Editing Standard enthalpy of reaction (section)

{{Short description|Enthalpy difference due to chemical reaction, reduced to standard states}}
The '''standard enthalpy of reaction''' (denoted <math>\Delta H_{\text {reaction}}^\ominus</math>) for a [[chemical reaction]] is the difference between total product and total reactant molar [[enthalpy|enthalpies]], calculated for substances in their [[standard state]]s. The value can be approximately interpreted in terms of the total of the chemical [[bond energy|bond energies]] for bonds broken and bonds formed.

For a generic chemical reaction

:<math>\nu_{\text {A}} \text {A} + \nu_{\,\text {B}} \text {B} ~+ ~... \rightarrow  \nu_{\,\text {X}} \text {X} + \nu_{\text {Y}} \text {Y} ~+ ~...</math>

the standard enthalpy of reaction <math>\Delta H_{\text {reaction}}^\ominus</math> is related to the [[standard enthalpy of formation]] <math>\Delta_{\text {f}} H^\ominus</math> values of the reactants and products by the following equation:<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 |page=247 |edition=8th}}</ref>

:<math>
\Delta H_{\text {reaction}}^\ominus = \sum_{\text{products},~p} \nu_p\Delta_{\text {f}} H_{p}^{\ominus} - \sum_{\text{reactants},~r} \nu_r\Delta_{\text {f}} H_{r}^{\ominus}
</math>

In this equation, <math>\nu_i</math> are the [[Stoichiometry#Stoichiometric coefficient and stoichiometric number|stoichiometric coefficients]] of each product and reactant. The [[standard enthalpy of formation]], which has been determined for a vast number of substances, is the change of enthalpy during the formation of 1 mole of the substance from its constituent elements, with all substances in their standard states. 

Standard states can be defined at any temperature and pressure, so both the standard temperature and pressure must always be specified. Most values of standard thermochemical data are tabulated at either (25°C, 1 bar) or (25°C, 1 atm). <ref>{{cite book |last1=Tinoco |first1=Ignacio Jr. |last2=Sauer |first2=Kenneth |last3=Wang |first3=James C. |title=Physical Chemistry: Principles and Applications in Biological Sciences |date=1995 |publisher=Prentice-Hall |isbn=0-13-186545-5 |page=125 |edition=3rd}}</ref> 

For ions in aqueous solution, the standard state is often chosen such that the aqueous H<sup>+</sup> ion at a concentration of exactly 1 mole/liter has a standard enthalpy of formation equal to zero, which makes possible the tabulation of standard enthalpies for cations and anions at the same standard concentration. This convention is consistent with the use of the [[standard hydrogen electrode]] in the field of [[electrochemistry]]. However, there are other common choices in certain fields, including a standard concentration for H<sup>+</sup> of exactly 1 mole/(kg solvent) (widely used in [[chemical engineering]]) and <math>10^{-7}</math> mole/L (used in the field of [[biochemistry]]). {{cn|date=November 2024}}