Edexcel A-Level Chemistry Paper 1, June 2022: Question 2

✅ Correct Answers (Any 2 for 2 marks)

• Universal Indicator changes colour from green to blue or purple.
• The water level in the test tube drops (or gas collects at the top of the test tube).

💡 Key Knowledge

Magnesium reacts slowly with cold water to form sparingly soluble magnesium hydroxide ( Mg(OH)₂ ) and hydrogen gas ( H₂ ). The alkalinity of the resulting solution turns the indicator blue/purple.

❌ Common Errors

• Stating the indicator turns "dark blue" from an incorrect starting colour (it starts green in neutral water).
• Describing the solution as "acidic" or claiming H⁺ ions increase.
• Vague observations like "magnesium disappears" or "bubbles form" without specifying where the gas collects or the water level change.

✅ Correct Answer (3 Marks)

• Point 1: Outer electrons are further from the nucleus (atomic radius increases).
• Point 2: There is increased shielding from inner electron shells.
• Point 3: First ionisation energy decreases, making it easier to lose electrons, so reactivity increases down the group.

🧠 Exam Technique

This is a classic "trend explanation" template question. Always structure your answer in a strict causal chain: Atomic Radius / Shielding → Ionisation Energy → Ease of cation formation → Reactivity.

❌ Common Errors

• Mentioning "charge density" incorrectly (not accepted by examiners for M2).
• Confusing nuclear charge with effective nuclear charge or forgetting to mention shielding.

✅ Correct Answers (2 Marks)

• Calcium is oxidised because it loses electrons (e.g., Ca → Ca²⁺ + 2e⁻ ).
• Chlorine ( Cl₂ ) is reduced because it gains electrons (e.g., Cl₂ + 2e⁻ → 2Cl⁻ ).

💡 Key Knowledge

Remember the acronym OIL RIG (Oxidation Is Loss, Reduction Is Gain of electrons). You must explicitly link the element to the specific electron transfer process to score.

❌ Common Errors

• Writing "calcium loses 1 electron" instead of 2 electrons (Calcium is in Group 2 and forms a 2+ ion).
• Failing to specify which substance undergoes oxidation and which undergoes reduction.

📐 Step-by-Step Calculation

1. Calculate moles of magnesium:
   Moles = Mass / Molar Mass
   Moles = 0.035 / 24.3 = 1.44 × 10⁻³ mol
2. Relate moles using stoichiometry:
   From equation Mg + 2HCl → MgCl₂ + H₂ , the reacting ratio is 1 : 1 .
   Therefore, moles of H₂ produced = 1.44 × 10⁻³ mol .
3. Calculate molar volume:
   Molar Volume = Volume / Moles
   = 32 cm³ / (1.44 × 10⁻³ mol)
   = 22220 cm³ mol⁻¹ (or 22.2 dm³ mol⁻¹ )

🧠 Exam Technique & Units

Ensure your units match your final output! Acceptable unit formats include cm³ mol⁻¹ , cm³ / mol , dm³ mol⁻¹ , or dm³ / mol . Always check whether your volume input needs conversion based on the unit requested.

❌ Common Calculation Traps

• Forgetting to factor in the 1:1 molar ratio correctly (though here it's 1:1, students often panic-multiply by 2).
• Premature rounding in step 1 leading to final answers outside the acceptable mark scheme tolerance window (e.g., 22000 to 22400 cm³ mol⁻¹ ).