9.3Thermodynamics and Electrochemistry

Gibbs Free Energy and Thermodynamic Favorability — AP CHEMISTRY Revision Notes & Topical Questions

5 min read 8 parts Lesson 6 of 15 1 quiz

0%read

Lesson summary

9.3 Gibbs Free Energy and Thermodynamic Favorability — AP CHEMISTRY notes: Use ΔG° = ΔH° − TΔS° to decide whether a process is thermodynamically favoured.ΔG° — the standard free-energy change; the maximum energy available from a reactionFavoured — ΔG° < 0; not favoured — ΔG° > 0S…

Second law link
From formation data
From ΔH and ΔS
Thermodynamically favoured — the preferred term for what was called spontaneous
Meaning — no energy input is required after mixing, which does not imply speed
Data — ΔH° = +135 kJ and ΔS° = +0.344 kJ K-1 at 298 K

Section 1 of 8Gibbs free energy

ΔG°

ΔG° the standard free-energy change; the maximum energy available from a reaction

Favoured

ΔG° < 0; not favoured — ΔG° > 0

Second law link

Second law link a process occurs when it increases the entropy of the universe

Section 2 of 8Equations

From formation data

ΔG° = ΣΔG°_f(products) − ΣΔG°_f(reactants)

From ΔH and ΔS

ΔG° = ΔH° − TΔS°
ΔS° in J K^-1 mol^-1 must be divided by 1000 to give kJ.

Section 3 of 8Signs of ΔH°, ΔS° and ΔG°

ΔH°	ΔS°	ΔG° < 0 (favoured) at	Description
Negative	Positive	All temperatures	Always favoured
Positive	Negative	No temperature	Never favoured
Positive	Positive	High temperatures	Favoured as temperature rises
Negative	Negative	Low temperatures	Favoured only at low temperature

Section 4 of 8Terminology

1Thermodynamically favoured

Thermodynamically favoured the preferred term for what was called spontaneous

2Meaning

Meaning no energy input is required after mixing, which does not imply speed

Section 5 of 8Pattern: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂

Data

ΔH° = +135 kJ and ΔS° = +0.344 kJ K^-1 at 298 K

Result

ΔG° = 135 − 298(0.344) = +32.5 kJ, so not favoured at 298 K

Section 6 of 8More from the specification

1Gravimetric filtration of precipitate solid

Gravimetric filtration of precipitate solid filtering, washing,
- and drying precipitates allows accurate mass measurement of the collected crystalline solid.

2Molecule

Two or more atoms held together by covalent bonds.

3Equilibrium constant

The reaction quotient at equilibrium.

4Enthalpy change

The thermal energy transferred during a reaction, ΔH.

5Kinetic control

Rate determined by the activation barrier rather than favourability.

6Activation energy

The minimum energy colliding particles need to react.

7Coupling reaction

Electrophilic substitution of a diazonium ion into an activated ring.

8Equilibrium

The state where forward and reverse transfer rates are equal.

9Dissolution

Process of a solute dissolving in a solvent.

10Exothermic

A reaction that transfers thermal energy to the surroundings.

11Endothermic

A reaction that takes in thermal energy from the surroundings.

12Disorder

Number of arrangements of particles and energy.

Section 7 of 8Key terms

Term	Meaning
Gibbs free energy	G = H − TS.
Thermodynamically favoured	ΔG° < 0.
ΔG°_f	Free energy of formation; zero for elements.
Second law	Entropy of the universe increases in a favoured process.

Section 8 of 8Exam technique

Avoid these mistakes

Using J instead of kJ for ΔS.

Using ΔG° to say a reaction is fast.

Assuming endothermic reactions are never favoured.

Last updated: 15 October 2026