The Temperature Dependence of Reaction Enthalpies Can Be Determined from Heat Capacity Data

7 The Temperature Dependence of Reaction Enthalpies Can Be Determined from Heat Capacity Data 

For any reaction, the standard enthalpy change at a particular temperature is 

To see how the standard enthalpy of reaction (changes with temperature, recall that dH = Cp dT (Equation 7.26) at constant pressure. Thus, 

AHUT2) = AH%,M + AQ., AT where we have defined the standard reaction heat capacity as 

Equation 7.49 is known as Kirchhoff s law, after the German physicist Gustav-Robert Kirchhoff. According to Kirchhoff s law, the difference between the enthalpies of a reaction at two different temperatures is just the difference in the enthalpies of heating the products and reactants from Ti to T2. Note that in deriving Equation 7.48 we have assumed that the constant-pressure heat capacities are independent of temperature. Otherwise, they must be expressed as functions of T, and the integral in Equation 7.48 must be done explicitly. 

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The temperature dependence of reaction enthalpies can be determined from heat capacity data using Kirchhoff s law. 

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