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

Topic 15: Group 7(17), the halogens

Why does the boiling point of the halogens increase down the group?

More electrons per molecule (Cl₂ 34, Br₂ 70, I₂ 106), so stronger van der Waals forces between molecules need more energy to overcome. The X–X bond does not break.

Trends down Group 7 in the oxidising ability of X₂ and the reducing ability of X⁻

Oxidising ability of the halogens decreases; reducing ability of the halide ions increases.

Chlorine water added to potassium iodide solution: result and ionic equation

Brown solution. Cl₂ + 2I⁻ → 2Cl⁻ + I₂

Concentrated H₂SO₄ with NaCl and with NaBr: observations

NaCl: steamy white fumes of HCl only (acid–base, not redox). NaBr: steamy fumes of HBr, then orange-brown fumes of Br₂, and SO₂ forms (S +6 → +4).

Reduction products of H₂SO₄ with sodium iodide, and their observations

SO₂ (S +4); S, a yellow solid (S 0); H₂S, a bad-egg smell (S −2). I₂ appears as a black solid or purple vapour.

Silver nitrate test: precipitate colours and ammonia solubility

Cl⁻: white, dissolves in dilute NH₃. Br⁻: cream, dissolves only in concentrated NH₃. I⁻: yellow, insoluble in concentrated NH₃. Acidify with dilute HNO₃ first.

Chlorine with cold, dilute NaOH: equation and type of reaction

Cl₂ + 2NaOH → NaCl + NaClO + H₂O. Disproportionation: Cl goes from 0 to −1 and to +1. The NaClO solution is bleach.

Why is chlorine added to drinking water despite being toxic?

It kills bacteria. The doses are controlled and the health benefits outweigh the toxic effects.