Question

In a reaction A -> B with Delta G < 0, why is there equilibrium instead of going to completion?

If products have lower free energy than reactants, why are there reactants left?

Answer 1

Equilibrium is the balance point.

Explanation:

Whether a reaction proceeds in one way or not is the thermodynamics. But before the kinetics finish their job the overall equilibrium MUST be satisfied.

This means that UNLESS you REMOVE a product from the system, it will establish an equilibrium with the reactants. Another way to look at it (experimentally, too) is to put the product by itself in a closed system. Eventually (depending on the thermodynamics) SOME of it will revert to the reactant materials in an attempt to reach equilibrium.

A more rapid and easily observed situation are reactions in which the relative amounts of reactants and products can be adjusted by system temperatures or pressures. Carbonyl sulfide production from hydrogen sulfide and carbon dioxide is one such reaction.

Dry `H_2S` and `CO_2` will spontaneously, produce `COS` and `H_2O`. `COS` in a moist container or solution will convert to `H_2S` and `CO_2`. Which combination occurs just depends on the relative concentration of each species.
`H_2S + CO_2 harr COS + H_2O`.