Thermochemistry Energy Flow and Chemical Change

The enthalpy change of a reaction, called the heat of reaction (AH ). is negative ( 0) if the reaction releases heat exothermic) and positive ( 0) if it absorbs heat endothermic)-, for example, the combustion of methane is exothermic (AH ,n 0). and the melting of ice is endothermic (AH n 0). 

The more heat a substance absorbs, the higher its temperature becomes, but each substance has its own heat capacity, the heat required for a given temperature rise. Knowing tNs capacity and measuring AT in a calorimeter, we can find AHn n- 

The quantity of heat lost or gained in a reaction is related stoichiomeiri-cally to the amounts of reactants and products. 

Because H is a state function, we can find AH of any reaction by imagining that it occurs as the sum of other reactions whose AH values we know or can measure (Hess s law of heat summation). 

Chemists define a set of conditions, caled standard stales, in order to compare heats of different reactions. Each substance has a standard heat of formation (AHf), the heat of reaction when the substance is formed from its elements under these conditions. AH values are used to calculate the standard heat of reaction (AH ). 

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CHAPTER 6 Thermochemistry Energy Flow and Chemical Change... 

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