OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2023: Question 13

✅ Correct Answer: B (Only 1 and 2)

Both ethane ( C₂H₆ ) and ice/water ( H₂O ) are molecular substances held together by intermolecular forces in the solid state, which always include London forces. Silicon ( Si ) is a giant covalent lattice.

💡 Key Knowledge

• London forces arise from random fluctuations of electron density, creating instantaneous and induced dipoles. They exist between all atoms and molecules containing electrons.
• C₂H₆ (Ethane) is a simple molecular compound containing only London forces.
• H₂O (Ice) has hydrogen bonds, permanent dipole-dipole interactions, and London forces.
• Si (Silicon) has a giant covalent macromolecular structure bonded entirely by strong covalent bonds, not intermolecular forces.

🧠 Exam Technique

• Go through each numbered statement systematically and classify its bonding/structure type.
• Never assume that because a substance has hydrogen bonding (like H₂O ), it lacks London forces. All molecules have London forces.
• Eliminate options quickly by identifying giant structures (like Si or SiO₂ ) which do not feature intermolecular forces.

❌ Common Errors

• The "Mutually Exclusive" Trap: Many students think a molecule can only have one type of intermolecular force (e.g., "water has hydrogen bonding so it doesn't have London forces"). This is incorrect.
• Structure Confusion: Treating silicon ( Si ) like a simple molecular lattice rather than recognising its giant covalent macromolecular structure (similar to diamond).