AQA A-Level Chemistry Paper 3, 2024: Question 20

Trends down Group 7: hydrogen halides, bond strength, E° and AgX solubility

What this question tests

Group 7 trend knowledge

How boiling points of HX change from HCl → HBr → HI.
How H–X bond strength changes down the group.
Direction of change of standard electrode potentials for X₂/ X⁻.
Trend in solubility of AgX in NH₃ (complex formation vs lattice enthalpy).

Multiple-choice skill

Compare each option against known trends; eliminate distractors fast.
Be careful: not all “down the group” trends move the same way.

Marks 1 mark total (AO1). You must select the single correct statement.

Part (a) (1 mark): Identify the correct trend from Cl to I

Question focus: which statement is true as X goes Cl → Br → I (X represents Cl, Br or I)

✅ Correct answer (from mark scheme)

A — The boiling point of HX increases.

Mark breakdown 1 mark (AO1) for choosing option A.

💡 Key knowledge: why option A is correct

Down Group 7, the halogen atom gets larger → more electrons → stronger London (dispersion) forces between HX molecules.
So boiling points increase: HCl < HBr < HI .
This question is specifically Cl to I, so you do not need to worry about HF behaving differently due to hydrogen bonding.

🧠 Exam technique: fast elimination for 1-mark trends

Read the trend direction carefully: “from Cl to I” means down the group.
Use “anchor facts”:
- Bond length increases → bond enthalpy usually decreases.
- Oxidising power decreases down Group 7 → E° becomes less positive.
- AgCl dissolves best in NH₃; AgI is essentially insoluble.
If one option matches a well-known trend exactly, select it and move on—don’t overthink.

❌ Common errors (what loses the mark)

Confusing “intermolecular forces” with “bond strength”: boiling point depends on intermolecular forces, not breaking the H–X covalent bond.
Mixing in HF: hydrogen bonding makes HF an anomaly, but HF is not included (the trend asked is Cl → I).
Assuming “down the group everything increases”: some properties decrease (e.g., bond dissociation enthalpy, E°).

Option-by-option check (to build confidence)

A: Boiling point of HX increases ✅

Correct: bigger X gives stronger London forces → higher boiling point.

B: Bond dissociation energy of H–X increases ❌

Incorrect: H–X bond length increases down the group, overlap is worse → bond dissociation energy decreases (HCl > HBr > HI).

C: E° for X₂(aq) + 2e⁻ → 2X⁻(aq) becomes more positive ❌

Incorrect: oxidising power of halogens decreases down the group, so E° becomes less positive (Cl₂/Cl⁻ more positive than Br₂/Br⁻, which is more positive than I₂/I⁻).

D: Solubility of AgX in ammonia increases ❌

Incorrect: solubility in NH₃ decreases down the group. AgCl dissolves in dilute NH₃, AgBr only in concentrated NH₃, AgI does not dissolve (very low tendency to form the soluble complex compared with lattice stability).

🧠 Examiner-style insight: what distinguishes top responses

Top students quickly link boiling point trends to London forces and increasing molecular size/electron count.
Weaker responses often pick B because they incorrectly associate “bigger atom” with “stronger bond”. In fact, bigger usually means longer bond and lower bond enthalpy for H–X.
For 1-mark MCQs, examiners expect recall of standard trends; no working is required, but understanding prevents traps.

Quick recap: the four key trends (Cl → Br → I)

💡 HX boiling point

Increases (stronger London forces).

HCl < HBr < HI

💡 H–X bond dissociation energy

Decreases (longer bond, poorer overlap).

💡 E° for X₂/ X⁻

Becomes less positive down the group (weaker oxidising agent).

💡 AgX solubility in NH₃

Decreases: AgCl dissolves best, AgI least.