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9.7Thermodynamics and Electrochemistry

Coupled Reactions — AP CHEMISTRY Revision Notes & Topical Questions

5 min read 9 parts Lesson 11 of 15 1 quiz

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Lesson summary

9.7 Coupled Reactions — AP CHEMISTRY notes: Explain how a thermodynamically favoured reaction can drive an unfavoured one.Unfavoured reaction — ΔG° > 0 and K < 1, so little product formsWays to make it occur — apply external energy or couple it to a favoured reactionElectrolysis …

  • Photosynthesis
  • Requirement
  • Overall ΔG°
  • Reaction 1
  • Reaction 2
  • Peptide formation

Section 1 of 9Unfavoured processes

1Unfavoured reaction
  • ΔG° > 0 and K < 1, so little product forms
2Ways to make it occur
  • Ways to make it occur apply external energy or couple it to a favoured reaction

Section 2 of 9External energy

Electrolysis
  • Electrolysis electrical energy drives an unfavoured redox reaction
Photosynthesis
  • Photosynthesis light drives conversion of carbon dioxide into glucose

Section 3 of 9Coupling through an intermediate

Requirement
  • Requirement the two reactions share an intermediate
Overall ΔG°
  • Overall ΔG° add the ΔG° values of the reactions
Effect
  • Effect removing the intermediate by the favoured reaction shifts the first equilibrium forward

Section 4 of 9Pattern: copper extraction

Reaction 1
  • Cu2S → 2Cu + S, ΔG° = +85.1 kJ
Reaction 2
  • S + O2 → SO2, ΔG° = −301.4 kJ
Overall
  • Cu2S + O2 → 2Cu + SO2, ΔG° = −216.3 kJ

Section 5 of 9Pattern: biological coupling

Peptide formation
  • Peptide formation alanine + glycine → dipeptide + H2O, ΔG° = +31 kJ
ATP hydrolysis
  • ΔG° = −33 kJ
Overall
  • ΔG° = −2 kJ, so the coupled reaction is favoured

Section 6 of 9Scaling

Multiplying an equation
  • Multiplying an equation multiply ΔG° by the same factor
Example
  • Fe2O3 + 3CO → 2Fe + 3CO2: 742.2 + 3(−283.5) = −108.3 kJ

Section 7 of 9More from the specification

1Insoluble sulfate precipitation as a solid
  • Insoluble sulfate precipitation as a solid combining aqueous barium ions with sulfate ions causes instantaneous precipitation of barium sulfate as a dense white solid.
2Exothermic
  • A reaction that transfers thermal energy to the surroundings.
3Endothermic
  • A reaction that takes in thermal energy from the surroundings.
4Molecule
  • Two or more atoms held together by covalent bonds.
5Equilibrium constant
  • The reaction quotient at equilibrium.
6Enthalpy change
  • The thermal energy transferred during a reaction, ΔH.
7Kinetic control
  • Rate determined by the activation barrier rather than favourability.
8Activation energy
  • The minimum energy colliding particles need to react.
9Exothermic reaction
  • Reaction that gives out thermal energy.
10Temperature
  • A measure of the average kinetic energy of the particles.
11Dissolution
  • Process of a solute dissolving in a solvent.
12Entropy
  • A measure of disorder, related to the number of ways of arranging particles and energy.
13Reactants
  • The substances that react together at the start of a reaction.
14Product
  • A substance formed in a reaction.

Section 8 of 9Key terms

TermMeaning
Coupled reactionTwo reactions combined so a favoured one drives an unfavoured one.
IntermediateSpecies produced in one reaction and consumed in the other.
ATP hydrolysisFavoured reaction used to drive biosynthesis.
Overall ΔG°Sum of ΔG° values of the coupled reactions.

Section 9 of 9Exam technique

Avoid these mistakes
Adding ΔG° values without cancelling the intermediate.
Forgetting to scale ΔG° when multiplying an equation.
Coupling reactions that share no species.

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