ΔH, ΔS, ΔG, and K: Predicting Reaction Favorability

With all this talk about product- and reactant-favored reactions, ΔG° sounds suspiciously like K, the equilibrium constant. In fact, there is a relation between ΔG° and K:

ΔG° = -RT ln(K)

in which R is the universal gas constant, 8.3145 J/K * mol, and T is the temperature in Kelvin. For use in situations involving only gases, K is replaced by K_p; in reactions involving solubility, K is K_sp. Again, there is a proof for this equation that we chose not to include, for reasons of usefulness and space. If you wish to examine the proof, other texts or Internet sites should make it available.

Using this equation, you can easily find K from ΔG° or vice-versa. This allows you to use the powerful Gibbs free energy equation, or use K for calculations involving the concentration of species in the reaction. The set of equations you have learned in the "Thermodynamics" chapter are among the most important you will encounter in your chemistry studies--use them wisely!

The next page will present practice problems to prepare you for a test on this material. Good luck!

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