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Thermochemistry
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<h2 id="history">History</h2> <p>Thermochemistry rests on two generalizations. Stated in modern terms, they are as follows:<a class="footnote-ref" id="fnref:1" href="#fn:1"><sup>1</sup></a> </p> <ol><li><a href="/facts/Antoine_Lavoisier/UheVQrhu">Lavoisier</a> and <a href="/facts/Pierre-Simon_Laplace/6ktp0txb">Laplace's</a> law (1780): The energy change accompanying any transformation is equal and opposite to energy change accompanying the reverse process.<a class="footnote-ref" id="fnref:2" href="#fn:2"><sup>2</sup></a></li> <li><a href="/facts/Hess%27_law/3fLwnwrj">Hess' law</a> of constant heat summation (1840): The energy change accompanying any transformation is the same whether the process occurs in one step or many.<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a></li></ol> <p>These statements preceded the <a href="/facts/First_law_of_thermodynamics/hjL95WCr">first law of thermodynamics</a> (1845) and helped in its formulation. </p><p>Thermochemistry also involves the measurement of the <a href="/facts/Latent_heat/QCIWSGln">latent heat</a> of <a href="/facts/Phase_transition/eLP2BY8R">phase transitions</a>. <a href="/facts/Joseph_Black/0HD6fdYR">Joseph Black</a> had already introduced the concept of latent heat in 1761, based on the observation that heating ice at its <a href="/facts/Melting_point/MY0w0h0k">melting point</a> did not raise the temperature but instead caused some ice to melt.<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a> </p><p><a href="/facts/Gustav_Kirchhoff/Ygfzlywy">Gustav Kirchhoff</a> showed in 1858 that the variation of the heat of reaction is given by the difference in <a href="/facts/Heat_capacity/I63nzSyp">heat capacity</a> 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.<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a><a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a> </p> <h2 id="calorimetry">Calorimetry</h2> <p>The measurement of heat changes is performed using <a href="/facts/Calorimetry/bzA3o7AN">calorimetry</a>, usually an enclosed chamber within which the change to be examined occurs. The temperature of the chamber is monitored either using a <a href="/facts/Thermometer/almVTBxI">thermometer</a> or <a href="/facts/Thermocouple/FW9cYU5D">thermocouple</a>, 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 <a href="/facts/Differential_scanning_calorimeter/Gj9u8Plc">differential scanning calorimeter</a>. </p> <h2 id="systems">Systems</h2> <p>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: </p> <ul><li>a (completely) <a href="/facts/Isolated_system/jheEBts7">isolated system</a> which can exchange neither energy nor matter with the surroundings, such as an insulated <a href="/facts/Bomb_calorimeter/Njp46oI8">bomb calorimeter</a></li> <li>a <a href="/facts/Thermally_isolated_system/n8J8SddS">thermally isolated system</a> which can exchange mechanical work but not heat or matter, such as an insulated closed <a href="/facts/Piston/z7pN46u7">piston</a> or <a href="/facts/Balloon/o9JQ2ykA">balloon</a></li> <li>a <a href="/facts/Mechanically_isolated_system/cHHt6uV9">mechanically isolated system</a> which can exchange heat but not mechanical work or matter, such as an uninsulated <a href="/facts/Bomb_calorimeter/Njp46oI8">bomb calorimeter</a></li> <li>a <a href="/facts/Closed_system/8KEjjgQE">closed system</a> which can exchange energy but not matter, such as an uninsulated closed piston or balloon</li> <li>an <a href="/facts/Thermodynamic_system/VUr0kpWu">open system</a> which it can exchange both matter and energy with the