AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Higher), June 2025: Question 5
13 marks · Standard Demand difficulty · Short Answer
Investigate factors affecting the rate of reaction using disappearing cross and gas collection methods, including explaining collision theory, identifying hydrogen gas, and calculating the rate from a tangent at 75 seconds.
Practise this questionQuestion
Question text
05 Students investigated the effect of changing the concentration on the rate of
chemical reactions.
A student reacted sodium thiosulfate solution with hydrochloric acid.
Figure 3 shows the apparatus.
Figure 3
This is the method used.
1. Measure 50 cm3 of sodium thiosulfate solution into a conical flask.
2. Put the conical flask on a black cross drawn on a piece of paper.
3. Add 10 cm3 of hydrochloric acid to the conical flask.
4. Start a timer.
5. Stop the timer when the cross is no longer visible.
6. Record the time taken.
7. Repeat steps 1 to 6 using different concentrations of sodium thiosulfate solution.
05.1 What is the dependent variable in this investigation?
[1 mark]
05.2 The equation for the reaction is:
*18* Na2S2O3(aq) + 2HCl(aq) → 2NaCl(aq) + S(s) + SO2(g) + H2O(l)
The mixture becomes cloudy as the reaction takes place.
Explain why the mixture becomes cloudy.
[2 marks]
05.3 Explain the effect of changing the concentration of the sodium thiosulfate solution on
the rate of reaction.
[3 marks]
A different student reacted magnesium with hydrochloric acid.
Figure 4 shows the apparatus.
Figure 4
This is the method used.
1. Measure 50 cm3 of hydrochloric acid into a conical flask.
2. Add 0.1 g of magnesium ribbon to the conical flask.
3. Insert the stopper into the conical flask as quickly as possible.
4. Start a timer.
5. Record the volume of gas collected every 20 seconds for 5 minutes.
6. Repeat steps 1 to 5 using different concentrations of hydrochloric acid.
05.4 Why is the stopper inserted into the conical flask as quickly as possible in step 3?
[1 mark]
05.5 The equation for the reaction of magnesium with hydrochloric acid is:
Mg + 2 HCl → MgCl2 + H2
Describe a test for the gas produced in the reaction.
Give the result of the test.
[2 marks]
Test
Result
05.6 Figure 5 shows the results for one concentration of hydrochloric acid.
Figure 5
Determine the rate of reaction at 75 seconds.
[4 marks]
Rate = cm3/s
Mark scheme
Show the mark scheme
Question 5
AO /
Question Answers Extra information Mark
Spec. Ref.
05.1 time taken (for the cross to be 1 AO1
no longer visible) 5.6.1.2
RPA11
AO /
Spec. Ref.
05.2 sulfur (is produced) 1 AO2
5.2.2.2
5.6.1.2
(which is a) solid allow (which is) insoluble 1 RPA11
allow (which is) a precipitate
AO /
Spec. Ref.
05.3 allow converse throughout AO1
5.6.1.2
5.6.1.3
increasing the concentration 1
increases the rate of reaction
(because) there are more allow (because) the particles are 1
particles in the same volume (of closer together
solution)
do not accept the particles have
more energy
(so) there are more frequent 1
collisions
AO /
Spec. Ref.
05.4 to reduce loss of gas allow to reduce loss of hydrogen 1 AO3
5.6.1.2
do not accept to stop loss of RPA11
– gas / hydrogenSCIENCE: TRILOGY – –
AO /
Spec. Ref.
18 05.5 (test) AO1
add a burning splint do not accept glowing splint 1 5.8.2.1
(result)
(hydrogen burns with) a pop 1
sound
MP2 is dependent upon MP1
being awarded
AO /
Spec. Ref.
05.6 tangent drawn at 75 s 1 AO2
5.6.1.1
RPA11
value of x-step and y-step from allow evidence of use of two 1
tangent points on tangent either on the
graph or in the text
allow a tolerance of ± ½ a small
square
value for y-step allow correct use of incorrectly 1
(rate=) determined value(s) for x-step
value for x-step
and/or y-step from a drawn
tangent
correct calculation of rate 1
Total Question 5 13
How to answer it
Rates of Reaction: Turbidity & Gas Syringe Methods
What this question tests
This question assesses practical investigation skills and theoretical knowledge from Topic 6: The Rate and Extent of Chemical Change:
- Identifying experimental variables in the disappearing cross practical.
- Interpreting chemical equations and state symbols to explain precipitation.
- Applying collision theory precisely to describe concentration effects.
- Evaluating experimental accuracy when collecting gas volumes.
- Recalling standard qualitative gas tests (hydrogen).
- Calculating the instantaneous rate of reaction by constructing a tangent on a curved graph.
Identifying the Dependent Variable
Sodium thiosulfate and hydrochloric acid practical
✅ Correct Answer
Time taken (for the cross to be no longer visible / to disappear).
