AQA GCSE Chemistry Chemistry Paper 2 (Higher), 2023: Question 9

📐 Step-by-Step Calculation

✅ Correct Answer

4.23 g

Note: The mark scheme also accepts 4.2 g or 4.225 g .

🧠 Exam Technique: Error Carried Forward (ECF)

If you make an arithmetic error in Step 1 (e.g., writing 4.68 - 2.99 = 1.79 g), you can still get the remaining 2 marks if you use your incorrect number correctly in the next steps. Always show your working clearly!

❌ Common Errors

Many students lose marks by trying to use complex molar mass calculations (Ar and Mr) from the periodic table. Because the formula of hydrated copper sulfate isn't fully specified here, you must use the simple experimental mass ratios provided in the question.

📐 Step-by-Step Calculation

✅ Correct Answer

7.37 kJ

Note: The mark scheme accepts any correctly rounded value from 7.37458... to at least 2 significant figures (e.g. 7.4 or 7.37 ).

💡 Key Knowledge: Reversible Energy Changes

If a reversible reaction is endothermic in the forward direction (takes in energy), it must be exothermic in the reverse direction (releases energy to the surroundings) by the exact same amount.

❌ Common Calculation Trap

Do not use the hydrated mass ( 4.68 g ) in your ratio calculation here. The question asks about 15.0 g of anhydrous copper sulfate, so you must pair it with the anhydrous mass from the prompt ( 2.99 g ).

✅ Correct Answer

☑ Shifts to the left

💡 Le Chatelier's Principle: Temperature

• An increase in temperature always shifts the equilibrium in the endothermic direction (to absorb the extra heat).
• Since the forward reaction is exothermic, the reverse reaction (left) is endothermic.

✅ Correct Answer

☑ The mixture is a lighter shade of brown.

🧠 Exam Technique: Step-by-Step Logic

1. Count gas moles: Left side has 2 moles of gas ( 2 NO₂ ). Right side has 1 mole of gas ( 1 N₂O₄ ).
2. Apply pressure rule: Increasing pressure shifts equilibrium to the side with fewer moles of gas (the right side).
3. Determine colour: Shifting right produces more colourless N₂O₄ , making the brown colour of NO₂ fade to a lighter shade.

✅ Correct Answer

No effect on the equilibrium position 1 Mark

Because there are equal numbers of gas moles/molecules on each side of the equation 1 Mark

❌ Common Errors

Students often write "no effect" but forget to state the reason. To get the second mark, you must explicitly mention that the number of moles of gas is the same on both sides (2 moles on the left vs 2 moles on the right).

✅ Correct Answer

No effect (equilibrium position stays the same)

💡 Key Knowledge: Catalysts & Equilibrium

A catalyst increases the rate of both the forward and reverse reactions equally. It helps the system reach equilibrium faster, but it does not change the final position of equilibrium.

✅ Correct Answer

☑ Add more hydrochloric acid

💡 Le Chatelier's Principle: Concentration

To make the mixture more yellow, we need to shift the equilibrium to the right (towards the yellow product). Adding more of a reactant (hydrochloric acid) forces the system to shift right to use it up.

✅ Correct Answer

Because the forward and reverse reactions are taking place at the exact same rate.

❌ Common Misconception

Do not say "the concentrations are equal". At equilibrium, the concentrations of reactants and products are constant (they don't change), but they are rarely equal to each other.