Examples of reactions that are always or never spontaneous

Processes that are never spontaneous

Common for all the processes that are never spontaneous is that they have a positive change in enthalpy, ΔH, and a negative change in entropy, ΔS.

Since the Gibbs free energy is calculated by the equation ΔG = ΔH - TΔS, if ΔH is positive and TΔS is negative, then ΔG will always be positive and the reaction will never be spontaneous.

Examples could be:

• Photosynthesis

• The forming of fuel from CO₂ and water

• Formation of diamonds from coal

Processes that are always spontaneous

Common for all the processes that are always spontaneous is that they have a negative change in enthalpy, ΔH, and a positive change in entropy, ΔS.

Since the Gibbs free energy is calculated by the equation ΔG = ΔH - TΔS, if ΔH is negative and TΔS is positive, then ΔG will always be negative and the reaction will always be spontaneous.

Examples could be:

• The burning of fuel