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1 Atomic Structure and Properties 2 Compound Structure and Properties 3 Properties of Substances and Mixtures 4 Chemical Reactions 5 Kinetics 6 Thermochemistry 7 Equilibrium 8 Acids and Bases 9 Thermodynamics and Electrochemistry

Thermodynamics and Electrochemistry

9.1 Entropy Introduction 9.2 Absolute Entropy and Entropy Change 9.3 Gibbs Free Energy and Thermodynamic Favorability 9.4 Thermodynamic and Kinetic Control 9.5 Free Energy and Equilibrium 9.6 Free Energy of Dissolution 9.7 Coupled Reactions 9.8 Galvanic (Voltaic) and Electrolytic Cells 9.9 Cell Potential and Free Energy 9.10 Cell Potential Under Nonstandard Conditions

Thermodynamic and Kinetic Control

Learning Objective 9.4.A Explain, in terms of kinetics, why a thermodynamically favored reaction might not occur at a measurable rate.

Quick Notes

  • Thermodynamic favorability (ΔG° < 0) tells us whether a process can occur.
  • Kinetics tells us how fast it occurs.
  • A thermodynamically favored process might not proceed if it has a high activation energy.
    • Such processes are said to be under kinetic control.

Full Notes

Just because a reaction is thermodynamically favored (-ΔG), it does not mean it will happen quickly or to any noticeable degree.

Thermodynamic favorability indicates that the products are lower in free energy than the reactants, but it says nothing about the pathway or rate.

Kinetic Control

A reaction is under kinetic control when it is thermodynamically favorable, but does not proceed at an observable rate.

This is usually due to a high activation energy (Ea) – the energy barrier that must be overcome for the reaction to proceed.

The molecules may not have enough energy or the correct orientation to react, even if the overall reaction would release energy.

For example The Conversion of Diamond into Graphite

AP Chemistry illustration showing diamond and graphite with negative ΔG° but large activation energy barrier—thermodynamically favored yet kinetically slow.

Standard Gibbs Free Energies of Formation (ΔG°f):
C(graphite) = 0 kJ/mol (this is the defined standard state of carbon)
C(diamond) = +2.9 kJ/mol

The conversion of diamond into graphite has a negative ΔG, meaning it is thermodynamically favourable. However, the reaction is extremely slow due to a very high activation energy. This means diamond is kinetically stable and does not spontaneously convert to graphite under normal conditions, even though the process is energetically favorable.

Worked Example

Calculation of ΔG° for diamond into graphite:

  1. ΔG° = ΔG°f(products) − ΔG°f(reactants)
  2. ΔG° = 0 − (+2.9) = −2.9 kJ/mol

The process is thermodynamically feasible (negative ΔG°) but proceeds imperceptibly slowly due to a large activation energy.


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Matt’s exam tip

It's important to distinguish between equilibrium and kinetic control.
Equilibrium: Forward and reverse reactions occur at equal rates; concentrations remain constant.
Kinetic control: A reaction is not happening at any significant rate – not because it's at equilibrium, but because the activation energy prevents it from starting in the first place.

Summary