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| The '''standard enthalpy of combustion''' is the [[enthalpy]] change when one [[mole (unit)|mole]] of a reactant completely burns in excess oxygen under [[Standard conditions for temperature and pressure|standard thermodynamic conditions]] (although experimental values are usually obtained under different conditions and subsequently adjusted). By definition, the combustion reactions are always particularly [[exothermic]] and so enthalpies of combustion are always negative, although the values for individual combustions may vary.
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| The most common way of calculating the enthalpy change of combustion (or formation) is by using a Hess cycle or by using numerical based bond enthalpies. It is commonly denoted as <math>\Delta H ^{\circ} _{\mathrm{comb}}</math> or <math>\Delta H ^{\circ}_{\mathrm{c}}</math>.
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| When the enthalpy required is not a combustion, it can be denoted as <math>\Delta H ^{\circ} _{\mathrm{total}}</math>. Enthalpies of combustion are typically measured using [[calorimeter|bomb calorimetry]], and have units of energy (typically kJ); strictly speaking, the enthalpy change per mole of substance combusted is the standard ''molar'' enthalpy of combustion (which typically would have units of kJ mol<sup>−1</sup>).
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| == Alcohols and alkanes ==
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| For alcohols and alkanes containing the same number of carbon atoms, e.g. [[methane]] (CH<sub>4</sub>) and [[methanol]] (CH<sub>3</sub>OH), for which the standard enthalpy change of the alkane would be more negative than the alcohol (<math>\Delta H ^{\circ}_{\mathrm{c}} [\mathrm{CH_{4 (g)}}]</math> = −890.3 kJ mol<sup>−1</sup>, <math>\Delta H ^{\circ}_{\mathrm{c}}[\mathrm{CH_3OH_{(l)}}]</math> = −726.0 kJ mol<sup>−1</sup>).
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| Since the complete combustion of both of these carbon compounds produce carbon dioxide (CO<sub>2</sub>) and water (H<sub>2</sub>O), there is more bond-breaking and bond-making when methane is burnt. The presence of an [[Hydroxide|-OH]] bond on methanol means that there is less bond-breaking and bond-making to produce water compared to methane.<ref>Edexcel AS Chemistry by Ann Fullick and Bob McDuell</ref>
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| ==References==
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| {{reflist}}
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| ==External links==
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| * [http://webbook.nist.gov/chemistry/ NIST Chemistry WebBook]
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| [[Category:Enthalpy]]
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| {{chem-stub}}
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Revision as of 18:02, 11 February 2014
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