Editing Thermochemistry (section)

==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>