The terms ΔH and TΔS are subtracted in the expression ΔG = ΔH − TΔS, so they must represent energy on the same unit scale. If enthalpy is reported in kilojoules per mole, the entropy term must also be converted to kilojoules per mole after multiplication by temperature. Matching units prevents invalid comparisons and incorrect free-energy values.
Temperature multiplies entropy in the TΔS term, so the entropy units must combine with temperature to produce energy per mole. A calculation using joules per mole for entropy therefore produces an energy term in joules per mole, whereas a kilojoule-based calculation requires the corresponding conversion. This keeps ΔH and TΔS directly comparable.
Reporting free energy in joules or kilojoules per mole relates the energetic change to a specified amount of substance. This format allows chemists to compare reactions or systems on a consistent basis, even when the samples contain different quantities. Total energy can still describe a particular amount, but molar units are especially useful for chemical comparison.
In ΔG° = −RT ln K, the energy unit assigned to ΔG° must match the unit used for the gas constant R. Using an R expressed in joules produces ΔG° in joules per mole, while a kilojoule form produces kilojoules per mole. Unit consistency therefore allows equilibrium data and standard free-energy values to correspond correctly.
First identify whether the desired result is total energy or energy per mole. Then place enthalpy and entropy on compatible joule or kilojoule scales, apply the temperature to the entropy term, and confirm that both terms share the same units before subtraction. The same check applies when using ΔG° and the equilibrium-constant relationship.
Consistent free-energy units support several kinds of chemical analysis. In thermodynamics, they help compare stability and reaction energetics; in equilibrium work, they connect standard free energy with K; and in electrochemistry and biochemical research, they provide a common energy basis for interpreting chemical change. Reporting units clearly makes results easier to compare across these contexts.