Kinetic Rates (Edexcel International A Level Chemistry)

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Kinetic Rates - Introduction

  • The rate of reaction refers to the change in the amount or concentration of a reactant or product per unit time
    • The units for rate of reaction are mol dm-3 s-1 
  • It can be found by measuring:
    • The mass lost over time
    • The volume of gas produced over time
    • Colour changes, including by the use of colorimetry
    • pH changes over time
    • Changes in electrical conductivity
  • The rate of reaction can be calculated by:

Rate of reaction = fraction numerator change space in space amout space of space reactants space or space products space left parenthesis mol space dm to the power of negative 3 end exponent right parenthesis over denominator time space left parenthesis straight s right parenthesis end fraction

Rate of Reaction

  • The following general reaction will be used as an example to study the rate of reaction

D (aq) → E (aq) + F (g) 

  • The rate of reaction at different concentrations of D is measured and tabulated

Rate of Reactions Table

Table to show rates [D], downloadable AS & A Level Chemistry revision notes

  • A directly proportional relationship between the rate of reaction and concentration of D is observed when the results are plotted on a graph:

Rates [D] graph, downloadable AS & A Level Chemistry revision notes

Rate of reaction over various concentrations of D

  • This leads to a very common rate expression:

Rate ∝ [D]       or       Rate = k[D]

  • This rate expression means that if the concentration of D is doubled, then the rate doubles
  • Equally, if the concentration of D halves, then the rate halves

Rate Equations

  • The following reaction will be used to discuss rate equations:

A (aq) + B (aq) → C (aq) + D (g) 

  • The rate equation for this reaction is:

Rate of reaction = k [A]m [B]n

  • Rate equations can only be determined experimentally and cannot be found from the stoichiometric equations
  • In the above rate equation:
    • [A] and [B] are the concentrations of the reactants
    • m and n are orders with respect to each reactant involved in the reaction

  • Products and catalysts may feature in rate equations
  • Intermediates do not feature in rate equations

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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.