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*Revision Materials* 1 Atomic Structure 2 Atoms, molecules and stoichiometry 3 Chemical Bonding 4 States of matter 5 Chemical energetics 6 Electrochemistry 7 Equilibria 8 Reaction kinetics 9 The Periodic Table, chemical periodicity 10 Group 2 11 Group 17 12 Nitrogen and sulfur 13 Organic 14 Hydrocarbons 15 Halogen compounds 16 Hydroxy compounds 17 Carbonyl compounds 18 Carboxylic acids and derivatives 19 Nitrogen compounds 20 Polymerisation 21 Organic synthesis 22 Analytical techniques 23 Chemical energetics 24 Electrochemistry 25 Equilibria 26 Reaction kinetics 27 Group 2 28 Chemistry of transition elements 29 Organic 30 Hydrocarbons 31 Halogen compounds 32 Hydroxy compounds 33 Carboxylic acids and derivatives 34 Nitrogen compounds 35 Polymerisation 36 Organic synthesis 37 Analytical techniques

23 Chemical energetics

23.1 Lattice energy and Born-Haber cycles 23.2 Enthalpies of solution and hydration 23.3 Entropy change, ΔS 23.4 Gibbs free energy change, ΔG

Gibbs free energy change, ΔG

Specification Reference Physical Chemistry, Chemical energetics 23.4

Quick Notes

  • Gibbs Free Energy Change (ΔG) can be used to determine whether a reaction is feasible (spontaneous).
  • Equation for Gibbs Free Energy:
    ΔG = ΔH – TΔS
    • ΔG = Gibbs Free Energy change (kJ mol⁻¹)
    • ΔH = Enthalpy change (kJ mol⁻¹)
    • T = Temperature (K)
    • ΔS = Entropy change (J K⁻¹ mol⁻¹) (converted to kJ K⁻¹ mol⁻¹ by dividing by 1000)
  • A reaction is spontaneous and feasible when ΔG < 0 (negative).
  • Feasibility can depend on temperature: some reactions become feasible only at high or low temperatures depending on the signs of ΔH and ΔS.
  • Just because a reaction can happen and is feasible doesn’t mean it will happen – a high activation energy barrier may prevent a feasible reaction from occurring.

Full Notes

Gibbs Free Energy has been outlined in more detail here.
This page is just what you need to know for CIE A-level Chemistry :)

A reaction that can happen, based on energy changes, is called feasible or spontaneous.

Feasible and spontaneous reactions don't require a net input of energy to occur.

Enthalpy change isn’t sufficient to explain why certain reactions are feasible – otherwise, endothermic reactions (+ΔH) would never happen.

To determine whether a reaction is feasible, an idea called Gibbs Free Energy Change (ΔG) is used.

The Gibbs Equation

Gibbs Free Energy can be calculated using:

CIE A-Level Chemistry equation showing Gibbs Free Energy equation ΔG° = ΔH° – TΔS°.

ΔG° = ΔH° – TΔS°

This equation links two key thermodynamic concepts – enthalpy and entropy – to determine whether a reaction is energetically possible (feasible).

For a reaction to be feasible, ΔG° must be negative. This means the system is losing useful energy, making the reaction thermodynamically favourable and able to proceed on its own.

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Matt’s Exam Tip

Just because a reaction is feasible in terms of ΔG, that doesn’t mean it will definitely happen. A high activation energy barrier may give such a slow rate of reaction, it prevents the reaction from occurring. ΔG values tell us whether reactions can happen, they don’t tell us that they actually will.

Example: Conversion of Diamond

The conversion of diamond into graphite has a negative ΔG, meaning it is thermodynamically feasible. In theory, this process should happen spontaneously.

CIE A-Level Chemistry diagram showing diamond converting to graphite with ΔG negative but high activation energy barrier.

However, in reality, the transformation does not occur noticeably because it has a very high activation energy (Ea). This large energy barrier prevents the reaction from proceeding at a measurable rate under standard conditions.

How to Perform a ΔG Calculation

Step-by-step process for ΔG Calculation:

  1. Check units: ΔH in kJ mol⁻¹, ΔS in J mol⁻¹ K⁻¹ (convert to kJ).
  2. Insert values into the formula.
  3. Multiply T × ΔS, then subtract that from ΔH.
  4. Interpret the result: negative = feasible, positive = not feasible.
Worked Example: ΔG Calculation

A reaction has the following:
ΔH = –150 kJ mol⁻¹
ΔS = –100 J mol⁻¹ K⁻¹ = –0.100 kJ mol⁻¹ K⁻¹
T = 298 K

  1. ΔG = ΔH – TΔS
  2. ΔG = –150 – (298 × –0.100)
  3. ΔG = –150 + 29.8 = –120.2 kJ mol⁻¹

So the reaction is feasible under standard conditions.


Photo of Matt
Matt’s Exam Tip

Always check your units. Remember to convert energy units for entropy change (ΔS) from J to kJ by dividing by 1000. This is because ΔH and ΔG both have units of kJ per mol. Every year I see students lose a mark from missing this small step.

What Does the Sign of ΔG Tell Us?

Predicting the Effect of Temperature on Feasibility

The temperature (T) can affect ΔG depending on whether ΔH and ΔS are positive or negative.

  1. If ΔH is negative and ΔS is positive:
    ΔG is always negative → Reaction is always feasible.
  2. If ΔH is positive and ΔS is negative:
    ΔG is always positive → Reaction is never feasible.
  3. If ΔH is negative and ΔS is negative:
    Reaction is feasible at low temperatures.
  4. If ΔH is positive and ΔS is positive:
    Reaction is feasible at high temperatures.

This helps explain why some reactions only work when heated or cooled.

To find the minimum temperature (T) where a reaction is feasible, we can set ΔG = 0:
T = ΔH / ΔS

Worked Example: Minimum temperature

A reaction has the following enthalpy and entropy changes. Find the minimum temperature, T, at which the reaction is feasible.
ΔH = +50 kJ mol⁻¹
ΔS = +100 J K⁻¹ mol⁻¹ (= +0.100 kJ K⁻¹ mol⁻¹)

  1. Use ΔG = ΔH – TΔS and set ΔG = 0
  2. 0 = ΔH – TΔS
  3. T = ΔH / ΔS
  4. T = 50 / 0.100
  5. T = 500 K

The reaction becomes feasible at T ≥ 500 K.

Summary