Rules of Thermochemistry

The internal energy and the enthalpy of a system depend only on the state of the system, as specified by parameters such as V, P, and T. When a system goes from an initial to a final state, ΔE and ΔH depend only on those two states, and are independent of the path taken between them. This path independence implies two important rules:

1. ΔE and ΔH for a reverse process are exactly the negatives of the values for the related forward process. For example, it requires 6.02 kJ of heat to melt 1 mole of ice to liquid water at 0°C, 1 atm. This is a constant pressure process and ΔH = +6.02 kJ/mol. Then for the reverse process, freezing liquid water to ice at 0°C, 1 atm, ΔH = -6.02 kJ/mol, i.e., we must transfer 6.02 kJ from 1 mole of liquid water to convert it to ice.

2. If a process can be imagined to occur in successive steps, ΔH for the overall process is equal to the sum of the enthalpy changes for the individual steps. This rule, sometimes called Hess's law of constant heat summation, has many applications in thermochemistry.