Enthalpy Changes

Measuring Enthalpy Changes

We can measure an enthalpy change by determining the amount of heat involved in a reaction when the only work done is PV work.

Calculating Enthalpy Changes

Enthalpy changes are calculated using Hess's law:  If a process can be written as the sum of several steps, the enthalpy change of the process equals the sum of the enthalpy changes of the individual steps.

1. If we know the enthalpy changes of a series of reactions that add up to give an overall reaction, we add these enthalpy changes to determine the enthalpy change of the overall rection.

Using the enthalpy change for the reaction of Fe with Cl2 to give FeCl2 and the enthalpy change for the reaction of FeCl2 with Cl2 to give FeCl3, we can determine the enthalpy change for the reaction of Fe with Cl2 to give FeCl3.

2. If we know the standard enthalpies of formation, Hof, of the reactants and products of a reaction we can calculate the enthalpy change of the reaction using the following shorthand version of Hess's law:

Many handbooks and textbooks have tables of standard enthalpies of formation.

Note: As an approximation, introductory courses assume that enthalpy changes do not change with temperature.

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