When a reaction is reversible, its entropy (ΔS) is equal to zero. This is because the process starts and ends in the same place, making the entropy remain unchanged. In order to determine whether ΔS is greater or lower than zero, we need to calculate the system entropy (ΔSsyst) and the surroundings entropy (ΔSsurr). Once we have these two values, we can calculate the universal entropy (ΔSuniv) and determine if the reaction is favored by the product or by the reagents.
For example, when oxygen is in a gaseous state, it has a higher entropy than when it binds with iron. However, at normal pressures, the binding energy between oxygen and iron is so great that oxygen passes through the gas phase and iron oxidizes. This is because the change in Gibbs free energy (ΔG) is negative, meaning that enthalpy wins over entropy.