AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Foundation), November 2020: Question 7

✅ Model 6-Mark Method

1. Accurate measurement: Use a measuring cylinder (or pipette) to measure 50 cm³ of sodium thiosulfate solution into a conical flask.
2. Separate apparatus: Use a clean, separate measuring cylinder to measure 10 cm³ of hydrochloric acid.
3. Heating: Heat the sodium thiosulfate solution gently using a water bath to a chosen temperature.
4. Place on cross: Place the flask over a black cross marked on white paper.
5. Timing & mixing: Add the acid, swirl once, and immediately start the stopwatch.
6. Temperature monitoring: Place a thermometer into the flask and record the temperature at the start and when the cross is obscured to calculate a mean temperature.
7. Stopping point: Look down through the flask and stop the stopwatch the exact moment the black cross disappears.
8. Range & Controls: Repeat the experiment across at least 4–5 different temperatures, keeping volumes, concentrations, and the cross identical each time.

💡 Flaws in the Student's Method

• Using a beaker for volume: Beakers are approximate; measuring cylinders are significantly more accurate.
• Using the same beaker for both liquids: Reusing the beaker contaminates the acid and triggers premature reaction before mixing.
• Measuring room temperature: They heated the liquid! You must measure the solution's temperature, not the room.
• Starting the timer too early: Step 4 starts the timer before the acid is even measured or added in Step 5.

🧠 Exam Technique: Securing Level 3 (5–6 marks)

• Identify and fix: Top marks require a coherent, logically sequenced step-by-step method that would produce a valid outcome.
• Control variables must be explicit: State "use the same concentration and volume of reactants each time".
• Describe temperature changes: Explicitly mention repeating at different temperatures (e.g., 20 °C, 30 °C, 40 °C, 50 °C).

❌ Common Student Traps

• Writing about safety: The prompt explicitly states: "You do not need to write about safety precautions." Writing about goggles or toxic SO₂ wastes valuable time and scores zero marks.
• Vague temperature recording: Saying "measure temperature with a thermometer" is not enough—you must state measuring the liquid, not the room.

❌ Common Errors in 07.2

• Dividing by 3 instead of 2: The question asks for the mean rate between 1 and 3 minutes. The elapsed time is 2 minutes, not 3!
• Misreading the y-axis scale: Each small square vertically represents 0.01 g. At 1 min the cross is at 18 small squares below 0 (-0.18 g), and at 3 mins it is at 33 small squares (-0.33 g).
• Worrying about negative signs: Rate is a positive scalar quantity representing how fast gas escapes; the mark scheme explicitly states "ignore signs".

🧠 Exam Tip: Error Carried Forward (ECF)

If you misread one graph value (e.g. you wrote 0.34 instead of 0.33), you lose mark 1, but if you correctly calculate (0.34 - 0.18) / 2 = 0.08 , you still earn the remaining 2 marks!

✅ What Your Curve Must Show

• Criterion 1 (1 mark): Starts at (0, 0) , lies to the left of the original curve, and is visibly steeper at all points initially.
• Criterion 2 (1 mark): Levels off horizontally at exactly -0.35 g (the exact same plateau value as the 40 °C curve), reaching this plateau before 4 minutes.

💡 Why Does It Look Like This?

• Why steeper? At 50 °C, particles have more kinetic energy, move faster, and collide more frequently. A greater proportion of collisions have energy ≥ activation energy (Eₐ), leading to more frequent successful collisions → higher initial rate.
• Why same final height (-0.35 g)? The amount of reactant (limiting reactant) has not changed. Therefore, the same total mass of gas will ultimately be produced.