Edexcel International A Level Chemistry Inorganic Chemistry of Group 7 Exam Questions

About Edexcel International A Level Chemistry Inorganic Chemistry of Group 7

The inorganic chemistry of Group 7 in Unit 2 of Edexcel International A Level Chemistry uses the halogens and halide ions to apply redox ideas. You explain trends in physical properties and oxidising power down the group, then study displacement reactions, disproportionation of chlorine in water and in cold and hot alkali, and the increasing reducing power of halide ions shown by their reactions with concentrated sulfuric acid. Tests for halide ions with silver nitrate and ammonia are also included, along with iodine-thiosulfate titrations. Examiners like questions linking observations to oxidation numbers, writing ionic equations and explaining trends in terms of atomic radius and shielding.

What you need to know

• Physical properties Down the group volatility decreases and melting temperatures increase because London forces get stronger with more electrons. Colours: chlorine is pale green gas, bromine is red-brown liquid, iodine is a grey solid giving a purple vapour.
• Oxidising power Oxidising power decreases down the group because the atoms get larger, with more shielding, so they attract an extra electron less strongly. A more reactive halogen displaces a less reactive halide from solution.
• Disproportionation Chlorine with cold dilute NaOH: Cl2 + 2NaOH gives NaCl + NaClO + H2O. With hot NaOH, NaClO3 forms. Chlorine with water gives HCl and HClO, used in water treatment.
• Halides with concentrated sulfuric acid Chloride gives HCl only. Bromide gives HBr, then Br2 and SO2. Iodide gives HI, then I2 with SO2, S and H2S, showing reducing power increases down the group.
• Testing for halides Acidify with dilute nitric acid, then add silver nitrate: AgCl white, AgBr cream, AgI yellow. AgCl dissolves in dilute ammonia, AgBr only in concentrated ammonia, AgI is insoluble in both.
• Iodine-thiosulfate titrations Iodine is titrated with sodium thiosulfate: I2 + 2S2O3 2- gives 2I- + S4O6 2-. Starch is added near the end point, and the blue-black colour disappears at the end point.

Common mistakes in Inorganic Chemistry of Group 7

• Saying chlorine is more reactive because it has fewer electrons. Explain using atomic radius and shielding, which affect attraction for an incoming electron.
• Forgetting to acidify with nitric acid before adding silver nitrate. The acid removes carbonate ions that would also give a precipitate; hydrochloric acid cannot be used as it adds chloride.
• Getting the oxidation numbers wrong in disproportionation. In NaClO chlorine is +1 and in NaClO3 it is +5.
• Adding starch at the start of an iodine titration. It is added near the end point when the solution is pale yellow.
• Stating that sulfuric acid is reduced by chloride ions. Chloride ions are too weak a reducing agent; only an acid-base reaction occurs.

Try three Inorganic Chemistry of Group 7 exam-style questions