Editing Enthalpy (section)

===Heat of reaction===
{{Main|Standard enthalpy of reaction}}
The total enthalpy of a system cannot be measured directly; the ''enthalpy change'' of a [[system (thermodynamics)|system]] is measured instead. Enthalpy change is defined by the following equation:
<math display="block">
 \Delta H = H_\text{f} - H_\text{i},
</math>
where
: {{math|Δ''H'' }} is the "enthalpy change",
: {{mvar|H}}{{sub|f}} is the final enthalpy of the system (in a chemical reaction, the enthalpy of the products or the system at equilibrium),
: {{mvar|H}}{{sub|i}} is the initial enthalpy of the system (in a chemical reaction, the enthalpy of the reactants).

For an [[exothermic reaction]] at constant [[pressure]], the system's change in enthalpy, {{math|Δ''H''}}, is negative due to the products of the reaction having a smaller enthalpy than the reactants, and equals the heat released in the reaction if no electrical or mechanical work is done. In other words, the overall decrease in enthalpy is achieved by the generation of heat.<ref name=Laidler-Meiser-1982>
{{cite book
 |last1=Laidler |first1=K. J. |author-link = Keith J. Laidler
 |last2=Meiser  |first2=John H. 
 |year=1982
 |title=Physical Chemistry
 |publisher=Benjamin / Cummings
 |isbn=978-0-8053-5682-3
 |page=53
}}
</ref>
Conversely, for a constant-pressure [[endothermic]] reaction, {{math|Δ''H''}} is positive and equal to the heat ''absorbed'' in the reaction.

From the definition of enthalpy as {{nobr|{{mvar|H {{=}} U + pV}},}} the enthalpy change at constant pressure is {{nobr|{{math|Δ''H'' {{=}} Δ''U'' + ''p''{{tsp}}Δ''V''}}.}} However, for most chemical reactions, the work term {{math|''p''{{tsp}}Δ''V''}} is much smaller than the internal energy change {{math|Δ''U''}}, which is approximately equal to {{math|Δ''H''}}. As an example, for the combustion of carbon monoxide {{nobr|2 CO(g) + O{{sub|2}}(g) → 2 CO{{sub|2}}(g),}} {{nobr|{{math|Δ''H'' {{=}} −566.0}} kJ}} and {{nobr|{{math|Δ''U'' {{=}} −563.5}} kJ.}}<ref>
{{cite book
 |first1=Ralph H.    |last1=Petrucci
 |first2=William S.  |last2=Harwood
 |first3=F. Geoffrey |last3=Herring
 |year=2002
 |title=General Chemistry  |edition=8th
 |publisher=Prentice Hall
 |isbn=978-0-13-014329-7
 |pages=[https://archive.org/details/generalchemistry00hill/page/237 237–238]
 |url=https://archive.org/details/generalchemistry00hill |url-access=registration
}}
</ref>
Since the differences are so small, reaction enthalpies are often described as reaction energies and analyzed in terms of [[bond energy|bond energies]].