Gibbs energy and spontaneity (HL)

Answer

$\Delta G = \Delta H - T\,\Delta S$ — it combines the enthalpy change and the entropy change into one test for spontaneity. (Given in the data booklet.)

Answer

When **ΔG < 0**. ΔG > 0 is non-spontaneous; ΔG = 0 is at equilibrium.

Answer

**No** — spontaneous means thermodynamically **feasible**. Speed is decided by **kinetics** (activation energy), not by ΔG.

Answer

ΔH is in **kJ** mol⁻¹ but ΔS is in **J** K⁻¹ mol⁻¹ — convert ΔS to kJ by **dividing by 1000** before combining.

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**Kelvin** (K = °C + 273) — never degrees Celsius.

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**Always spontaneous** (at every temperature) — ΔG is negative at all T.

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**Never spontaneous** — ΔG is positive at all T.

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Spontaneous only at **low** temperature (the +TΔS term wins once T is large).

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Spontaneous only at **high** temperature (the −TΔS term wins once T is large).

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Set **ΔG = 0**, so $T = \dfrac{\Delta H}{\Delta S}$ — use matching units (both in kJ) so T comes out in K.

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**Same** signs on ΔH and ΔS ⇒ **temperature decides**. **Opposite** signs ⇒ same answer at every temperature.

Answer

A large increase in **disorder** (ΔS > 0) makes −TΔS very negative, so ΔG can be negative even when ΔH > 0 (e.g. dissolving ammonium nitrate).