surroundings, such as a pot of boiling water</li></ul> <h2 id="processes">Processes</h2> <p>A system undergoes a process when one or more of its properties changes. A process relates to the change of state. An <a href="/facts/Isothermal_process/6mkdESuS">isothermal</a> (same-temperature) process occurs when temperature of the system remains constant. An <a href="/facts/Isobaric_process/SRx2Pd15">isobaric</a> (same-pressure) process occurs when the pressure of the system remains constant. A process is <a href="/facts/Adiabatic_process/rJj42U5h">adiabatic</a> when no heat exchange occurs. </p> <h2 id="see-also">See also</h2> <ul> <li>Science portal</li></ul> <ul><li><a href="/facts/Calorimetry/bzA3o7AN">Calorimetry</a></li> <li><a href="/facts/Chemical_kinetics/D8vyX4aE">Chemical kinetics</a></li> <li><a href="/facts/Cryochemistry/eTgdQJXM">Cryochemistry</a></li> <li><a href="/facts/Differential_scanning_calorimetry/Gj9u8Plc">Differential scanning calorimetry</a></li> <li><a href="/facts/Isodesmic_reaction/6mK1Q0lx">Isodesmic reaction</a></li> <li><a href="/facts/List_of_important_publications_in_chemistry/Yb4MRGGW">Important publications in thermochemistry</a></li> <li><a href="/facts/Photoelectron_photoion_coincidence_spectroscopy/ofQ78nKd">Photoelectron photoion coincidence spectroscopy</a></li> <li><a href="/facts/Principle_of_maximum_work/kErV8e9F">Principle of maximum work</a></li> <li><a href="/facts/Reaction_Calorimeter/n8lcU4Gt">Reaction Calorimeter</a></li> <li><a href="/facts/Thermodynamic_databases_for_pure_substances/bGBIX6XC">Thermodynamic databases for pure substances</a></li> <li><a href="/facts/Thermodynamics/q4hIx3yP">Thermodynamics</a></li> <li><a href="/facts/Thomsen-Berthelot_principle/kPWkuvs2">Thomsen-Berthelot principle</a></li> <li><a href="/facts/Julius_Thomsen/QINr2v4E">Julius Thomsen</a></li></ul> <h2 id="external-links">External links</h2> <ul><li><a href="/facts/James_Walker_(chemist)/ta1C2Ira">Walker, James</a> (1911). <a href="https://en.wikisource.org/wiki/1911_Encyclop%C3%A6dia_Britannica/Thermochemistry">"Thermochemistry" </a>. <i><a href="/facts/Encyclop%C3%A6dia_Britannica_Eleventh_Edition/mPElbCRY">Encyclopædia Britannica</a></i>. Vol. 26 (11th ed.). pp. 804–808.</li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>Perrot, Pierre (1998). A to Z of Thermodynamics. Oxford University Press. ISBN 0-19-856552-6. <a href="0-19-856552-6" target="_blank">0-19-856552-6</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>See page 290 of Outlines of Theoretical Chemistry by Frederick Hutton Getman (1918) <a href="https://archive.org/details/bub_gb_En83AAAAMAAJ_2/page/n308" target="_blank">https://archive.org/details/bub_gb_En83AAAAMAAJ_2/page/n308</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>Petrucci, Ralph H.; Harwood, William S.; Herring, F. Geoffrey (2002). General Chemistry (8th ed.). Prentice Hall. pp. 241–3. ISBN 0-13-014329-4. <a href="0-13-014329-4" target="_blank">0-13-014329-4</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>Chisholm, Hugh, ed. (1911). "Black, Joseph" . Encyclopædia Britannica. Vol. 4 (11th ed.). Cambridge University Press. <a href="/wiki/Hugh_Chisholm" target="_blank">/wiki/Hugh_Chisholm</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>Laidler K.J. and Meiser J.H., "Physical Chemistry" (Benjamin/Cummings 1982), p.62 <a href="/wiki/Keith_J._Laidler" target="_blank">/wiki/Keith_J._Laidler</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>Atkins P. and de Paula J., "Atkins' Physical Chemistry" (8th edn, W.H. Freeman 2006), p.56 <a href="/wiki/Peter_Atkins" target="_blank">/wiki/Peter_Atkins</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> </ol>