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First teaching 2014

Last exams 2024

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Rate of Reaction (DP IB Chemistry: SL)

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Rate of Reaction

Reaction rate

  • Some reactions take place instantly, but most are much slower and it is possible to measure how long these reactions take to reach a certain stage
  • As a chemical reaction proceeds, the concentration of the reactants decreases and the concentration of the products increases
  • The rate of a reaction is the speed at which a chemical reaction takes place and has units mol dm-3 s-1
  • The rate of a reaction can be calculated by:

  • Graphically we can represent the rate of reaction as:

Rate of reaction graphs, downloadable IB Chemistry revision notes

Rate of reaction graphs

Worked example

Iodine and methanoic acid react in aqueous solution.

I2 (aq) + HCOOH (aq) → 2I (aq) + 2H+ (aq) + CO2 (g)

The rate of reaction can be found by measuring the volume of carbon dioxide produced per unit time and plotting a graph as shownRate of reaction - Worked Example Question, downloadable IB Chemistry revision notesCalculate the rate of reaction at 20 seconds

Answer:

    • Draw a tangent to the curve at 20 seconds:

Rate of reaction - Worked Example Answer, downloadable IB Chemistry revision notes

    • Complete the triangle and read off the values of x and y
    • Determine the gradient of the line using ∆y / ∆x
    • Rate of reaction = 24 ÷ 40 = 0.60 cm3 s-1

Examiner Tip

When drawing the tangent to a curve make the triangle large and try to intersect with gridlines if you can. This minimises errors of precision and reduces the chance you will accidently misread the graph values

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Stewart

Author: Stewart

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Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Exam Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.