1.19 Enthalpy Change in Chemical Reactions(2) S.J.T.U. Phase Transformation and Applications In general,the enthalpy change for a chemical reaction at a specified temperature may be expressed as the sum of the enthalpies of formation of the products less the enthalpies of formation of the reactions at that temperature. 1.e., AH=∑nAH.r-∑n,AHf.r products reactants where np and n,are the stoichiometric coefficients of the reaction. Enthalpy of combusion the enthalpy change for methane when it is reacted with oxygen,or the heat of combustion of methane. Low heat of combustion:gas water;High heat of combustion:liquid water. SJTU Thermodynamics of Materials Spring 2010 ©X.J.Jin Lecture 3 Second law IPhase Transformation and Applications S. J. T. U. SJTU Thermodynamics of Materials Spring 2010 © X. J. Jin Lecture 3 Second law I 1.19 Enthalpy Change in Chemical Reactions (2)        reac ts r f T products HT np H f T n H tan , , i. e., In general, the enthalpy change for a chemical reaction at a specified temperature may be expressed as the sum of the enthalpies of formation of the products less the enthalpies of formation of the reactions at that temperature. where np and nr are the stoichiometric coefficients of the reaction. Enthalpy of combusion : the enthalpy change for methane when it is reacted with oxygen, or the heat of combustion of methane. Low heat of combustion: gas water; High heat of combustion: liquid water