AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Higher), 2023: Question 6

💡 Key knowledge

• Hydrogen peroxide decomposes: 2H₂O₂ → 2H₂O + O₂
• Oxygen is tested with a glowing splint
• Faster rate = steeper gradient
• Systematic error is caused by equipment that is consistently wrong
• Rate from a curve at one point is found using a tangent

🧠 Exam technique

• Use the exact words from standard tests where possible
• When explaining accuracy, compare the size of the error to the total reading
• For graph questions, always mention gradient or gas produced per unit time
• For tangent questions, show the tangent, triangle values, and calculation

✅ Correct answer

Test: glowing splint

Result: relights

💡 Key knowledge

Hydrogen peroxide breaks down to make oxygen gas. Oxygen makes a glowing splint relight.

❌ Common errors

• Writing lit splint or burning splint – the mark scheme says this is not accepted.
• Saying the splint “pops” – that is the test for hydrogen, not oxygen.

🧠 Exam technique

For gas tests, learn the exact pair: test + result. If you only give one, you only get one mark.

✅ Correct answer

Using 10 cm³ produces a smaller volume of gas (per unit time).

So any error in reading the gas volume is a larger percentage of the total volume.

💡 Key knowledge

If the measured value is small, the same reading uncertainty matters more. That means the result is less accurate.

🧠 Exam technique

This is an accuracy question, not a rate question. Focus on measurement error, not just “less reactant so slower reaction”.

❌ Common errors

• Saying only “less oxygen is produced” without linking it to bigger percentage error.
• Talking about concentration changing – the question says the concentration is the same.

✅ Correct answers

Any one of these gains the mark:

• measuring vessel for hydrogen peroxide or gas is wrongly calibrated
• balance is wrongly calibrated
• timer is slow or fast

💡 Key knowledge

A systematic error is a consistent error that shifts all results in the same direction, often due to faulty calibration.

❌ Common errors

• Giving a random error instead, such as difficulty reading the syringe once.
• Saying “human reaction time” – that is usually random, not systematic.

🧠 Exam technique

If you see systematic error, think equipment calibrated wrongly.

✅ Correct plotting points

The first point is already plotted: (0, 0)

Plot the remaining points from Table 3:

• (30, 22)
• (60, 38)
• (90, 41)
• (120, 54)
• (150, 58)
• (180, 60)

Then draw a sensible line of best fit through the plotted points.

🧠 Exam technique

• Plot points carefully using the correct time and gas volume.
• AQA allows a tolerance of ± ½ a small square.
• The line of best fit should follow the overall trend, not join each point with a ruler unless the data clearly requires that.

💡 Examiner insight

Students often lost marks by misreading scales or plotting time and volume the wrong way round. Strong answers used neat crosses and a sensible best-fit line.

❌ Common errors

• Joining the dots exactly rather than drawing a best-fit line.
• Plotting (90, 14) -type mistakes from reading the graph scale wrongly.
• Forgetting the units are seconds and cm³.

✅ Correct answer

Catalyst: manganese dioxide

Reason: it has the steepest gradient on the graph, so it produces the most gas per unit time.

💡 Key knowledge

On a volume of gas vs time graph, the rate is shown by the slope. Steeper slope = faster reaction.

🧠 Exam technique

The question says use the completed Figure 3, so your reason should mention the graph. “Fastest” alone is not enough for full marks.

❌ Common errors

• Saying manganese dioxide because “it made the most gas” without referring to per unit time.
• Confusing final amount made with rate.

✅ Correct answer

There is the same number of particles per unit volume.

So there is the same frequency of collisions with the catalyst.

💡 Key knowledge

Rate depends on how often reacting particles collide successfully. If concentration stays the same, each cm³ contains the same number of particles.

🧠 Exam technique

This is really a concentration idea. The total volume can change, but if concentration stays the same, the rate at the start stays the same.

❌ Common errors

• Saying “same amount of particles” instead of same number of particles per unit volume.
• Not mentioning collisions at all.

📐 Calculations

1. Draw a tangent to the curve at 20 s.
2. Choose two points far apart on your tangent line.
3. Work out the vertical change: y step in cm³.
4. Work out the horizontal change: x step in s.
5. Calculate rate using:
   rate = y step ÷ x step
6. Give your answer in cm³/s to 3 significant figures.

✅ What scores the marks

• Tangent drawn at 20 s
• Values taken for x step and y step from the tangent
• Correct formula used: rate = y step / x step
• Answer calculated correctly from the student's tangent
• Answer given to 3 significant figures

💡 Key knowledge

Because the graph is curved, the rate is changing. At one moment in time, the rate is the gradient of the tangent, not the gradient between two points on the curve.

❌ Common calculation traps

• Using the curve instead of drawing a tangent.
• Using points that are not actually on the tangent.
• Doing x ÷ y instead of y ÷ x .
• Forgetting the unit cm³/s.
• Not rounding to 3 significant figures.

✅ Quick answers

• 06.1: glowing splint; relights
• 06.2: smaller gas volume, so larger percentage error
• 06.3: e.g. wrongly calibrated balance / measuring vessel / timer fast or slow
• 06.4: plot all points correctly and draw best-fit line
• 06.5: manganese dioxide; steepest gradient / most gas per unit time
• 06.6: same particles per unit volume; same collision frequency
• 06.7: tangent at 20 s, calculate gradient, 3 s.f.

🧠 How to score highly

• Use exact scientific wording for tests and definitions.
• Link graph shape to rate using the word gradient.
• In explanations, give both the scientific idea and the consequence.
• In calculations, show each step so method marks can still be awarded.