AQA GCSE Chemistry Chemistry Paper 1 (Foundation), June 2022: Question 10

✅ Model 6-Mark Method

1. Measure a set volume (e.g. 25 cm³) of hydrochloric acid using a measuring cylinder.
2. Pour the acid into a polystyrene cup placed inside a glass beaker for stability.
3. Measure and record the initial temperature of the acid using a thermometer.
4. Weigh an exact mass (e.g. 1.0 g) of sodium carbonate powder using a digital balance.
5. Add the powder to the acid, place a lid on the cup, and stir the mixture continuously.
6. Record the highest temperature reached on the thermometer.
7. Repeat the experiment using different known masses of sodium carbonate (e.g. 2.0 g, 3.0 g, 4.0 g, 5.0 g).
8. Keep key control variables constant: initial starting temperature, volume of acid, and acid concentration.

🧠 Exam Technique: Level 3 Criteria

This is a 6-mark banded response question. To achieve Level 3 (5–6 marks), your method must be:

• Valid: Leads to the required outcome (investigates mass vs highest temperature).
• Logically sequenced: Step-by-step from start to finish.
• Specific equipment named: Mention a measuring cylinder (not "pot"), a balance (not "scales"), and a thermometer.
• Control variables stated: Acid volume and concentration must stay constant so it is a fair test.

💡 Key Knowledge: Why a Polystyrene Cup?

Polystyrene is a thermal insulator. It reduces the rate of energy transfer to the surroundings, ensuring measured maximum temperatures are closer to true theoretical values.

❌ Common Errors

• Forgetting to state that the acid's initial temperature must be measured before adding powder.
• Omitting the word highest (writing "measure final temperature" instead of "highest temperature reached").
• Vague equipment names: writing "measure 25 ml of acid" without stating with a measuring cylinder.

✅ Final Answers

Gradient: 1.6

Unit: °C/g

❌ Common Errors

• Misreading the y-axis: counting 1 small square as 0.1 °C instead of 0.2 °C.
• Using points that are too close together or not actually on the line.
• Writing inverted units (e.g. g/°C instead of °C/g).

✅ Correct Methods & Answers

Method 1 (Graphical extrapolation):

Place a ruler on the straight line in Figure 17 and extend it backwards to cross the vertical axis at mass = 0.0 g. The line crosses at 3 small squares above 20.0 °C:

Initial temperature = 20.6 °C

Method 2 (Algebraic):

y = mx + c → c = y - mx

c = 22.2 - (1.6 × 1.0) = 20.6 °C

🧠 Exam Technique: "Show your working on Figure 17"

Always draw working lines on the graph with a sharp pencil and a long clear ruler.

✅ Correct Answer

Box C is correct.

💡 Understanding Limiting Reactants & Plateau

Initially, hydrochloric acid is in excess and sodium carbonate is the limiting reactant. Adding more powder releases more thermal energy, causing the maximum temperature to rise steadily (upward slope).

Once all the acid has reacted, sodium carbonate is now in excess and hydrochloric acid is limiting. No further reaction occurs, so no additional heat is released. The temperature reaches a plateau (horizontal flat line).

✅ 10.5 Labels X and Y (2 marks)

• X: Energy (the vertical axis label)
• Y: Overall energy change (or ΔH)

✅ 10.6 Explaining Exothermic from the Profile (1 mark)

The profile shows the reaction is exothermic because:

The products are at a lower energy level than the reactants

(Accept: Energy has been released/transferred to the surroundings / overall energy decreases).

💡 Reaction Profile Features

• Reactants to Peak: Activation energy (Eₐ).
• Reactants to Products (Arrow Y): Overall energy change (ΔH). For exothermic reactions, this arrow points downwards.
• Products below Reactants: Energy is given out to the surroundings as heat, which increases the thermometer reading.

❌ Common Misconceptions

• Confusing Y with Activation Energy: Activation energy is measured from reactants up to the highest point (hump), not down to products.
• In 10.6: Giving bond-breaking/bond-forming explanations. The question specifically says "Use Figure 18", so you must refer to the relative levels of reactants and products on the diagram!