AQA GCSE Chemistry Chemistry Paper 2 (Foundation), June 2024: Question 5

📐 Step-by-Step Volume Check

1. Volume of sodium thiosulfate = 75 cm³
2. Volume of hydrochloric acid = 15 cm³
3. Total volume of reactants = 75 + 15 = 90 cm³

✅ Mark Scheme Answers

• Mark 1: The total volume of reactants is 90 cm³ (or greater than 50 cm³).
• Mark 2: The flask will overflow (or the liquids will not fit in the flask).

🧠 Exam Technique

Always inspect the method steps for numbers! Whenever an exam question asks why a container is "unsuitable," check the total volume of liquids added together before writing an answer.

❌ Common Errors

• Vague answers such as "it is too small" without stating that the solutions will spill/overflow or calculating 90 cm³ .
• Assuming a 50 cm³ flask would make the reaction "too fast" or "hard to see".

✅ Accepted Answers

• Measuring cylinder (most common GCSE answer)
• Burette
• Pipette (volumetric or graduated)

❌ Common Errors

• Beaker: Beakers only have approximate graduation lines and are not accurate enough for quantitative measurement in a rate experiment.
• Flask / Test tube: Not measuring instruments.

✅ Correct Pairings

• Dependent variable → Time for pencil cross to become no longer visible
• Independent variable → Concentration of sodium thiosulfate solution

💡 Key Knowledge

• Independent: What the experimenter intentionally changes (the concentrations listed in Step 7).
• Dependent: What is measured as the outcome (the time recorded on the stopclock).
• Control: What must be kept constant (e.g., total volume, temperature, cross thickness).

✅ Correct Answer

☑ The time taken will increase.

🧠 Examiner Insight

The cross disappears because solid sulfur precipitate forms, making the mixture cloudy (turbid). A darker cross requires a denser precipitate cloud to fully block it from sight, meaning the reaction must run for longer to produce sufficient sulfur.

📐 Data Analysis (Table 2)

• 8 g/dm³ → 120 seconds
• 16 g/dm³ → 60 seconds
• 24 g/dm³ → 40 seconds
• 32 g/dm³ → 30 seconds (Shortest time)

✅ Correct Answer

☑ 32 g/dm³

❌ Common Error: Time vs. Rate

Many students confuse time with rate. A short time means the reaction is fast, so the rate is highest. Selecting 8 g/dm³ is incorrect because taking 120 s represents the slowest reaction.

💡 Golden Rule

Rate = 1 ÷ Time

Shortest time = Highest reaction rate.

✅ Correct Statements (Tick 2)

• ☑ The particles are closer together. [1 mark]
• ☑ The particles collide more frequently. [1 mark]

💡 Why Concentration Increases Rate

Higher concentration means more reactant particles in a given volume. Because they are packed closer together, they collide with each other more often (increased frequency of collisions).

❌ Common Misconception: "Moving Faster"

Particles do NOT move faster when concentration increases! Kinetic energy and particle speed depend entirely on temperature, not concentration.

🧠 Exam Language Tip

Always use the phrase "collide more frequently" or "more collisions per second" rather than just "more collisions".

✅ Completed Sentences

If the temperature of the hydrochloric acid is increased, the time taken for the cross to disappear decreases. [1 mark]

If the concentration of the hydrochloric acid is decreased, the time taken for the cross to disappear increases. [1 mark]

🧠 Two-Step Thinking Strategy

1. Step 1: What happens to the rate?
   Higher temperature → Faster rate.
   Lower concentration → Slower rate.
2. Step 2: What happens to the timer?
   Faster reaction → less time (decreases).
   Slower reaction → more time (increases).