AQA GCSE Chemistry Chemistry Paper 2 (Foundation), June 2022: Question 6

✅ Correct Answer

The missing coefficient is 2.

Na₂S₂O₃ + 2HCl → 2NaCl + H₂O + SO₂ + S

💡 Key Knowledge

• Conservation of mass: The number of atoms of each element must be equal on both sides.
• Reactants side has 2 Na atoms (in Na₂S₂O₃) and 2 Cl atoms (in 2HCl).
• Therefore, 2NaCl is needed to balance both sodium and chlorine atoms.

✅ Correct Answer

Tick: (s)

💡 Key Knowledge

• (s) = solid
• (l) = liquid
• (g) = gas
• (aq) = aqueous (dissolved in water)
• Sulfur precipitates out of solution as an insoluble solid (s), causing the mixture to turn cloudy.

❌ Common Errors

Confusing (aq) with (s). While the mixture is in water, the sulfur itself is insoluble and suspended as fine solid particles.

✅ Correct Answer

Tick: A cross on a piece of paper

🧠 Exam Technique

• Recall the standard laboratory method for RPA 5: the reaction flask is placed over a black cross drawn on white paper.
• The timer is stopped when the cross is no longer visible from above due to the solid sulfur precipitate.
• A light sensor achieves the exact same goal, but objectively measures light transmission instead of relying on the human eye.

❌ Common Errors

Selecting gas syringe or balance. Although SO₂ is produced, it dissolves readily in water and is not typically collected with a syringe in this practical.

✅ Correct Answer

Tick: The rate of reaction decreased.

💡 Key Knowledge

• On a rate graph, the gradient (steepness) represents the rate of reaction.
• Between 40 s and 60 s, the curve becomes less steep (flatter).
• A decreasing gradient means the reaction rate is slowing down.

❌ Common Errors

Thinking the rate is zero. The curve does not become completely flat until around 80 seconds. Between 40 s and 60 s, the light percentage is still dropping, meaning reaction is still taking place.

✅ Correct Answer

Tick: One of the reactants was used up.

💡 Key Knowledge

• When the graph levels off completely (horizontal line), the reaction has finished.
• Reactions stop because the limiting reactant has been completely consumed.
• No more sulfur precipitate forms, so the percentage of transmitted light remains constant.

❌ Common Errors

Choosing "No light was reaching the sensor". Look closely at Figure 8: the line flattens at approximately 24% light, NOT at 0%.

✅ Correct Answer

Tick: Initially the line of best fit would be steeper.

💡 Key Knowledge

• Higher concentration = more reactant particles per unit volume.
• Particles collide more frequently, leading to a faster initial rate.
• A faster rate always corresponds to a steeper initial gradient on the graph.

🧠 Exam Technique

Whenever an exam question asks about increasing concentration, surface area, temperature, or adding a catalyst, the initial slope of the curve is always steeper.

✅ Correct Answers (Choose Two)

• ☑️ Fewer reactant particles have the activation energy.
• ☑️ The reactant particles move more slowly.

💡 Key Knowledge: Collision Theory

• An ice bath lowers the temperature of the reactants.
• Kinetic energy decreases: Particles move more slowly and have less energy.
• Fewer successful collisions: Fewer particles have energy equal to or greater than the activation energy (Eₐ).
• Collisions also occur less frequently.