Rate-Concentration Graphs (OCR A Level Chemistry)

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Rate-Concentration Graphs

Reaction order using rate-concentration graphs

  • In a zero-order reaction, the rate doesn’t depend on the concentration of the reactant
    • The rate of the reaction therefore remains constant throughout the reaction
    • The graph is a horizontal line
    • The rate equation is rate = k

Reaction Kinetics - Zero Order Rate, downloadable AS & A Level Chemistry revision notes

Rate-concentration graph of a zero-order reaction

  • In a first-order reaction, the rate is directly proportional to the concentration of a reactant
    • The rate of the reaction increases as the concentration of the reactant increases
    • This means that the rate of the reaction decreases as the concentration of the reactant decreases when it gets used up during the reaction
    • The graph is a straight line
    • The rate equation is rate = k[A]

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

Rate-concentration graph of a first-order reaction

  • In a second-order reaction, the rate is directly proportional to the square of concentration of a reactant
    • The rate of the reaction increases more as the concentration of the reactant increases
    • This means that the rate of the reaction decreases more as the concentration of the reactant decreases when it gets used up during the reaction
    • The graph is a curved line
    • The rate equation is rate = k[A]2

Reaction Kinetics - Second Order Rate, downloadable AS & A Level Chemistry revision notes

Rate-concentration graphs of a second-order reaction

Examiner Tip

Careful - sometimes when asked to complete calculations for the rate constant, k, the exam question will give you a graph as well as tabulated data. Do not ignore the graph as this demonstrates the order of one of the reactants, while the tabulated data allows you to determine the order for the other reactants.

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Richard

Author: Richard

Expertise: Chemistry

Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.