AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Higher), 2019: Question 5

✅ Correct answer

Mg + 2H⁺ → Mg²⁺ + H₂

This is the only option that shows magnesium reacting with hydrogen ions to make magnesium ions and hydrogen gas.

💡 Key knowledge

• Hydrochloric acid provides H⁺ ions.
• Magnesium is oxidised to Mg²⁺ .
• Hydrogen ions are reduced to H₂ .
• Only the ions that change are shown in an ionic equation.

🧠 Exam technique

• Check the question asks for an ionic equation, not a full symbol equation.
• Use ion charges carefully: magnesium becomes Mg²⁺ , so you need 2H⁺ to balance.

❌ Common errors

• Writing Mg + 2HCl → MgCl₂ + H₂ — that is a full equation, not ionic.
• Using chloride ions in the ionic equation when they are spectators.
• Forgetting the charges on ions.

✅ Correct answer

Electron transfer

💡 Key knowledge

• Magnesium atoms lose electrons.
• Hydrogen ions gain electrons.
• This is a redox reaction.

🧠 Exam technique

For AQA, if the options are electron sharing / electron transfer / proton transfer, the correct choice here is electron transfer.

❌ Common errors

• Choosing electron sharing — that is for covalent bonding.
• Choosing proton transfer — not the key idea being tested here.

✅ Correct answer

Plot these points accurately:

• (0, 0.0)
• (10, 12.5)
• (35, 36.0)
• (50, 43.5)
• (95, 59.0)
• (120, 60.0)
• (140, 60.0)

Then draw a smooth curve that rises steeply at first and then levels off at 60 cm³.

💡 Key knowledge

• The volume increases quickly at the start.
• The graph then levels off because the reaction finishes.
• A line of best fit should follow the trend, not join dot-to-dot with sharp corners.

🧠 Exam technique

• Use the correct axes: time on the x-axis, volume of gas on the y-axis.
• Plot carefully — AQA allows only a small tolerance.
• For full marks, you need all points correctly plotted and a sensible line of best fit.

❌ Common errors

• Mixing up the axes.
• Plotting the points too far off the grid square.
• Joining points with straight lines instead of drawing a smooth curve.
• Forgetting the plateau at 60 cm³.

✅ Correct answer

The rate:

1. decreases as time goes on,
2. decreases more slowly as the reaction continues,
3. and eventually becomes zero when the reaction stops.

💡 Key knowledge

• Rate is shown by the slope of the graph.
• Steeper line = faster rate.
• Flatter line = slower rate.
• Horizontal line = no change in volume, so rate is zero.

🧠 Exam technique

• Use the graph to describe the trend in words.
• It is good to mention fastest at the beginning.
• You can also say the reaction stops when the graph levels off.

❌ Common errors

• Saying the rate increases — this is the opposite of the graph.
• Only describing the final point and missing the change over time.
• Writing about gas volume, not rate.

✅ Correct answer

As the reaction progresses, there are fewer reactant particles in the mixture, so there are less frequent collisions. Eventually a reactant is used up, so the reaction stops.

💡 Key knowledge

• Collision theory: particles must collide to react.
• Fewer particles means fewer collisions per second.
• The concentration of acid decreases during the reaction.
• When the limiting reactant is used up, the reaction ends.

🧠 Exam technique

• Best answers link cause → effect: fewer particles → fewer collisions → lower rate.
• Use the phrase limiting reactant if you can.
• Do not mention energy unless the question asks about it.

❌ Common errors

• Saying the rate decreases because particles have less energy — this does not score here.
• Saying “less collisions” without explaining why.
• Not stating that one reactant gets used up, so the reaction stops.

💡 What distinguished top responses

• Used the exact ionic equation with correct charges.
• Identified electron transfer correctly.
• Plotted every point accurately and drew a smooth curve.
• Described the rate change in the correct order.
• Explained the change using collision theory and a limiting reactant.

🧠 Quick memory check

• Ionic equation: Mg + 2H⁺ → Mg²⁺ + H₂
• Reaction type: electron transfer
• Graph shape: steep rise, then plateau
• Why rate falls: fewer particles, fewer collisions, reactant used up