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9.4Thermodynamics and Electrochemistry

Thermodynamic and Kinetic Control — AP CHEMISTRY Revision Notes & Topical Questions

5 min read 7 parts Lesson 8 of 15 1 quiz

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Lesson summary

9.4 Thermodynamic and Kinetic Control — AP CHEMISTRY notes: Distinguish whether a reaction is favoured (thermodynamics) from how fast it proceeds (kinetics).AspectThermodynamicsKineticsQuestionIs the reaction favoured?How fast does it proceed?QuantityΔG°Activation energy EaCataly…

  • Definition
  • Misconception
  • Peak height
  • Eventually
  • Thermodynamic control
  • Reaction — H2O2(l) → H2O(l) + ½O2(g) is favoured at 298 K

Part 1 of 5Two different questions

AspectThermodynamicsKinetics
QuestionIs the reaction favoured?How fast does it proceed?
QuantityΔG°Activation energy Ea
CatalystNo effectLowers Ea

Part 2 of 5Kinetic control

Definition
  • Definition a thermodynamically favoured reaction that is slow or immeasurable because of a high activation energy
Misconception
  • Misconception favoured reactions do not necessarily occur quickly

Part 3 of 5Example: hydrogen peroxide

Reaction
  • H2O2(l) → H2O(l) + ½O2(g) is favoured at 298 K
Rate
  • Rate very slow without a catalyst because of a large Ea
Catalyst
  • MnO2 provides a lower-Ea route

Part 4 of 5Energy profile

ΔH and ΔG
  • ΔH and ΔG unchanged by a catalyst
Peak height
  • Peak height lowered by a catalyst
Eventually
  • Eventually given long enough, an uncatalysed favoured reaction would proceed

Part 5 of 5More from the specification

1Precipitation of insoluble crystalline solid
  • Precipitation of insoluble crystalline solid exceeding ionic product values drives instantaneous crystallization of aqueous ions into an insoluble solid precipitate.
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.
7Coupling reaction
  • Electrophilic substitution of a diazonium ion into an activated ring.
8Exothermic reaction
  • Reaction that gives out thermal energy.
9Equilibrium
  • The state where forward and reverse transfer rates are equal.
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.

Key terms

TermMeaning
Kinetic controlRate determined by the activation barrier rather than favourability.
Thermodynamic controlOutcome determined by relative stability.
Activation energyMinimum energy for reaction.
CatalystSubstance lowering Ea without being consumed.

Avoid these mistakes

Avoid these mistakes
Believing ΔG° < 0 means the reaction is fast.
Believing a catalyst makes an unfavoured reaction favoured.
Confusing activation energy with ΔH.

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