Low Net and High Gross Heating Values

Therefore either in pure oxygen or dry air, it is possible to apply Hess s law to the previous combustion equations. The standard molar enthalpy and the internal energy of combustion 

Note that the standard molar enthalpy of combustion corresponds to the heat of combustion at constant pressure Q, while the standard molar internal energy of combustion corresponds to the heat of combustion at constant volume Q,. The two quantities are related as follows  

Since the difference between isochore and isobare heat of reactions is often small, in most cases it can be neglected. Actually, both Q,and are about several kJ.moT while the additional term is J.moT . 

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