Editing Standard enthalpy of reaction (section)

== Evaluation of reaction enthalpies==
There are several methods of determining the values of reaction enthalpies, involving either measurements on the reaction of interest or calculations from data for related reactions.

For reactions which go rapidly to completion, it is often possible to measure the heat of reaction directly using a [[calorimeter]]. One large class of reactions for which such measurements are common is the [[combustion]] of [[organic compound]]s by reaction with molecular oxygen (O<sub>2</sub>) to form [[carbon dioxide]] and water (H<sub>2</sub>O). The heat of combustion can be measured with a so-called [[Calorimeter#Bomb calorimeters|bomb calorimeter]], in which the heat released by combustion at high temperature is lost to the surroundings as the system returns to its initial temperature.<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=227–229 |edition=8th}}</ref><ref>{{cite book |last1=Engel |first1=Thomas |last2=Reid |first2=Philip |title=Physical Chemistry |date=2006 |publisher=Pearson Benjamin Cummings |isbn=0-8053-3842-X |pages=72–73}}</ref> Since enthalpy is a [[state function]], its value is the same for any path between given initial and final states, so that the measured Δ''H'' is the same as if the temperature stayed constant during the combustion.<ref>Engel and Reid p.65</ref>

For reactions which are incomplete, the [[equilibrium constant]] can be determined as a function of temperature. The enthalpy of reaction is then found from the [[van 't Hoff equation]] as <math> \Delta_{\text {rxn}} H^\ominus = {RT^2}\frac{d}{dT} \ln K_\mathrm{eq}</math>. A closely related technique is the use of an electroanalytical [[voltaic cell]], which can be used to measure the [[Gibbs energy]] for certain reactions as a function of temperature, yielding  <math>K_\mathrm{eq}(T)</math> and thereby <math> \Delta_{\text {rxn}} H^\ominus </math>.<ref>{{cite book |last1=Chang |first1=Raymond |last2=Thoman, Jr. |first2=John W. |title=Physical Chemistry for the Chemical Sciences |date=2014 |publisher=University Science Books |pages=356–360}}</ref>

It is also possible to evaluate the enthalpy of one reaction from the enthalpies of a number of other reactions whose sum is the reaction of interest, and these not need be formation reactions. This method is based on [[Hess's law]], which states that the enthalpy change is the same for a chemical reaction which occurs as a single reaction or in several steps. If the enthalpies for each step can be measured, then their sum gives the enthalpy of the overall single reaction. <ref>Petrucci, Harwood and Herring, pages 241–243</ref>

Finally the reaction enthalpy may be estimated using [[Bond energy|bond energies]] for the bonds which are broken and formed in the reaction of interest. This method is only approximate, however, because a reported bond energy is only an average value for different molecules with bonds between the same elements.<ref>Petrucci, Harwood and Herring, pages 422–423</ref>