The 2nd Law, entropy, and the chemists

The Thomsen–Berthelot principle together with the thermal theory of affinity theorized that the change in enthalpy of a reaction (dHrxn) is the sole determiner of reaction spontaneity. Exothermic reactions are spontaneous. This theory worked well except for the fact that some endothermic reations are also spontaneous. Gibbs’ work incorporating entropy showed that the true determiner of reaction spontaneity, as well demonstrated in the electrochemical cell, is the change in Gibbs free energy (dGrxn) as quantified by dHrxn − TdSrxn. Included is a discussion of the Gibbs–Helmholtz equation, which quantifies the the impact of temperature on dGrxn and thus on equilibrium.