Group 17

• Bigger atom, weaker hold: the halogen gains electrons less readily, the halide loses them more readily, every bond to the halogen weakens — and only the forces between molecules get stronger.
• Bigger atom, weaker hold. Volatility is about forces between molecules, not bonds inside them. White soluble, cream partially soluble, pale yellow insoluble.

• 11.1.1 I can describe — describe the colours and the trend in volatility of chlorine, bromine and iodine
• 11.1.2 I can describe — describe and explain the trend in the bond strength of the halogen molecules
• 11.1.3 I can interpret — interpret the volatility of the elements in terms of instantaneous dipole-induced dipole forces
• 11.2.1 I can describe — describe the relative reactivity of the elements as oxidising agents
• 11.2.2 I can describe — describe the reactions of the elements with hydrogen and explain their relative reactivity in these reactions
• 11.2.3 I can describe — describe the relative thermal stabilities of the hydrogen halides and explain these in terms of bond strengths
• 11.3.1 I can describe — describe the relative reactivity of halide ions as reducing agents
• 11.3.2 I can describe — describe and explain the reactions of halide ions with: (a) aqueous silver ions followed by aqueous ammonia (the formation and formula of the [Ag(NH3)2]+ complex is not required) (b) concentrated sulfuric acid, to include balanced chemical equations
• 11.4.1 I can describe — describe and interpret, in terms of changes in oxidation number, the reaction of chlorine with cold and with hot aqueous sodium hydroxide and recognise these as disproportionation reactions
• 11.4.2 I can explain — explain, including by use of an equation, the use of chlorine in water purification to include the production of the active species HOCl and ClO- which kill bacteria

Units and significant figures are marked. The syllabus states that failure to quote units, the inclusion of units in quantities defined as ratios, and answers given to an inappropriate number of significant figures are all liable to be penalised. Give a calculated answer to the same number of significant figures as the least precise data, or one more; keep full precision in the working and round only at the end. A fifth of the qualification is experimental: Papers 3 and 5 test AO3 only, and their questions may be set in contexts outside the syllabus content, so the practical work in this chapter is set out as procedure, recording and evaluation rather than as theory.

• “Iodine is the least volatile because the I–I bond is the strongest.” Correct Volatility is about the forces between molecules, not the bond inside them. No X–X bond breaks when a halogen boils. Iodine is least volatile because its 106-electron molecules have the strongest instantaneous dipole–induced dipole forces; the I–I bond is in fact the weakest of the three (151 kJ mol⁻¹).
• “Bond strength increases down the group because the atoms are bigger.” Correct It decreases, Cl–Cl 242 > Br–Br 193 > I–I 151 kJ mol⁻¹: in a larger atom the shared pair is further from the nucleus and more shielded, so it is attracted less strongly.
• “Iodide is the weakest reducing agent because iodine is the weakest oxidising agent.” Correct The two trends are mirror images. The atom that holds an incoming electron least strongly also holds its own outer electron least strongly, so iodide is the strongest reducing agent of the three ions.
• “Silver bromide is insoluble in ammonia.” Correct Silver bromide is partially soluble: it does not dissolve in dilute aqueous ammonia but does dissolve in concentrated. Insoluble in both is silver iodide.
• “Concentrated sulfuric acid oxidises chloride to chlorine.” Correct Chloride is too weak a reducing agent. Only the acid–base reaction happens, and the only gas is hydrogen chloride (steamy fumes).
• “Chlorine kills the bacteria in drinking water.” Correct Chlorine reacts with the water first. The species that kill bacteria are chloric(I) acid, HOCl, and the chlorate(I) ion, ClO⁻; the answer needs the equation Cl₂ + H₂O ⇌ HCl + HOCl.
• “Bromine is a brown gas.” Repair At room temperature bromine is a red-brown liquid; it is its vapour that is red-brown and gaseous.
• “Iodine is less volatile because the I–I bond is stronger.” Repair The I–I bond is the weakest of the three (151 kJ mol⁻¹), and it is not broken on boiling. Iodine is least volatile because its 106-electron molecules have the strongest instantaneous dipole–induced dipole forces.
• “Bond strength increases down the group because the atoms are bigger.” Repair It decreases: the shared pair is further from both nuclei and more shielded, so the attraction is weaker.
• Br₂(aq) + 2Cl⁻(aq) → 2Br⁻(aq) + Cl₂(aq) Repair A halogen oxidises only the halide ions of halogens below it. Bromine cannot oxidise chloride: no reaction.
• “HI is the most stable hydrogen halide because iodine is the least reactive.” Repair HI is the least stable: the H–I bond (299 kJ mol⁻¹) is the weakest and breaks most easily.
• “Iodide is the weakest reducing agent because iodine is the weakest oxidising agent.” Repair The trends are mirror images: iodide is the strongest reducing agent, because its outer electrons are furthest from the nucleus and most shielded.
• “Silver bromide is insoluble in ammonia.” Repair Partially soluble: insoluble in dilute, soluble in concentrated aqueous ammonia. Insoluble in both is silver iodide.
• Adding silver nitrate straight to the unknown solution. Repair Add dilute nitric acid first, or carbonate and hydroxide ions give precipitates of their own that confuse the result.
• “Concentrated sulfuric acid oxidises chloride to chlorine.” Repair Chloride is too weak a reducing agent; only HCl fumes form. Bromide gives Br₂ and SO₂; iodide gives I₂ with SO₂, S and H₂S.
• “Chlorine with hot NaOH gives NaOCl.” Repair Cold dilute alkali gives NaCl + NaOCl (chlorine +1); hot concentrated alkali gives 5NaCl + NaClO₃ (chlorine +5).
• “Chlorine kills bacteria in water.” Repair Chlorine reacts with water to form HOCl and, from it, ClO⁻; those species kill bacteria. Write Cl₂ + H₂O ⇌ HCl + HOCl and HOCl ⇌ H⁺ + ClO⁻.

