CIE A Level Chemistry (9701) exams from 2022

Revision Notes

5.6.3 Half-Life

First Order Reaction Half-life

  • The half-life of a first-order reaction is independent of the concentration of reactants
    • This means that despite the concentrations of the reactants decreasing during the reaction
    • The amount of time taken for the concentrations of the reactants to halve will remain the same throughout the reaction
    • The graph is a straight line going downwards
  • The rearrangement of the methyl group (CH3) in ethanenitrile (CH3CN) is an example of a first-order reaction with rate equation rate = k [CH3CN]

CH3CN (g) → CH3NC (g)

Reaction Kinetics - Rearrangement of Methyl Group, downloadable AS & A Level Chemistry revision notes

Rearrangement of the CH3 group in CH3CN

  • Experimental data of the changes in concentration over time suggests that the half-life is constant
    • Even if the half-lives are slightly different from each other, they can still be considered to remain constant
  • This means that no matter what the original concentration of the CH3CN is, the half-life will always be around 10.0 minutes

Half-life table

Reaction Kinetics - First Order Reaction Half-life table, downloadable AS & A Level Chemistry revision notes

Reaction Kinetics - Half-Life First-Order, downloadable AS & A Level Chemistry revision notes

In a first-order reaction, the time taken for the concentration to halve remains constant

Worked Example: Using the half-life of first-order reactions in calculations

Reaction Kinetics - Worked Example - Using the half-life of first-order reactions in calculations, downloadable AS & A Level Chemistry revision notes

Answer

  • Step 1: Plot the concentration-time graph using appropriate scales and labels for the axis

Reaction Kinetics - Step 1 graph, downloadable AS & A Level Chemistry revision notes

  • Step 2: Find the first and second half-lives by determining when the concentrations halve using the graph

Step 2 table

Reaction Kinetics - Step 2 table, downloadable AS & A Level Chemistry revision notes

  • Step 3: Determine the reaction order
    It is s a first-order reaction as the successive half-lives remain reasonably constant (around 450 seconds) throughout the reaction

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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