Question

Would someone be able to explain the difference between ∆G_rxn = -RTLnK_eq and ∆G = ∆H - T∆S? I am not sure on the difference in context when they are used. Thanks

Answer 1

is the Gibbs-Helmholtz equation. It can be applied to all types of reactions, and is used to determine if a given chemical process is spontaneous or not. If the value of is (-)ve, the reaction is said to be spontaneous.

For reversible reactions, we write , where is the standard free energy change for the reaction, also written as .

Now, at equilibrium, , which gives

This can only be applied at equilibrium, to reversible reactions. The equation allows us to determine if a given (reversible) reaction favours completion, when observed at equilibrium. The value of K obtained (if the other quantities are known) tells us if a given reaction will favour formation of the products, i.e., the forward reaction (K>>1), or formation of the reactants, i.e., the backward reaction (K<<1). Please remember: This is when the reaction is at equilibrium.

To summarize:

• If you need to check if a given reaction is spontaneous, use the Gibbs-Helmholtz equation.
• If the process is reversible and you need to predict which direction the reaction will favour, use the other equation.