AQA GCSE Chemistry Chemistry Paper 1 (Foundation), June 2025: Question 2

✅ Correct Answers

• 02.1: Magnesium chloride [1 mark]
• 02.2: Hydrogen [1 mark]

💡 Key Knowledge

Learn the word equation rule:

Metal + Acid → Salt + Hydrogen

• Hydrochloric acid always forms chloride salts.
• Sulfuric acid forms sulfate salts.
• Nitric acid forms nitrate salts.

❌ Common Errors

• Selecting chlorine or oxygen gas instead of hydrogen. Only hydrogen gas (H₂) is released when a metal reacts with an acid.
• Confusing the acid used (e.g. ticking magnesium sulfate when the question clearly states hydrochloric acid).

🧠 Exam Technique

Split the salt name into two parts:

1. First word comes from the metal: Magnesium.
2. Second word comes from the acid: Hydrochloric → chloride.

✅ Correct Answers

• 02.3:
  • Dependent variable → Temperature change [1 mark]
  • Independent variable → Type of metal [1 mark]
• 02.4: A polystyrene cup [1 mark]
• 02.5: 2 °C [1 mark]
• 02.6: Increases [1 mark]

📐 Working Out Resolution (02.5)

1. Find two numbered major divisions: e.g., 40 °C and 20 °C.
2. Calculate the difference: 40 − 20 = 20 °C.
3. Count the number of small tick intervals between them: 10 divisions.
4. Divide total difference by intervals: 20 °C ÷ 10 = 2 °C.

Each single small mark on this scale represents exactly 2 °C.

💡 Equipment & Exothermic Definitions

• Insulation: Polystyrene is an excellent thermal insulator. It dramatically reduces heat conduction to the surroundings compared to glass or copper.
• Exothermic reaction: Releases heat energy to the surroundings, which causes the temperature of the reaction mixture and surroundings to increase.
• Variables:
  • Independent: What I change (metal type).
  • Dependent: What is measured (temperature change).
  • Control: Kept constant (mass of metal, volume of acid).

❌ Common Errors

• 02.3: Drawing multiple lines from one box on the left. Mark schemes explicitly state: do not accept more than one line from a box on the left.
• 02.4: Choosing a copper can. Copper is a metal and a very good thermal conductor; it would increase energy loss.
• 02.6: Writing that temperature decreases because students confuse "energy lost to surroundings" with a temperature drop. Energy leaving the chemicals means temperature of the solution goes up!

✅ Correct Answers

• 02.7 (Reactants): A [1 mark]
• 02.8 (Activation energy): C [1 mark]
• 02.9 (Overall energy change): B [1 mark]

💡 Reading Reaction Profiles

• A (Horizontal line on left): Energy level of Reactants.
• D (Horizontal line on right): Energy level of Products.
• C (Arrow from reactants to peak): Activation energy (Eₐ) — the minimum energy colliding particles need to react.
• B (Vertical difference between reactants and products): Overall energy change (ΔH). Because products are lower than reactants, energy has been released (exothermic).
• E: Total height from product level to peak.

🧠 Memory Aid for Reaction Profiles

Always measure vertical arrows starting from the reactant line (A):

• From Reactants UP to Peak = Activation Energy (C).
• From Reactants DOWN to Products = Overall Energy Change (B).

❌ Common Errors

• Confusing C and E: Activation energy must start at the reactants' energy level (A), never from the products (D).
• Confusing letters A and D: Reactants are on the left (reaction start); products are on the right (reaction finish).