How to Determine whether a Process Is Thermodynamically Favored

The term ‘spontaneous’ has historically been used to describe processes for which the change in Gibbs free energy in the standard state, ΔG^Ѳ < 0.

The phrase ‘thermodynamically favored’ is preferred instead so that common misunderstandings equating ’spontaneous’ with ‘suddenly’ or ‘without cause’ can be avoided.

Knowing the values of ΔH^Ѳ & ΔS^Ѳ (change in enthalpy and entropy in the standard state respectively) for a process at a given temperature T (absolute value) allows ΔG^Ѳ to be calculated directly:

ΔG^Ѳ = ΔH^Ѳ ─ TΔS^Ѳ

If ΔH^Ѳ < 0 & ΔS^Ѳ > 0, no calculation of ΔG^Ѳ is necessary to determine that the process is thermodynamically favored (ΔG^Ѳ < 0). If ΔH^Ѳ > 0 & ΔS^Ѳ < 0, no calculation of ΔG^Ѳ is required either to conclude that the process is thermodynamically unfavored (ΔG^Ѳ > 0). However, in some cases, it is necessary to consider both enthalpy and entropy to determine if a process will be thermodynamically favored. The temperature conditions for a process to be thermodynamically favored are tabulated below with examples:

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References:
(1)    Wikipedia Contributors. Laws of thermodynamics. Wikipedia. https://en.wikipedia.org/wiki/Laws_of_thermodynamics.

(2)    Ncert. Chemistry : Textbook for Class XI - Part.I; National Council Of Educational Research And Training: New Delhi.