Entropy and Gibbs energy: favorable does not mean fast
Thermodynamic spontaneity depends in part on ΔH, ΔS and temperature through ΔG = ΔH − TΔS. Kinetics answers another question: how fast does the change occur?
The essential idea
Thermodynamic spontaneity depends in part on ΔH, ΔS and temperature through ΔG = ΔH − TΔS. Kinetics answers another question: how fast does the change occur?
Example
A change can have ΔG < 0 yet remain very slow if its activation barrier is high.
Mini problem
Does ΔG < 0 mean a reaction is instantaneous?
Show the solution
Answer: No. It indicates a thermodynamically favorable direction under the stated conditions, not a high reaction rate.
Explanation: Before moving on, explain “Entropy and Gibbs energy: favorable does not mean fast” in two sentences without rereading the page. A change can have ΔG < 0 yet remain very slow if its activation barrier is high.
Method: Start from the relation used, substitute each quantity with its unit, then check the order of magnitude and the final unit.
Check: A yes/no answer is complete only if you can justify the direction of the relation or state the mechanism that requires it.
Common mistake
Do not replace reasoning with a memorized rule. Always check geometry, units, coefficients or model assumptions.
Guided exercise
At 298 K, a change has ΔH = −20 kJ·mol⁻¹ and ΔS = −50 J·mol⁻¹·K⁻¹. What is ΔG approximately?
Show the detailed solution
Convert ΔS to kJ: −0.050 kJ·mol⁻¹·K⁻¹. Then ΔG = −20 − 298×(−0.050) ≈ −5.1 kJ·mol⁻¹. The change is thermodynamically favorable under these conditions.
Avoid this
Watch the units: mixing joules and kilojoules produces a result wrong by a factor of 1000.