Gibbs Free Energy and Spontaneity
Learn how Gibbs free energy determines spontaneity, how to calculate it, and its dependence on temperature, pressure, and other thermodynamic parameters.
Why this shows up in the exam
Predicting spontaneity and equilibrium using Gibbs energy is a key skill for NEET and JEE.
How NEET tests this
Learn the idea
Gibbs free energy links enthalpy, entropy and temperature to decide if a process occurs spontaneously.
🧠 Memory hook: G = H – T S
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- ΔG = ΔH – TΔS
- A process is spontaneous when ΔG is negative under the given conditions
- The sign of ΔG indicates the relative stability of oxidation states
- Temperature influences spontaneity through the TΔS term
How to approach it
- 1Write the expression ΔG = ΔH – TΔS and substitute the given data
- 2Check the sign of ΔG; negative means spontaneous, positive means non‑spontaneous
Worked example — watch it click
PbO₂ → PbO; ΔG₂₉₈ < 0 SnO₂ → SnO; ΔG₂₉₈ > 0 Most probable oxidation state of Pb and Sn will be
- A)Pb⁴⁺, Sn⁴⁺
- B)Pb⁴⁺, Sn²⁺
- C)Pb²⁺, Sn²⁺
- ✅Pb²⁺, Sn⁴⁺
The concept behind this problem
The example compares ΔG signs for reduction of Pb⁴⁺ and Sn⁴⁺, showing that a negative ΔG makes the lower oxidation state more stable, while a positive ΔG keeps the higher state stable.
Step by step
- 1PbO₂ → PbO has ΔG < 0 (spontaneous), meaning Pb⁴⁺ reduces to Pb²⁺ easily, so Pb²⁺ is more stable.
- 2SnO₂ → SnO has ΔG > 0 (non-spontaneous), meaning Sn⁴⁺ does not reduce to Sn²⁺ easily, so Sn⁴⁺ is more stable.
- 3Most probable states: Pb²⁺ and Sn⁴⁺.
Watch out
Do not treat a positive ΔG as indicating a reaction will never occur; it may become spontaneous at another temperature or pressure.
Common slip-ups that cost marks
- •Confusing a negative ΔS with increased disorder; a negative ΔS reduces the TΔS contribution
- •Assuming ΔG equals ΔH and ignoring the entropy term
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Practise it
These are real questions from past NEET papers that test this exact idea.
PbO₂ → PbO; ΔG₂₉₈ < 0 SnO₂ → SnO; ΔG₂₉₈ > 0 Most probable oxidation state of Pb and Sn will be
Push further
More challenging26 harder questions built from the past papers above — a step up in difficulty, with distractors designed so you can't get there by elimination. Written and checked by our reviewers, not from a real paper.
For a certain reaction, the enthalpy change (ΔH) is +120 kJ and the entropy change (ΔS) is +200 J K⁻¹. At what minimum temperature will this reaction become spontaneous?
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