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Rate of Reaction (HL IB Chemistry)

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Caroline

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Rate of Reaction

What is rate of reaction?

  • 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
    • Reversible reactions are an exception to this, where the concentration of reactants and products remains constant once dynamic equilibrium has been achieved
  • The rate of a reaction is the speed at which a chemical reaction takes place and can be expressed as the change in concentration of a particular reactant or product per unit time
  • The rate of reaction typically has the units mol dm-3 s-1

How to calculate rate of reaction

  • The rate of reaction formula is:

rate space of space reaction space open parentheses mol space dm to the power of negative 3 end exponent space straight s to the power of negative 1 end exponent close parentheses space equals space fraction numerator change space in space concentration space of space reactants space or space products space open parentheses mol space dm to the power of negative 3 end exponent close parentheses over denominator time space open parentheses straight s close parentheses end fraction

  • Experimental data from reactions can be shown graphically and used to calculate the rate of reaction

Rate of reaction graphs

Graphs to show how the concentration of reactants decreases with time and the concentration of products increases with time

As the reaction proceeds, the concentration of reactants and products change with time

  • The steeper the gradient, the quicker the rate of reaction
  • The rate of reaction at a particular time can be found by calculating the gradient of the curve at that time
  • To find the gradient of a curve, draw the tangent to the curve and calculate the gradient of the tangent by using the equation:

gradient space equals space fraction numerator straight capital delta straight y over denominator straight capital delta straight x end fraction

Examiner Tip

  • The rate of reaction is a positive value
  • The graph of the reactants' concentration over time shows a negative gradient
    • So, the sign of the rate should be altered to be a positive value
    • E.g. gradient = –20.6 mol dm-3 s-1 → rate = 20.6 mol dm-3 s-1 
  • The graph of the products' concentration over time shows a positive gradient
    • So, the sign of the rate will already be a positive value and not need altering 

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 shown:

Graph to show how the volume of gas collected increases with timeCalculate the rate of reaction at 20 seconds.

Answer:

  • Draw a tangent to the curve at 20 seconds:

Graph to show how the tangent of the curve can be used to calculate the rate of the reaction

  • 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, you should:
    • Make the triangle large
    • Try to intersect with gridlines if you can
  • These points should minimise errors of precision and reduce the chance you will accidentally misread the graph values

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    Caroline

    Author: Caroline

    Expertise: Physics Lead

    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 creating high-quality resources to help students achieve their full potential.