Campbell\u27s Rule for Estimating Entropy Changes in Gas-Producing and Gas-Consuming Reactions and Related Generalizations about Entropies and Enthalpies

Learning how to estimate thermodynamic quantities and how to use estimates extends the usefulness of thermodynamic analyses of chemical reactions. Estimating helps chemists understand why chemical reactions occur and why the outcomes change with temperature. A new generalization with the suggested name of Campbell\u27s rule, where ΔrS° ≈ 140 J/(K mol rxn) per net mole of gas produced in a reaction, is introduced. This rule enables estimating entropies of reaction from the net change in moles of gas whenever the formation or combination of aqueous ions is not involved. A number of patterns in molar entropies are identified. These patterns are the basis for making good estimates of molar entropies of unknown or transient species from entropies of known species of the same phase, same bond type, same shape, similar composition, and similar mass. For several other types of processes, including reactions involving ions, the sign of ΔrS° is considered. For molar enthalpies and enthalpies of reaction we have fewer patterns than for entropies on which to build. The principal exceptions are recognizing the patterns in bond dissociation enthalpies and estimating enthalpies of reaction for gas phase processes. A simple scheme for estimating enthalpies of reaction for the gas phase is described