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4.1.4Physical Chemistry

Factors Affecting the Rate of a Reaction — AQA AS Chemistry Revision Notes & Topical Questions

5 min read 6 parts Lesson 98 of 102

📚 AQA AS Chemistry Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 5Measuring Rate

Measuring Rate
Definition
  • Definition Rate is the change in concentration of a reactant or product divided by the time taken, in mol dm⁻³ s⁻¹.
Changing rate
  • Changing rate The rate falls as reactants are used up, so it is not constant during a reaction.
Tangent
  • Tangent The rate at a particular time is the gradient of a tangent drawn to a concentration–time graph at that time.
Proportional measure
  • Proportional measure For a clock reaction, 1/t is proportional to the average rate, where t is the time to reach a fixed point.

Part 2 of 5Variables and Safety

Independent variable
  • Independent variable The factor you deliberately change, such as the temperature or the concentration of one reactant.
Dependent variable
  • Dependent variable The time taken for the cross to disappear, which is measured.
Control variables
  • Control variables Volumes, concentrations of the other reagent, depth of solution and the same observer must be kept constant.
Hazard
  • Hazard Sulfur dioxide is toxic and an irritant, so work in a well-ventilated room or fume cupboard.
Why a solid
  • Why a solid The method needs a product that forms an insoluble solid which clouds the solution.

Part 3 of 5Analysing the Results

Analysing the Results
Temperature
  • Temperature A graph of 1/t against temperature rises steeply, showing that rate increases quickly with temperature.
Concentration
  • Concentration A graph of 1/t against concentration shows how rate depends on the amount of the reactant varied.
Reliability
  • Reliability Repeating each run and comparing results helps to spot anomalies.
Limitations
  • Limitations Judging when the cross disappears is subjective, so the same person should always do it.

Part 4 of 5Worked Example

Question

  • A tangent to a concentration–time graph for a reactant passes through (0 s, 0.80 mol dm⁻³) and (40 s, 0.20 mol dm⁻³).
  • Calculate the rate at that point.
  • Also, find 1/t for runs where the cross disappears after 40 s and after 20 s.

  1. Gradient = change in concentration ÷ change in time = (0.20 − 0.80) ÷ (40 − 0) = −0.015 mol dm⁻³ s⁻¹.
  2. The rate is the size of the gradient, so rate = 0.015 mol dm⁻³ s⁻¹.
  3. For t = 40 s, 1/t = 1 ÷ 40 = 0.025 s⁻¹.
  4. For t = 20 s, 1/t = 1 ÷ 20 = 0.050 s⁻¹, so the second reaction is twice as fast.

Answer

  • The rate is 0.015 mol dm⁻³ s⁻¹;
    • 1/t is 0.025 s⁻¹ and 0.050 s⁻¹ respectively.

Part 5 of 5Exam Checklist & Tips

4.1.4 – 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, whose gradient gives the rate at that point.
  • Independent variable The variable that the experimenter changes.
  • Control variable A variable kept constant so that a fair comparison can be made.

Common Mistakes

  • Taking the gradient of the whole curve rather than a tangent at the chosen point.
  • Plotting the time itself instead of 1/t as the measure of rate.
  • Changing more than one variable at once, such as using different volumes of acid.
  • Ignoring the safety hazard of sulfur dioxide.

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