Bromine water decolourises when it reacts with propene. A student investigates how the initial concentration of bromine affects the initial rate of this reaction at constant temperature. When the concentration of bromine is doubled, the initial rate of reaction doubles. Explain what this tells the student about how bromine concentration affects the rate of this reaction, and describe one other factor, besides concentration, that the student would need to keep constant to make the investigation a fair test, explaining why it must be controlled.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Bromine water decolourises when it reacts with propene. A student investigates how the initial concentration of bromine affects the initial rate of this reaction at constant temperature. When the concentration of bromine is doubled, the initial rate of reaction doubles.
Model answer (5 marks)
The rate is directly proportional to the concentration of bromine, i.e. the reaction is first order with respect to bromine.
When the concentration of bromine is doubled, the number of bromine molecules per unit volume doubles, so the collision frequency between bromine and propene molecules also doubles.
Because the rate of successful collisions is proportional to the collision frequency, the initial rate of reaction doubles.
One other factor that must be kept constant is temperature.
Temperature must be controlled because an increase in temperature raises the kinetic energy of the molecules, increasing the proportion of collisions that have energy equal to or greater than the activation energy, which would otherwise increase the rate and confound the effect of bromine concentration.
When the concentration of bromine is doubled, the number of bromine molecules per unit volume doubles, so the collision frequency between bromine and propene molecules also doubles.
Because the rate of successful collisions is proportional to the collision frequency, the initial rate of reaction doubles.
One other factor that must be kept constant is temperature.
Temperature must be controlled because an increase in temperature raises the kinetic energy of the molecules, increasing the proportion of collisions that have energy equal to or greater than the activation energy, which would otherwise increase the rate and confound the effect of bromine concentration.
Examiner tips
- Use the word ‘directly proportional’ and state ‘first order’ to gain full marks for the rate law. Show the logical chain: double concentration → double collision frequency → double rate. Name a control variable (temperature) and give a clear reason why it must be constant.
- Remember to mention the collision theory explanation – students who skip this lose marks.
Mark scheme (5 marks)
- The rate is directly proportional to the concentration of bromine / the reaction is first order with respect to bromine
- When concentration doubles, the number of bromine particles per unit volume doubles / collision frequency between bromine and propene molecules doubles
- This leads to a proportional increase in the rate of successful collisions / rate of reaction
- Named control variable: temperature / concentration of propene / surface area (if propene is a liquid or solid form) — one acceptable named variable
- Correct explanation of why the named variable must be controlled — e.g. increasing temperature increases the kinetic energy of particles / increases the proportion of particles with energy greater than or equal to the activation energy, which would also increase the rate, making results unreliable; OR changing propene concentration would change collision frequency independently of bromine concentration
Key terms in this question
rate of reaction · concentration
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Rate equations (A-Level only) questions
- A student investigates how changing the concentration of hydrochloric acid affec…
- Explain how increasing the temperature of a reaction mixture increases the rate …
- A student carries out experiments to find the order of reaction with respect to …
- Explain how increasing the concentration of sodium thiosulfate solution increase…