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2.6.2Unit 2. Energetics, Group Chemistry, Halogenoalkanes and Alcohols

Rates of Reaction — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 5 parts Lesson 38 of 53 Unit quiz

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Part 1 of 5Defining and Calculating Rate

2.6.2 Rates of Reaction

Rate is the change in the amount of a reactant or product per unit time.

1
Rate = change in concentration ÷ time taken, so the units are mol dm⁻³ s⁻¹.
2
Reactant concentration decreases with time, while product concentration increases with time.
3
The rate is largest at the start, when reactant concentration is highest, and falls as the reaction proceeds.
4
For a curved graph, draw a tangent at the chosen time and calculate its gradient.
5
The gradient of the tangent, Δy ÷ Δx, equals the rate at that moment, such as the initial rate at time zero.

Part 2 of 5Methods of Following a Reaction

Gas volume
  • Collect a gas in a gas syringe or an upturned measuring cylinder, then plot volume against time.
Mass loss
  • If a gas escapes, measure the fall in mass on a balance at intervals, which suits dense gases such as CO₂.
Colour change
  • Time how long a coloured solution takes to fade or appear, or use a colorimeter for a continuous record.
Precipitate
  • Time how long it takes for a cross seen through the mixture to disappear as a precipitate forms.
Sampling and quenching
  • Remove samples at intervals, stop the reaction suddenly, and titrate to find the concentration.

Part 3 of 5Choosing and Evaluating a Method

Choosing and Evaluating a Method
Match to the reaction
  • Pick a measurable property that changes, such as gas volume, mass or colour intensity.
Hydrogen gas
  • Light gases like hydrogen are measured by volume because the mass lost is too small for a balance.
Control variables
  • Keep temperature, volumes and concentrations constant so that only one factor is changed.
Repeats
  • Repeat the experiment and use averages to reduce the effect of random errors.

Part 4 of 5Worked Example

Question

  • The concentration of a reactant falls from 0.200 to 0.150 mol dm⁻³ in 25 s.
  • Find the mean rate.
  • In another experiment a tangent to a volume-time graph rises by 36 cm³ over 24 s.
  • Find the rate.

  1. Change in concentration = 0.200 - 0.150 = 0.050 mol dm⁻³.
  2. Mean rate = 0.050 ÷ 25 = 2.0 × 10⁻³ mol dm⁻³ s⁻¹.
  3. Gradient of the tangent = 36 ÷ 24 = 1.5 cm³ s⁻¹.

Answer

  • Mean rate = 2.0 × 10⁻³ mol dm⁻³ s⁻¹;
    • rate from the tangent = 1.5 cm³ s⁻¹.

Part 5 of 5Exam Checklist & Tips

2.6.2 – Key Points for the Exam

  • Rate of reaction The change in concentration of a reactant or product per unit time.
  • Tangent A straight line touching a curve at one point, used to find the gradient there.
  • Initial rate The rate at the start of the reaction, found from the gradient at time zero.
  • Quenching Stopping a reaction suddenly so that a sample can be analysed.

Common Mistakes

  • Using the gradient of a chord instead of a tangent to find a rate at one point.
  • Forgetting that the units of rate include time, such as mol dm⁻³ s⁻¹.
  • Reading values from the axes with the wrong scale when working out Δy and Δx.
  • Measuring the mass loss of a very light gas such as hydrogen on a low-precision balance.

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