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.
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Mini problem
Does ΔG < 0 mean a reaction is instantaneous?
Show the solution
No. It indicates a thermodynamically favorable direction under the stated conditions, not a high reaction rate.
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.
The entropy contribution is multiplied by absolute temperature, so its importance changes with T.
Key idea
Entropy and Gibbs energy: favorable does not mean fast
Key idea
Entropy and Gibbs energy: favorable does not mean fast
ΔG = ΔH − TΔS
Thermodynamic spontaneity depends in part on ΔH, ΔS and temperature through ΔG = ΔH − TΔS. Kinetics answers another question: how fast does the change occur?
Guided example
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?
Put it into practice
Entropy and Gibbs energy: favorable does not mean fast
Choose a concrete case linked to Entropy and Gibbs energy: favorable does not mean fast. Write down the known information, what you are trying to find, and the units. Then state the rule, model or trend you will use. If a calculation is involved, keep the units visible at every line; if it is conceptual, state what changes and what remains fixed.
Check your answer
A strong answer contains four things: the starting situation, the variable that changes, the direction or numerical result, and one check. The check can be a unit, an order of magnitude, a limiting case or a comparison with a familiar example.