• Read the command word before you decide how much to write. This syllabus has twenty-two of them: analyse, calculate, compare, consider, contrast, deduce, define, demonstrate, describe, determine, discuss, evaluate, examine, explain, give, identify, justify, predict, show (that), sketch, state and suggest. Comment, estimate, name and outline are not among them: where a question wants something named it says identify, which the syllabus glosses as “name/select/recognise”. State and give want a fact and nothing more. Describe wants the points or the features. Explain wants the reasons and the relationships — a describe-level answer to an explain question is incomplete however well written it is. Deduce and determine want a conclusion reached from the information given, with the reasoning visible.
• Interleave with the chapters that use this one. Topic 15 (halogen compounds) uses the same bond-strength argument for C–X: re-answer “why is HI the least stable?” when you get there. Topic 24 measures the oxidising-power order with numbers: re-answer the displacement grid then. Recalling a topic inside a new context is worth more than another pass over this chapter on its own; at A Level, Paper 4 assumes the whole of the AS content, so nothing here is ever finished with.

• Cambridge International AS & A Level Chemistry 9701 has five components. Topic 11 is AS Level content, so it is examined in Papers 1, 2 and 3. AS Level content: examined in Paper 1 (multiple choice), Paper 2 (AS structured) and, as practical context, Paper 3. Assumed knowledge for Papers 4 and 5. AS Level candidates take Papers 1, 2 and 3; A Level candidates take all five, either staged over two years (Papers 1–3 in year one, Papers 4 and 5 in year two) or together in one series. Examinations are available in the June and November series, and in March in India.
• Across both the AS Level and the A Level the assessment objectives are weighted AO1 40% (knowledge and understanding), AO2 40% (handling, applying and evaluating information) and AO3 20% (experimental skills and investigations). AS candidates are graded a–e; A Level candidates A*–E.
• A Paper 1 item on this topic can turn on one trend read the right way round (oxidising power down, reducing power up), one colour word, or one oxidation number. A Paper 2 question can ask you to describe an observation in the syllabus’s words, write a balanced equation with state symbols, and explain a trend from atomic structure or bond energy.
• The Data-section bond energies (Cl–Cl 242, Br–Br 193, I–I 151, H–Cl 431, H–Br 366, H–I 299, H–H 436 kJ mol⁻¹) are supplied, not recalled; you use them to calculate ΔH and to explain stability. Oxidation numbers and electron counts check every redox equation. Boiling points are not in the Data section; a question that needs them gives them.
• Paper 3 can set the halide tests as an observation exercise, recorded in the qualitative-analysis words including “no change”. The quantitative procedure is the thiosulfate–iodine titration: burette readings to 0.05 cm³, concordant titres within 0.10 cm³, and in Paper 5 a plan for analysing a bleach with its errors and improvements.
• Read the command word before you decide how much to write. This syllabus has twenty-two of them: analyse, calculate, compare, consider, contrast, deduce, define, demonstrate, describe, determine, discuss, evaluate, examine, explain, give, identify, justify, predict, show (that), sketch, state and suggest. Comment, estimate, name and outline are not among them: where a question wants something named it says identify, which the syllabus glosses as “name/select/recognise”. State and give want a fact and nothing more. Describe wants the points or the features. Explain wants the reasons and the relationships — a describe-level answer to an explain question is incomplete however well written it is. Deduce and determine want a conclusion reached from the information given, with the reasoning visible.