A LevelChemistryEdexcelRevision Notes5. Advanced Physical ChemistryKinetics IIReaction Order Graphs

Reaction Order Graphs (Edexcel A Level Chemistry): Revision Note

Exam code: 9CHO

Richard Boole

Written by: Richard Boole

Reviewed by: Caroline Carroll

Updated on

Reaction Order Graphs

Reaction Order Using Concentration-Time Graphs

  • In a zero-order reaction, the concentration of the reactant decreases at a constant rate, so it is a linear function of time

    • This means that the reactant concentration decreases as time increases

    • The graph is a straight line going down as shown:

Concentration-time graph of a zero-order reaction: a straight line sloping downwards at a constant gradient

Concentration-time graph of a zero-order reaction

  • The gradient of the line is the rate of reaction

    • Calculating the gradient at different points on the graph, will give a constant value for the rate of reaction

  • When the order with respect to a reactant is 0, a change in the concentration of the reactant has no effect on the rate of the reaction

  • Therefore:

Rate = k

  • This equation means that the gradient of the graph is the rate of reaction as well as the rate constant, k

  • In a first-order reaction, the concentration of the reactant decreases with time

    • The graph is a curve going downwards and eventually plateaus:

Concentration-time graph of a first-order reaction: a curve falling steeply at first then more gently, never quite reaching zero

Concentration-time graph of a first-order reaction

  • In a second-order reaction, the concentration of the reactant decreases more steeply with time

    • The concentration of reactant decreases more with increasing time compared to a first-order reaction

    • The graph is a steeper curve going downwards:

Concentration-time graph of a second-order reaction: a curve falling more steeply at first than the first-order curve, then flattening

Concentration-time graph of a second-order reaction

Order of reaction from half-life

  • The order of a reaction can also be deduced from its half-life (t1/2 )

  • For a zero-order reaction the successive half-lives decrease with time

    • This means that it would take less time for the concentration of reactant to halve as the reaction progresses

  • The half-life of a first-order reaction remains constant throughout the reaction

    • The amount of time required for the concentration of reactants to halve will be the same during the entire reaction

  • For a second-order reaction, the half-life increases with time

    • This means that as the reaction is taking place, it takes more time for the concentration of reactants to halve

Three concentration-time graphs showing successive half-lives: zero order has decreasing half-lives, first order has constant half-lives, second order has increasing half-lives

Half-lives of zero, first and second-order reactions

Examiner Tips and Tricks

Make sure that you know the correct shapes for the concentration-time graphs. It can be easy to confuse some concentration-time graphs with the following rate-concentration graphs, particularly;

  • The straight line of a zero-order concentration-time graph with the straight line of a first-order rate-concentration graph.

  • The curve of a first-order concentration-time graph with the curve of a second-order rate-concentration graph.

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

Rate-concentration graph of a zero-order reaction: a horizontal line, rate independent of [A]

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]

Rate-concentration graph of a first-order reaction: a straight line through the origin, rate directly proportional to [A]

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

Rate-concentration graph of a second-order reaction: a curve through the origin that gets steeper as [A] increases, rate proportional to [A] squared

Rate-concentration graphs of a second-order reaction

Examiner Tips and Tricks

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 Boole

Author: Richard Boole

Expertise: Curriculum Expert

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.

Caroline Carroll

Reviewer: Caroline Carroll

Expertise: Head of Content Delivery

Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about delivering high-quality resources to help students achieve their full potential.