OxfordAQA International A Level Chemistry 3.2.3 Group 7(17), the Halogens Exam Questions

About OxfordAQA International A Level Chemistry 3.2.3 Group 7(17), the Halogens

Group 7(17), the halogens, is an inorganic chemistry topic examined in Unit 1. You need to explain the trends in electronegativity and boiling point, show the decreasing oxidising ability of the halogens using displacement reactions, and explain the increasing reducing ability of the halide ions, including their reactions with concentrated sulfuric acid. You also need the silver nitrate test for halide ions, the reactions of chlorine with water and with cold dilute sodium hydroxide, and the use of chlorine in water treatment. Required practical 3, test-tube reactions to identify cations and anions, is listed here and is commonly examined.

What you need to know

• Physical trends Electronegativity decreases down the group because atomic radius and shielding increase. Boiling points increase because larger molecules have more electrons and stronger van der Waals forces.
• Oxidising ability Oxidising ability decreases down the group. A halogen displaces a less reactive halide, for example Cl2 + 2Br- → 2Cl- + Br2.
• Reducing ability of halides Reducing ability increases down the group. With concentrated sulfuric acid, NaCl gives HCl only; NaBr gives HBr and also SO2 and Br2; NaI gives HI, I2, SO2, S and H2S.
• Silver nitrate test Add dilute nitric acid, then silver nitrate. Chloride gives a white precipitate that dissolves in dilute ammonia; bromide gives a cream precipitate that dissolves in concentrated ammonia; iodide gives a yellow precipitate insoluble in ammonia.
• Chlorine reactions Cl2 + H2O ⇌ HCl + HClO, and in sunlight 2Cl2 + 2H2O → 4HCl + O2. Cl2 + 2NaOH → NaCl + NaClO + H2O, forming bleach. Chlorine kills bacteria in water treatment, but its toxicity must be balanced against its benefits.
• Required practical 3 Identify cations (NH4+ and Group 2 ions) and anions (halides, hydroxide, carbonate and sulfate) using test-tube reactions.

Common mistakes in 3.2.3 Group 7(17), the Halogens

• Saying fluorine is the least reactive halogen. Oxidising ability decreases down the group, so fluorine is the most reactive.
• Forgetting to add nitric acid before silver nitrate. It removes carbonate ions that would give a false precipitate.
• Giving only one product for NaI with concentrated sulfuric acid. Iodide is a strong reducing agent and gives several reduction products of sulfuric acid.
• Calling the reaction of chlorine with water a displacement. It is a disproportionation, where chlorine is both oxidised and reduced.

Try three 3.2.3 Group 7(17), the Halogens exam-style questions