AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Foundation), 2021: Question 6
10 marks · Standard Demand difficulty · Short Answer
Analyze an investigation into the reaction between magnesium and hydrochloric acid, including identifying variables, plotting data, interpreting reaction rates, and explaining the effect of temperature on reaction rate using collision theory.
Practise this questionQuestion
Question text
06 A student investigated the reaction between magnesium and
excess hydrochloric acid.
Figure 5 shows the apparatus.
Figure 5
This is the method used.
1. Pour 50 cm3 of hydrochloric acid into a conical flask.
2. Add a piece of magnesium.
3. Insert stopper and delivery tube and start a timer.
4. Collect the gas produced in a gas syringe.
5. Record the volume of gas produced every 20 seconds for 2 minutes.
6. Repeat steps 1 to 5 with higher concentrations of hydrochloric acid.
06.1 Give the independent variable and one control variable in this investigation.
[2 marks]
Independent variable
Control variable
Table 3 shows the results from the first experiment using hydrochloric acid with a
low concentration.
Table 3
Time in seconds 0 20 40 60 80 100 120
Volume of gas in cm3 0 48 72 90 97 98 98
*25* Complete Figure 6.
06.2
You should:
• plot the data from Table 3 (the point 0,0 has been plotted for you)
• draw a line of best fit.
[3 marks]
Figure 6
06.3 How does the rate of this reaction change with time?
Use Table 3.
[1 mark]
Tick ( ) one box.
The rate decreases.
The rate stays the same.
The rate increases.
06.4 The student repeated the experiment using hydrochloric acid with
a higher concentration.
Which statement is correct?
[1 mark]
Tick ( ) one box.
The activation energy for the reaction was higher.
The magnesium reacted more quickly.
The reaction finished at the same time.
The total volume of gas collected was smaller.28
06.5 Temperature also affects the rate of the reaction.
Explain how increasing the temperature affects the rate of the reaction.
You should refer to particles and collisions.
[3 marks]
Mark scheme
Show the mark scheme
AO /
Question Answers Extra information Mark Spec.
R f
06.1 independent 1 AO1
concentration (of hydrochloric 5.6.1
acid) RPA11
control 1
any one from:
• temperature (of hydrochloric
acid)
• volume of (hydrochloric) acid
• length of magnesium
• surface area of magnesium
allow same mass of magnesium
allow same form of magnesium
ignore amount
06.2 all points correctly plotted allow a tolerance of ± ½ a small 2 AO2
square 5.6.1
allow 1 mark for 4 or 5 points RPA11
correctly plotted
line of best fit must include 0,0 1
06.3 the rate decreases 1 AO3
5.6.1
RPA11
06.4 the magnesium reacted more 1 AO3
quickly
5.6.1.2
06.5 rate increases allow reaction happens faster 1 AO1
5.6.1.2
(because) particles have more allow (because) particles move 1 5.6.1.3
energy faster
allow (because) more particles
have energy greater than the
activation energy
(so) more frequent collisions 1
Total 10
How to answer it
Investigating Rates: Magnesium & Acid
This question assesses your ability to identify variables in a Required Practical (RPA 11), plot and interpret rate graphs, and use Collision Theory to explain how temperature and concentration affect the speed of a chemical reaction.
Part (a) — Question 06.1
Variables in Investigation
✅ Correct Answers
- Independent: Concentration (of hydrochloric acid)
- Control (Any one): Temperature, Volume of acid, Mass/Length/Surface area of magnesium.
💡 Key Knowledge
Independent Variable: The factor you change (the "cause").
Control Variable: Factors kept the same to ensure a fair test.
❌ Common Errors
Avoid using the word "amount" when describing magnesium. Examiners want specific measurable quantities like mass, length, or surface area. Also, "Volume of gas" is the dependent variable, not a control!
Part (b) — Question 06.2
Graphing the Results
🧠 Exam Technique: Drawing the Graph
- Plotting: Use a sharp pencil and mark points with a small 'x'. You are allowed a tolerance of half a small square.
- Line of Best Fit: For rate experiments, this is almost always a smooth curve. It must pass through the origin (0,0) because at 0 seconds, 0 gas has been made.
- The Plateau: Notice the data stays at 98 cm³ from 100s to 120s. Your curve should level off horizontally here.
Parts (c) & (d) — Questions 06.3 & 06.4
Interpreting Rate Data
✅ Correct Answers
06.3: The rate decreases.
06.4: The magnesium reacted more quickly.
💡 Why does the rate decrease?
As the reaction progresses, reactant particles (acid and magnesium) are used up. There are fewer particles available to collide, so the reaction slows down until it stops.
Part (e) — Question 06.5
Temperature & Collision Theory
📐 Step-by-Step Explanation
To get full marks on "Explain" questions about rates, you must follow this logical chain:
- State the effect: The rate of reaction increases.
- Energy: Particles have more (kinetic) energy and move faster.
- Collisions: There are more frequent collisions (or more collisions per second).
❌ The "Frequency" Trap
Never just say "there are more collisions." This will often lose the mark. You must say more frequent collisions or more collisions per unit of time.
💡 Advanced Insight
At higher temperatures, more particles also have energy greater than the activation energy, meaning a higher percentage of collisions result in a reaction.
Summary Equation
Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)
Magnesium + Hydrochloric Acid → Magnesium Chloride + Hydrogen Gas
Topics
Chemistry · C6: The Rate and Extent of Chemical Change
Question and mark scheme from the AQA GCSE Combined Science: Trilogy examination, Chemistry Paper 2 (Foundation), 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.