AQA A-Level Chemistry AS Paper 1, November 2021: Question 5

✅ Correct Answer (3 Marks)

• (Random) movement of electrons in one molecule creates a temporary (instantaneous) dipole / imbalance in electron density.
• This induces a dipole in a neighbouring molecule.
• These temporary dipoles attract / there is a temporary electrostatic attraction between delta-positive (δ+) and delta-negative (δ−) regions.

💡 Key Knowledge

• Halogens exist as simple covalent diatomic molecules (Cl₂).
• The forces holding molecules together in the liquid/solid state are London forces (also known as instantaneous dipole-induced dipole forces).
• These are distinct from the strong covalent bonds holding the two chlorine atoms together *within* the molecule.

🧠 Exam Technique

This is a classic 3-step explanation question. To secure all 3 marks, you must follow the chronological chain of events: movement of electrons/temporary dipole → induction of a dipole in a neighbour → electrostatic attraction. Missing any step drops your mark.

❌ Common Errors

• Confusing intermolecular London forces with covalent bonds inside the Cl₂ molecule.
• Vague terminology such as "chlorine molecules break" instead of referring to the forces between molecules.
• Failing to mention the word induced when describing the second molecule.

✅ Correct Answer (2 Marks)

Equation (1 mark):
Cl₂ + H₂O ⇌ HCl + HClO
(Accept balanced full reaction: 2 Cl₂ + 2 H₂O → 4 HCl + O₂ )

Reason (1 mark):
Kills bacteria / kills microorganisms / kills microbes / kills pathogens (Allow: sterilise water / disinfect water).

💡 Key Knowledge

• This reaction is an example of a disproportionation reaction where chlorine is both oxidized and reduced.
• Chloric(I) acid ( HClO ) and hydrochloric acid ( HCl ) are formed.
• HClO acts as the active sterilising agent that destroys microorganisms.

🧠 Exam Technique

State symbols are not strictly penalised here unless incorrect, but equilibrium arrows ( ⇌ ) or standard arrows ( → ) must match your chosen correct equation format. Make sure you learn both acceptable equations from the mark scheme.

❌ Common Errors

• Writing Cl instead of Cl₂ for the reactant.
• Stating that chlorine is added to "clean" the water or "remove taste/odour" rather than the specific scientific reason: killing bacteria/pathogens.

✅ Correct Answer (1 Mark)

Cl₂ + 2 NaOH → NaCl + NaClO + H₂O

💡 Key Knowledge

• This is another crucial disproportionation reaction of halogens.
• Cold dilute NaOH reacts with Cl₂ to produce sodium chloride ( NaCl ), sodium chlorate(I) ( NaClO - the active ingredient in bleach), and water ( H₂O ).
• Contrast this with hot alkali, which forms chlorate(V) instead.

🧠 Exam Technique

Always double-check your balancing for disproportionation reactions. Ensure you include the stoichiometric coefficient 2 in front of NaOH to balance both sodium and oxygen/hydrogen atoms effectively.

❌ Common Errors

Forgetting the coefficient of 2 for NaOH , leading to unbalanced sodium, hydrogen, or oxygen atoms on either side of the equation.