=Petrucci |first1=Ralph H. |last2=Harwood |first2=William S. |last3=Herring |first3=F. Geoffrey |title=General Chemistry |date=2002 |publisher=Prentice Hall |isbn=0-13-014329-4 |pages=241–3 |edition=8th}}</ref>

These statements preceded the [[first law of thermodynamics]] (1845) and helped in its formulation.

Thermochemistry also involves the measurement of the [[latent heat]] of [[phase transition]]s. [[Joseph Black]] had already introduced the concept of latent heat in 1761, based on the observation that heating ice at its [[melting point]] did not raise the temperature but instead caused some ice to melt.<ref>{{cite EB1911|wstitle=Black, Joseph|volume=4}}</ref>

[[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 = ΔC<sub>p</sub>. Integration of this equation permits the evaluation of the heat of reaction at one temperature from measurements at another temperature.<ref>[[Keith J. Laidler|Laidler K.J.]] and Meiser J.H., "Physical Chemistry" (Benjamin/Cummings 1982), p.62</ref><ref>[[Peter Atkins|Atkins P.]] and de Paula J., "Atkins' Physical Chemistry" (8th edn, W.H. Freeman 2006), p.56</ref>

==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 '''un'''insulated [[bomb calorimeter]]
* a [[closed system]] which can exchange energy but not matter, such as an '''un'''insulated closed piston or balloon
* an [[Thermodynamic system#Open system|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 process|isothermal]] (same-temperature) process occurs when temperature of the system remains constant. An [[Isobaric process|isobaric]] (same-pressure) process occurs when the pressure of the system remains constant. A process is [[Adiabatic process|adiabatic]] when no heat exchange occurs.

==See also==
{{Portal|Science}}
*[[Calorimetry]]
*[[Chemical kinetics]]
*[[Cryochemistry]]
*[[Differential scanning calorimetry]]
*[[Isodesmic reaction]]
*[[List of important publications in chemistry#Thermochemistry|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==
<references />

==External links==
*{{cite EB1911 |wstitle=Thermochemistry |volume=26 |pages=804–808 |first=James |last=Walker |author-link=James Walker (chemist) |short=x}}

{{BranchesofChemistry}}

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[[Category:Thermochemistry| ]]
[[Category:Physical chemistry]]
[[Category:Branches of thermodynamics]]