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Rate of Reaction (DP IB Chemistry): Revision Note

Caroline Carroll

Written by: Caroline Carroll

Reviewed by: Philippa Platt

Updated on

DP Chemistry: HLDP Chemistry: SL

Rate of reaction

What is rate of reaction?

  • Some chemical reactions happen instantly, but most occur more slowly

    • It's possible to measure how long a reaction takes to reach a certain stage

  • As a reaction progresses, the concentration of reactants decreases while the concentration of products increases

    • Note: Reversible reactions behave differently, once dynamic equilibrium is reached, the concentrations of reactants and products remain constant

  • The rate of a reaction refers to how quickly a chemical reaction occurs. It is usually defined as the change in concentration of a reactant or product per unit of time.

  • The units of rate of reaction are typically 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 Tips and Tricks

  • 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 time

Calculate 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 Tips and Tricks

  • 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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    Author
    Reviewer
    Caroline Carroll

    Author: Caroline Carroll

    Expertise: Physics & Chemistry Subject 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.

    Philippa Platt

    Reviewer: Philippa Platt

    Expertise: Chemistry Content Creator

    Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener