Concentration-Time Graphs (OCR A Level Chemistry)

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Concentration-Time Graphs

Reaction Order Using Concentration-Time Graphs

  • In a zero-order reaction, the concentration of the reactant is inversely proportional to time
    • This means that the reactant concentration decreases as time increases
    • The graph is a straight line going down as shown:

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

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:

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

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:

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

Concentration-time graph of a second-order reaction

Concentration-Time Graphs & Half-Life

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

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

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

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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.