🧠 Exam Technique
Always give the exact quantity measured. Writing just "the cross" or "turbidity" gains zero marks because neither is a measurable quantity. Writing simply "time" is often insufficient—state what event the time measures.
Explaining Turbidity (Precipitate Formation)
Interpreting the chemical equation: Na₂S₂O₃(aq) + 2HCl(aq) → 2NaCl(aq) + S(s) + SO₂(g) + H₂O(l)
✅ Correct Answer
- Sulfur is produced [1 mark]
- which is a solid (or insoluble / forms a precipitate) [1 mark]
💡 Key Knowledge
Look carefully at state symbols in the equation:
- NaCl(aq) is soluble salt solution.
- SO₂(g) is a toxic, soluble gas.
- S(s) is solid sulfur, which stays suspended as tiny particles that scatter light, turning the solution opaque yellow/cloudy.
❌ Common Errors
- Naming sulfur dioxide as the solid (it is a gas!).
- Saying "a precipitate is formed" without identifying that it is sulfur.
Explaining the Effect of Concentration on Rate
Collision theory explanation (3-mark structure)
✅ Correct Answer (Full 3 Marks)
- Increasing concentration increases the rate of reaction. [1 mark]
- There are more particles in the same volume (or particles are closer together). [1 mark]
- Therefore, there are more frequent collisions (or a higher collision frequency). [1 mark]
❌ Collision Theory Traps
- The Energy Trap: Stating "particles have more energy" or "particles move faster". That is only true for temperature! Concentration does NOT change kinetic energy.
- The "Time" Omission: Writing "there are more collisions". You must include a time reference: "more frequent collisions" or "more collisions per second".
Practical Technique: Gas Loss Minimisation
Magnesium ribbon reacting with hydrochloric acid
✅ Correct Answer
To reduce loss of gas (or reduce loss of hydrogen).
❌ Common Errors
Do NOT write: "To stop gas escaping" or "to prevent any gas escaping".
Examiners strictly reject "stop" because the reaction starts instantaneously; some tiny amount of gas inevitably escapes before the bung seals the flask. You can only reduce or minimise the loss.
Chemical Test for Hydrogen Gas
Equation: Mg + 2HCl → MgCl₂ + H₂
✅ Correct Answer
- Test: Place a burning (lit) splint near the mouth of the test tube. [1 mark]
- Result: Burns with a squeaky pop sound. [1 mark]
❌ Splint Confusion
- Lit splint: Used for Hydrogen (gives squeaky pop).
- Glowing splint: Used for Oxygen (relights).
- Note: The second mark is strictly dependent on getting the test correct. If you write "glowing splint", you get 0/2.
Determining Rate from a Graph: Tangent Method
Determine the rate of reaction at t = 75 seconds
📐 Step-by-Step Calculation Guide
- Step 1: Locate 75 s on the x-axis and plot your point.
Find 75 s on the x-axis, follow it up to the curve (at 75 s, volume collected is approximately 58 cm³). - Step 2: Draw a tangent line [1 mark] .
Place a ruler so it touches the curve only at (75 s, 58 cm³) with equal space between ruler and curve on both sides. Extend the straight line across the grid so it intersects easily readable grid lines (e.g. from x = 0 to x = 150). - Step 3: Read coordinates to determine Δy and Δx [1 mark] .
Choose two points far apart on your tangent line, for example:- Point 1: (0 s, 20 cm³)
- Point 2: (140 s, 90 cm³)
- Δy = 90 − 20 = 70 cm³
- Δx = 140 − 0 = 140 s
- Step 4: Calculate the gradient (Rate) [2 marks] .
$\text{Rate} = \frac{\Delta y}{\Delta x} = \frac{70\text{ cm}³}{140\text{ s}} = 0.50\text{ cm}³\text{/s}$
🧠 Tangent Checklist
- Draw a single, thin, continuous straight pencil line. Do NOT sketch or double-line.
- Make your Δx triangle large (at least half the width of the graph) to minimise percentage reading error.
- Always check the grid scale: here, 10 small squares = 50 s on the x-axis (each small square = 5 s), and 10 small squares = 10 cm³ on the y-axis (each small square = 1 cm³).
✅ Expected Final Answer Range
Depending on minor variations in your hand-drawn tangent:
Rate ≈ 0.48 to 0.54 cm³/s
Units are already provided on the answer line: cm³/s .
• 1 mark: Tangent drawn at 75 s.
• 1 mark: Value of x-step and y-step determined from tangent (tolerance ± ½ small square).
• 1 mark: Rate = (y-step) ÷ (x-step).
• 1 mark: Correct numerical answer evaluated from candidate's readings.
Topics
Chemistry · C6: The Rate and Extent of Chemical Change · C8: Chemical Analysis
Question and mark scheme from the AQA GCSE Combined Science: Trilogy examination, Chemistry Paper 2 (Higher), June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.