Edexcel A-Level Chemistry AS Paper 1, June 2019: Question 8

✅ Correct Answer

• MgO has doubly charged ions (Mg²⁺ and O²⁻), whereas KBr has singly charged ions (K⁺ and Br⁻).
• Mg²⁺ and O²⁻ ions are smaller in ionic radius than K⁺ and Br⁻ ions.
• More energy is required to overcome the stronger electrostatic attractions between ions in MgO compared to KBr.

💡 Key Knowledge

Melting point in ionic lattices depends on the magnitude of attraction between oppositely charged ions. Use F = Q₁Q₂ / r² conceptually: higher charges (Q) and smaller ionic radii (r) drastically increase the lattice energy and melting temperature.

❌ Common Errors

Students frequently lose marks by referring to atomic radii instead of ionic radii, or by mistakenly mentioning intermolecular forces, London forces, or covalent bonding within ionic lattices.

✅ Correct Answer

• Solid KBr does not conduct electricity because ions are in fixed positions within the giant ionic lattice and cannot move.
• Aqueous KBr is a good electrical conductor because the ions are mobile (free to move and carry charge) when dissolved.

🧠 Exam Technique

Always link electrical conductivity explicitly to charged particles and their mobility. State clearly whether particles are free to move.

❌ Common Errors

Fatal Error: Mentioning the movement of electrons or free electrons in an ionic compound results in an immediate loss of all marks. Ionic compounds conduct via ion movement, not delocalised electrons!

✅ Correct Answer (Indicative Content)

• Graphene: Single layer/sheet of carbon atoms bonded in hexagons; contains delocalised electrons that are mobile.
• Graphite: Consists of layers/sheets where each carbon is bonded to three others; contains delocalised electrons between layers.
• Diamond: Each carbon is bonded to four other carbon atoms in a tetrahedral arrangement with no delocalised electrons (all outer electrons are localised in single covalent bonds).

🧠 Quality of Written Communication & Linkages

This is a starred (*C) question marked using levels of response. To hit top marks (Level 2 structure + 4-6 indicative points), you must explicitly link structure to bonding and then to conductivity. E.g., "Because graphite has delocalised electrons free to move parallel to the layers, it can conduct electricity, whereas diamond's localized bonding prevents this."

✅ Correct Answer

• Iron atoms have a greater relative atomic mass / molar mass than carbon atoms.
• Iron atoms pack much closer together in a metallic lattice than carbon atoms do across the widely spaced layers in graphite.

💡 Key Knowledge

Density depends on two factors: the mass of the individual constituent particles and how closely packed they are in space ( Density = Mass / Volume ).

❌ Common Errors

Do not mention "iron molecules" or "graphite molecules". Iron is metallic and exists as a giant lattice of atoms; graphite is a giant covalent macromolecule.

✅ Correct Answer

• The lower value (2.3 GPa) relates to overcoming weak London forces / van der Waals forces between the parallel layers.
• The higher value (15.3 GPa) relates to breaking strong covalent C-C bonds within each individual sheet of carbon atoms.

💡 Key Knowledge

Anisotropic materials display different physical properties depending on the direction of measurement. Compressive strength tests applied perpendicular vs parallel to graphite planes yield vastly different force requirements because inter-layer forces are orders of magnitude weaker than intra-layer covalent bonds.