Key points about enthalpy changes

AH values are meaningless unless we state how many moles of product are made, or else include a chemical equation to which that enthalpy change applies. 

The enthalpy change of a reaction is an extensive property. This means that the enthalpy change depends upon the amounts of reactants consumed, which, in turn, control the quantities of product made. In the absence of any other information, we always assume that the AH value for a reaction is produced when the number of moles of reactants that combine are those indicated by the chemical equation. So, when we write the thermochemical equation 

The physical states of the reactants and products must be clearly shown. 

The importance of specifying the physical states of the substances involved in a reaction may be illustrated by referring to the reaction of hydrogen and oxygen gases, with gaseous water as the product  

Aid for this reaction is — 483.64 kJ mol k Experiments show that AH for the reaction 

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