# 8: Enthalpy and Thermochemical Cycles

• 8.1: Enthalpy
• 8.2: Using Thermochemical Cycles to Find Enthalpy Changes
• 8.3: How Enthalpy Depends on Pressure
• 8.4: Standard States and Enthalpies of Formation
• 8.5: The Ideal Gas Standard State
The ideal gas standard state is a useful invention, which has additional advantages that emerge as our development proceeds. For permanent gases—gases whose behavior is approximately ideal anyway—there is a negligible difference between the enthalpy in the ideal gas state and the enthalpy at 1 bar.
• 8.6: Standard Enthalpies of Reaction
If we have the enthalpies of formation, we can compute the enthalpy change for the reaction. We can demonstrate this by writing out the chemical equations corresponding to the formation of A, B, C, and D from their elements. When we multiply these chemical equations by the appropriately signed stoichiometric coefficient and add them, we obtain the chemical equation for the indicated reaction of A and B to give C and D. Because enthalpy is a state function.
• 8.7: Standard State Heat Capacities
We have observed that CV depends on volume and temperature, while CP depends on pressure and temperature. Compilations of heat capacity data usually give values for CP , rather than CV . When the temperature-dependence of CP is known, such compilations usually express it as an empirical polynomial function of temperature.
• 8.8: How The Enthalpy Change for a Reaction Depends on Temperature
We use tabulated enthalpies of formation to calculate the enthalpy change for a particular chemical reaction. Such tables typically give enthalpies of formation at a number of different temperatures, so that the enthalpy change for a given reaction can also be calculated at these different temperatures; it is just a matter of repeating the same calculation at each temperature.
• 8.9: Calorimetry
Calorimetry is the experimental science of measuring the heat changes that accompany chemical or physical changes. The accurate measurement of small amounts of heat is experimentally challenging. Nevertheless, calorimetry is an area in which great experimental sophistication has been achieved and remarkably accurate measurements can be made. Numerous devices have been developed to measure heat changes.
• 8.10: Problems