Group 7 Halogens Exam Questions

Examiner note: Bromine is the only non-metal that is liquid at room temperature. The trend is gas → liquid → solid down the group because intermolecular forces increase with molecular size. Know the colours — they are frequently tested.

• Boiling point increases down the group [1]
• The molecules get larger / have more electrons going down the group [1]
• This means stronger intermolecular forces (van der Waals forces) between molecules, requiring more energy to separate them [1]

Examiner note: Halogens are simple molecular substances (X2). Their boiling points depend on the strength of intermolecular forces, not covalent bonds. Larger molecules have stronger van der Waals forces.

• Halogens react by gaining one electron to form a 1- ion (halide ion) [1]
• Going down the group, the atomic radius increases / the outer shell is further from the nucleus [1]
• There is more shielding from inner electrons [1]
• The attraction between the nucleus and the incoming electron is weaker, so the electron is gained less easily, and the element is less reactive [1]

Examiner note: This is the opposite trend to Group 1. Group 1 metals lose electrons (larger = easier = more reactive). Group 7 non-metals gain electrons (larger = harder = less reactive). Link to atomic radius and shielding for full marks.

(a) Chlorine is more reactive than bromine [1], so it displaces bromide ions from solution [1]

(b) Cl2 + 2KBr → 2KCl + Br2 [1]

(c) Cl2 + 2Br- → 2Cl- + Br2 [1]

Examiner note: The orange colour is due to dissolved bromine. This is a displacement reaction — a more reactive halogen displaces a less reactive halide from solution. Chlorine displaces bromine and iodine; bromine displaces iodine; iodine cannot displace either.

(a) The solution turns brown/dark brown [1]. Bromine is more reactive than iodine, so it displaces iodide ions, producing iodine: Br2 + 2KI → 2KBr + I2 [1]

(b) No reaction occurs [1] because iodine is less reactive than chlorine, so it cannot displace chloride ions

Examiner note: A more reactive halogen always displaces a less reactive halide. Since reactivity decreases down the group: Cl > Br > I. Iodine is the least reactive and cannot displace any other halide.

• Add dilute nitric acid followed by silver nitrate solution [1]
• Chloride ions: white precipitate (AgCl) [1]
• Bromide ions: cream precipitate (AgBr); iodide ions: yellow precipitate (AgI) [1]

Examiner note: Dilute nitric acid is added first to remove any carbonate or sulfite ions that would also produce precipitates with silver nitrate and interfere with the test. Do not use hydrochloric acid — it contains Cl- ions.

(a) H2 + Cl2 → 2HCl [1]

(b) Going down the group, the halogen atom is larger and gains an electron less easily [1]. The bond formed between hydrogen and the halogen (H-X) becomes weaker going down the group (the bond gets longer), so less energy is released [1].

Examiner note: H-F is the strongest hydrogen-halide bond; H-I is the weakest. This trend in bond strength mirrors the trend in reactivity. The reaction of H2 with F2 is explosive; with I2 it is slow and reversible.

(a) Cl2 + H2O → HCl + HClO (or HOCl) [1]

(b) The chloric(I) acid (HClO) formed is an oxidising agent that kills bacteria by oxidising their cell components [1]

(c) Chlorine and its compounds can form harmful by-products / some people are concerned about the toxicity of chlorine compounds in water / chlorine has an unpleasant taste and smell [1]

Examiner note: Chlorination of water is a balance between the benefit (killing harmful bacteria) and the risk (possible harmful by-products). This is a common evaluation question.