Flashcards · 9620 · AS Topic 15

Group 7(17), the halogens (AS Level) flashcards

10 cards on AS Topic 15, subtopics 3.2.3.1 to 3.2.3.2. Pick a subtopic or study them all.

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Trends down Group 7 (F → I): electronegativity and boiling point: Electronegativity falls: larger atom, more shielding, so the bonding pair is held less tightly. Boiling point rises: more electrons per molecule, so stronger van der Waals forces.
Trends down Group 7: oxidising ability of X₂ and reducing ability of X⁻: Oxidising ability of the halogen falls. Reducing ability of the halide ion rises (its outer electron is further out and more shielded, so easier to lose).
Which halogens displace which halide ions in aqueous solution?: A halogen oxidises the halide ions of any halogen below it: Cl₂ displaces Br⁻ and I⁻; Br₂ displaces I⁻ only. For example: Cl₂ + 2I⁻ → 2Cl⁻ + I₂
Products of solid NaCl, NaBr and NaI with concentrated H₂SO₄: All: HX (steamy fumes), e.g. NaCl + H₂SO₄ → NaHSO₄ + HCl. Br⁻ also reduces S to SO₂ (orange-brown Br₂). I⁻ reduces S to SO₂, S and H₂S (purple vapour / black solid I₂, yellow solid, bad-egg smell).
Silver nitrate test results for Cl⁻, Br⁻ and I⁻ (with ammonia): Acidify with HNO₃, add AgNO₃(aq). Cl⁻: white, dissolves in dilute NH₃. Br⁻: cream, dissolves in concentrated NH₃ only. I⁻: yellow, insoluble in both.
Why is the sample acidified with nitric acid (not HCl) before adding silver nitrate?: To remove ions such as CO₃²⁻ and OH⁻ that would also form a precipitate with Ag⁺. HCl would add Cl⁻ ions.

Reaction of chlorine with water (in the dark): Cl₂ + H₂O ⇌ HClO + HCl A reversible disproportionation: chlorine goes from 0 to +1 and −1.
Reaction of chlorine water in sunlight: 2Cl₂ + 2H₂O → 4H⁺ + 4Cl⁻ + O₂ Chlorine is reduced and oxygen is oxidised. This is not disproportionation.
Reaction of chlorine with cold, dilute sodium hydroxide, and a use of the product: Cl₂ + 2NaOH → NaClO + NaCl + H₂O (disproportionation). The solution of sodium chlorate(I) is bleach (also a disinfectant).
Why is chlorine added to drinking water, despite its risks?: It kills bacteria and so prevents water-borne disease. Chlorine is toxic and can form chlorinated organic compounds, but on balance the health benefits outweigh the toxic effects.