Commonly the chemical equilibrium is defined through Eq. 19a with the Gibbs free energy of reaction ∆𝐺 = 𝐺 𝑃 -𝐺 𝑅 . The Gibbs free energy 𝐺 𝑅/𝑃 itself is defined through the enthalpy 𝐻 𝑅/𝑃 and entropy 𝑆 𝑅/𝑃 (Eq. 19b). At constant pressure and volume, any change in enthalpy, ∆𝐻 𝑅/𝑃 , can be written as a change in inner energy, ∆𝑈 𝑅/𝑃 , and I obtain Eq. 19c. For ∆𝐺 I can then write the total differential as in Eq. 19d. 𝐺, 𝐻, 𝑈, and 𝑆 should be understood as intensive quantities.