Edexcel A-Level Chemistry AS Paper 1, June 2025: Question 3

✅ Correct Answer

C: London forces, permanent dipoles and hydrogen bonds

💡 Key Knowledge

• London forces (dispersion forces): Present between all atoms and molecules due to temporary fluctuations in electron density causing instantaneous induced dipoles.
• Permanent dipole-dipole forces: Present because H₂O has a non-linear (bent) shape and asymmetrical charge distribution (O is much more electronegative than H).
• Hydrogen bonding: An especially strong type of permanent dipole attraction occurring because H is directly bonded to the highly electronegative oxygen atom with lone pairs.

❌ Common Errors & Misconceptions

• Selecting Option A (Hydrogen bonds only): The most common student error. Students remember water forms strong hydrogen bonds and forget that London forces exist in every molecular system without exception.
• Treating hydrogen bonding as mutually exclusive: Hydrogen bonding is simply an exceptionally strong category of permanent dipole attraction; it does not replace or cancel out underlying London forces or basic permanent dipole interactions.

🧠 Exam Technique

Whenever an exam asks "What forces exist between molecules?", start from the bottom up:

1. Does it have electrons? → Always has London forces.
2. Is the molecule polar? → Also has permanent dipoles.
3. Is H bonded to N, O, or F with lone pairs available? → Also forms hydrogen bonds.

✅ Correct Answer

C: ethanol forms hydrogen bonds with water but ethanethiol does not

💡 Key Knowledge

• For a covalent compound to dissolve well in water, the energy released when forming new solute-solvent interactions must be sufficient to compensate for breaking the strong hydrogen bonds between water molecules.
• The -O-H group in ethanol (CH₃CH₂OH) readily forms hydrogen bonds with water molecules.
• Although sulfur has lone pairs, it is not electronegative enough to participate in hydrogen bonding. Hence, ethanethiol (CH₃CH₂SH) can only form weak London and dipole-dipole forces with water, which cannot overcome the water-water hydrogen bonds.

❌ The "True Statement" Distractor Trap

The examiner intentionally included statements that are factually correct, but do not provide the underlying chemical explanation for solubility:

• Option B ("sulfur is less electronegative than oxygen"): This is true, but it is an atomic property, not the direct intermolecular reason that explains dissolution into solvent water.
• Option A: Ethanethiol actually has more electrons than ethanol, so its London forces are indeed stronger, but stronger London forces would favour mixing rather than explain insolubility!
• Option D: Ethanol does have a larger dipole, but this is an incomplete explanation because simple dipole interactions are not strong enough to disrupt the hydrogen-bonded network of water.

🧠 Examiner Advice

When an Edexcel question asks for the "best explanation" regarding solubility in water:

• Always focus directly on hydrogen bonding to water.
• Remember the strict criteria for H-bonding: Hydrogen must be covalently bonded to Nitrogen, Oxygen, or Fluorine (N, O, F). Sulfur does not form hydrogen bonds!