AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Foundation), November 2020: Question 4

✅ Correct Answer

hydrogen peroxide → water + oxygen

💡 Key Knowledge

• Symbol equation given: 2 H₂O₂ → 2 H₂O + O₂
• H₂O is water.
• O₂ is oxygen gas.
• A "decomposition" reaction is when one compound breaks down into two or more simpler substances.

❌ Common Errors & Examiner Warnings

• Writing formulas: The question specifically asked for a word equation. The mark scheme states ignore H₂O and ignore O₂ — chemical symbols score zero!
• Writing "hydrogen oxide" instead of water (not standard naming).

🧠 Exam Technique

Always double-check whether the question specifies a word equation or a symbol equation. Underline the word "word" in the question to remind yourself not to write symbols.

✅ Correct Selections

• ☑ Concentration of hydrogen peroxide [1 mark]
• ☑ Mass of catalyst [1 mark]

💡 Key Knowledge

• Independent variable: Type of catalyst (the thing you change).
• Dependent variable: Volume of gas collected over time (the thing you measure).
• Control variables: Factors kept constant so only the catalyst affects the rate (e.g. concentration, volume, temperature, mass/surface area of catalyst).

❌ Why Other Options Are Wrong

• Size of gas syringe: A syringe only records volume; changing its capacity does not alter how fast the reaction proceeds.
• Type of catalyst: This is the independent variable being investigated!
• Volume of gas collected: This is the dependent variable being measured.

✅ Correct Answer

Volume = 42 cm³ [1 mark]

🧠 How to Read the Scale

1. Find the major numbered marks: the plunger sits between 40 and 50.
2. Count the smaller subdivisions: there are 5 spaces per 10 cm³, meaning each small graduation represents 2 cm³.
3. The edge of the rubber plunger ring lines up exactly with one tick past 40 → 40 + 2 = 42 cm³ .

❌ Common Misconceptions

Assuming each small mark represents 1 cm³ and incorrectly writing 41 cm³. Always calculate the value of each minor division first: (50 - 40) ÷ 5 = 2 cm³ .

✅ Required Steps

• Plotting (2 marks): Plot all 5 points from Table 3 accurately within ±½ small square:
  • (0.0 min, 0 cm³) — don't forget the origin!
  • (0.5 min, 34 cm³)
  • (1.0 min, 54 cm³)
  • (1.5 min, 68 cm³)
  • (2.0 min, 78 cm³)
  (4 or 3 correct plots awards 1 mark; 5 correct awards 2 marks)
• Line of Best Fit (1 mark): A single, smooth, continuous curve that starts at (0,0), passes smoothly through or evenly balances all plotted points, and levels off at approximately 98 cm³.

🧠 Best Fit Curve Guidance

What the graph should look like:

• A curved line showing a steep initial slope from (0,0) that gradually flattens out to a plateau around 98 cm³ at 4.5–5.0 minutes.
• Do not use a ruler to join dot-to-dot. Chemical reactions produce continuous curves.
• Avoid "feathery" lines or multiple overlapping pencil strokes. Draw in one smooth arc.

❌ Common Errors

• Forgetting to plot the origin (0.0, 0).
• Using blunt pencils causing plots to exceed the ±½ small square tolerance.
• Drawing straight point-to-point lines like a zigzag.

✅ Acceptable Answers (Any One)

• The curve is the steepest (at the start / beginning).
• It has the highest initial gradient.
• It produces the greatest volume of gas in a given/stated time (e.g. highest volume at 1 minute).
• It levels off in the shortest amount of time / finishes quickest.

💡 Key Knowledge

The gradient (slope) of a volume-time graph equals the rate of reaction.

A more effective catalyst provides an alternative pathway with a lower activation energy, speeding up the reaction. Hence, the initial slope will be steeper.

✅ Acceptable Answers (Any One)

• The same volume / concentration of hydrogen peroxide was used each time.
• The same amount / number of moles of reactant was present.
• A catalyst does not change the yield (it only affects the rate, not the amount of product formed).

❌ Examiner Pitfalls

• Vague phrases like "same amount of chemicals" are ignored. You must specify that the reactant (hydrogen peroxide) or its volume/concentration was the same.
• Saying "the catalyst ran out" is chemically incorrect; catalysts are not used up in a chemical reaction!

✅ Correct Selections

• ☑ The particles have more energy. [1 mark]
• ☑ There are more particle collisions per second. [1 mark]

💡 Collision Theory Essentials

When temperature increases:

1. Particles gain kinetic energy → move faster.
2. They collide more frequently (more collisions per unit time / per second).
3. A greater proportion of colliding particles have energy ≥ activation energy, leading to a much higher frequency of successful collisions.

❌ Why Other Options Are False

• "The particles are moving more slowly" — False, heating increases kinetic energy.
• "There are more particles per unit volume" — False, that defines higher concentration, not higher temperature.
• "The concentration of hydrogen peroxide decreases" — This does not explain why heating causes a faster initial rate.

🧠 Exam Tip on Frequency

Always look for time phrases when discussing collision theory: "per second", "per unit time", or "frequency of collisions". Simply stating "more collisions" without a time frame often loses marks in longer written questions!