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Topic 3.1.5: Kinetics - Exam Practice

AQA A-Level Chemistry • Physical Chemistry • Exam Practice

Exam Practice

Topic 3.1.5: Kinetics Exam Practice

Test your understanding of collision theory, Maxwell-Boltzmann distributions, factors affecting reaction rates, and catalysis.

Kinetics › Exam Practice

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📋 Structured Questions

Complete each question on paper, then check your answers against the mark scheme.

Question 1: Temperature and Energy Distributions

8 marks

(a) Describe how a Maxwell-Boltzmann distribution curve changes when the temperature is increased. Explain your answer with reference to the peak of the curve and the area under the curve. [4]

(b) With reference to collision theory and the Maxwell-Boltzmann distribution, explain why the rate of reaction increases when the temperature is increased. [4]

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(a)

  • The peak of the curve shifts to the right [1]
  • and shifts downwards. [1]
  • The area under the curve remains the same (since the total number of molecules is constant). [1]
  • The curve is flatter, and the tail of the curve is higher at high energy. [1]

(b)

  • As temperature increases, particles gain kinetic energy and move faster, which increases the collision frequency. [1]
  • Crucially, the average energy of the particles increases, so a much larger fraction of molecules now possess energy greater than or equal to the activation energy (Ea). [2]
  • This leads to a significantly higher frequency of successful collisions, which increases the rate of reaction. [1]
Examiner tip: In part (b), there are two factors: collision frequency and successful collision frequency. A common mistake is focusing purely on particles moving faster. The key reason for the rate increase is that a much larger proportion of the collisions have energy greater than Ea, resulting in a higher frequency of successful collisions.

Question 2: Catalysis

5 marks

(a) Define the term catalyst. [1]

(b) Explain how a catalyst increases the rate of a chemical reaction. [2]

(c) Explain, in terms of the Maxwell-Boltzmann distribution, how the addition of a catalyst affects the fraction of molecules that can react successfully. [2]

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(a)

  • A substance that increases the rate of a chemical reaction without being used up in the process (chemically unchanged at the end). [1]

(b)

  • It provides an alternative reaction pathway [1]
  • which has a lower activation energy (Ea) than the uncatalysed reaction. [1]

(c)

  • On the Maxwell-Boltzmann diagram, the activation energy shifts to the left (Ea catalysed). [1]
  • Therefore, a much larger fraction of molecules now possess energy greater than or equal to this new, lower activation energy, increasing the frequency of successful collisions. [1]
Examiner tip: Avoid saying a catalyst "lowers the activation energy of the reaction". Technically, it provides an alternative route which has a lower activation energy. The original pathway still exists but is bypassed. AQA is very strict on this distinction.

Question 3: Practical Measurement of Rates

4 marks

(a) Suggest a suitable experimental method to measure the rate of the reaction between a strip of magnesium ribbon and excess dilute hydrochloric acid: Mg(s) + 2HCl(aq) -> MgCl2(aq) + H2(g). [2]

(b) Explain, in terms of collision theory, why the rate of this reaction decreases over time. [2]

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(a)

  • Measure the volume of hydrogen gas collected over time [1] using a gas syringe / gas collection tube. [1]
    Alternative: Place the reaction flask on a balance and record the decrease in mass over time as hydrogen gas escapes. [2]

(b)

  • As the reaction proceeds, the concentration of hydrochloric acid (HCl) decreases because the reactant is being used up. [1]
  • This means there are fewer acid particles per unit volume, which decreases the frequency of collisions between H+ ions and the magnesium surface. [1]
Examiner tip: In part (b), always link concentration to "particles per unit volume". Just saying "fewer particles" is not sufficient. You must explicitly state that the collision frequency decreases, causing the rate to fall.

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