4CH1

Rates of Reaction

Physical Chemistry · 3 question types

Practise

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 8% of your exam marks.

stable
Low
Stable8%

Factors affecting rate and collision theory are high-frequency multi-mark questions.

What activation energy is

  • Activation energy, Ea, is the minimum energy that colliding reactant particles must have in order to react
  • Below Ea, even a head-on collision just bounces apart; above Ea, the existing bonds can start to break and new bonds form
  • Different reactions have different activation energies: rusting has a small Ea (rust forms slowly at room temperature) while burning natural gas has a large Ea (needs a spark or flame to start)

Reaction profiles

  • A plots the energy of the reactants, the products, and the in between as the reaction progresses
  • The horizontal axis is "progress of reaction"; the vertical axis is "energy"
  • Two key features:
    • The vertical distance from the reactants line up to the peak of the profile is the Ea
    • The vertical distance from the reactants line to the products line is the overall enthalpy change ΔH
  • For an exothermic reaction, products sit lower than reactants and ΔH is negative
  • For an endothermic reaction, products sit higher than reactants and ΔH is positive
  • In both cases the activation-energy hump rises above the starting energy of the reactants
An exothermic reaction profile comparing the uncatalysed path (higher peak, activation energy Ea) with the catalysed path (lower peak, activation energy Ea(cat)); both curves start at the reactants line and end at the lower products line, and the overall enthalpy change ΔH is negative and unchanged by the catalyst
Source: What is activation energy? by Save My Exams

Catalysts on the reaction profile

  • A catalyst lowers the activation-energy hump without changing the energies of the reactants or the products
  • On the reaction profile, the curve has a lower peak but the same starting and ending levels
  • The overall ΔH is unchanged — a catalyst does not change how exothermic or endothermic a reaction is, only how fast it gets there