Question

Why is it more convenient to predict the direction of reaction in terms of Δ$G_{sys}$ instead of Δ$S_{total}$ ? Under what conditions can Δ$G_{sys}$ be used to predict the spontaneity of a reaction?

Answer 1

ΔG is change in Gibb’s free energy and ΔS is change in entropy. Free energy of the system can be expressed as:

                                                 G = H – TS

Sometimes it is difficult to measure the change in entropy – for example, if it occurs too fast like in explosion or too slow like formation of diamond in nature.

But very easily we can measure Gibb’s free energy as it also depends on the concentration of the reactants and products.

Taking the concentrations of reactants and products as variable, we can determine ΔG as

                                     ΔG = ΔG° + RT($ln C_{product}/ C_{reactant}$)

where R is the universal gas constant, T is temperature, and $C_{product}$, $C_{reactant}$ are the initial concentrations of the products and reactants and ΔG° is the standard free energy of the reaction.

When ΔG < 0, forward reaction ( reactant →product) occurs spontaneously. When ΔG > 0, backward reaction occurs spontaneously.

When ΔG = 0, the reaction is at equilibrium and no change in concentration occurs.