Edexcel A-Level Chemistry AS Paper 1, June 2016: Question 8

Structure, Bonding, Stoichiometry, and Redox

What this question tests

This question assesses your core chemical understanding across multiple AS topics: identifying lattice types based on physical properties, calculating empirical formulas from percentage mass data, balancing inorganic chemical equations, assigning oxidation numbers to prove redox reactions, and linking physical properties of metals directly to their giant metallic lattice structure and delocalised electrons.

Part (a): Structure and Bonding of Magnesium Oxide

Question 8(a)

✅ Correct Answer

A: giant ionic

💡 Key Knowledge

Magnesium oxide (MgO) is formed from a metal (Mg) and a non-metal (O).
It forms a giant ionic lattice held together by strong electrostatic forces of attraction between oppositely charged ions (Mg²⁺ and O²⁻).

Marks: 1 mark

Part (b)(i): Empirical Formula Calculation

Question 8(b)(i)

📐 Step-by-Step Calculation

Find moles of each element:
Moles of S = 45.79 ÷ 32.1 = 1.426 mol
Moles of F = 54.21 ÷ 19.0 = 2.853 mol
Find the simplest whole-number ratio:
Divide by the smallest value (1.426):
S = 1.426 ÷ 1.426 = 1
F = 2.853 ÷ 1.426 = 2.00 (Ratio 1 : 2)
State empirical formula: SF₂

❌ Common Calculation Traps

Using atomic numbers instead of relative atomic masses (Aᵣ values) results in 0 marks overall.
Rounding intermediate numbers too aggressively before finding the ratio can throw off the final whole-number ratio. Keep at least 3 significant figures during intermediate steps.

Marks: 2 marks (1 for moles evaluation, 1 for ratio & formula)

Part (b)(ii): Balancing Equations

Question 8(b)(ii)

✅ Correct Equation

S₈ + 8AgF₂ → 4S₂F₂ + 8AgF

🧠 Exam Technique

Tackle complex or unfamiliar equations systematically. Balance elements that appear in the fewest species first (like sulfur, starting with S₈ on the left and S₂F₂ on the right), and leave single elements or fluorine balancing until the end.

Marks: 1 mark (Multiples are accepted by the mark scheme)

Part (b)(iii): Explaining Redox via Oxidation Numbers

Question 8(b)(iii)

💡 Key Knowledge & Answer

Sulfur (S): Oxidation number increases from 0 in S₈ to +1 in S₂F₂ (oxidation).
Silver (Ag): Oxidation number decreases from +2 in AgF₂ to +1 in AgF (reduction).
Fluorine (F): Remains at -1 throughout.
Conclusion: Because both oxidation and reduction occur simultaneously, it is a redox reaction.

❌ Where Students Lost Marks

Examiners note that omitting explicit statements for *both* oxidation and reduction (or failing to state that it is a redox reaction at the end) caps marks at 1 or 2. You must explicitly link changes in oxidation numbers to the terms oxidation and reduction.

Marks: 3 marks

Part (c): Physical Properties of Metals

Question 8(c)

✅ Valid Marking Pairs (Choose any TWO)

High melting/boiling temperature: Strong electrostatic attraction between metal ions and delocalised electrons.
Electrical/thermal conductivity: Mobile delocalised electrons are free to move and carry charge/thermal energy.
Malleability/ductility: Layers of metal ions/atoms can easily slide over each other without disrupting the metallic bond.
High density: Ions/atoms are tightly packed due to strong attractive forces.

❌ Common Errors & Pitfalls

Incomplete answers: Stating the property (e.g. "it conducts electricity") without explaining *why* (mentioning delocalised electrons) scores 0 for that pair. The explanation mark is strictly dependent on stating the correct property first.
Contradictory chemistry: If you list more than two properties, any incorrect chemistry included in extra answers will negate your correct marks. Stick to exactly two!
Avoid mentioning general appearance (like lustrous) or reactivity with water as physical properties linked to structure in this context.

Marks: 4 marks (2 marks per valid property-and-explanation pair)