You will determine the value of for one mole of MgO by using Hess's Law and the enthalpy changes of several other solution reactions that are more easily performed and observed.
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Some chemical reactions are endothermic and proceed with absorption of heat while others are exothermic and proceed with an evolution of heat. The magnitude of the heat change is determined by the particular reaction of interest, as well as by the amount of reactants consumed. These units emphasize that enthalpy is an extensive property, which depends upon the amount of reactants consumed. Since enthalpy is a state function, it is not necessary for the reaction mixture to stay at constant temperature during the entire reaction. T 1 is not always an easy condition to satisfy in the laboratory.