CIE A Level Chemistry Group 17 Exam Questions

About CIE A Level Chemistry Group 17

Group 17 in Cambridge International 9701 is the study of the halogens chlorine, bromine and iodine and their compounds. You need to describe trends in colour, volatility and bond strength, explain the decreasing oxidising power of the halogens down the group, and use displacement reactions as evidence. The topic covers the thermal stability of the hydrogen halides, the reactions of halide ions with aqueous silver ions followed by ammonia, and with concentrated sulfuric acid. You also study the disproportionation of chlorine in cold and hot sodium hydroxide and the use of chlorine in water treatment. It links closely to redox, and examiners frequently test oxidation number changes.

What you need to know

• Physical properties Chlorine is a pale green gas, bromine a red-brown liquid and iodine a grey-black solid giving a purple vapour. Volatility decreases down the group as van der Waals forces increase with more electrons.
• Oxidising power Oxidising power decreases down the group because atoms are larger with more shielding, so they attract an extra electron less strongly. A halogen displaces a less reactive halide, for example Cl2 + 2Br- → 2Cl- + Br2.
• Hydrogen halides Thermal stability decreases from HCl to HI because the H-X bond energy decreases. HI is decomposed into purple iodine vapour by a hot wire.
• Testing for halide ions With AgNO3(aq): AgCl white, AgBr cream, AgI yellow. AgCl dissolves in dilute NH3, AgBr only in concentrated NH3, and AgI is insoluble in both.
• Halides with concentrated H2SO4 NaCl gives HCl fumes only. NaBr gives HBr then Br2 and SO2, and NaI gives I2, SO2, S and H2S, because the reducing power of the halide ions increases down the group.
• Disproportionation With cold dilute NaOH: Cl2 + 2NaOH → NaCl + NaClO + H2O. With hot NaOH: 3Cl2 + 6NaOH → 5NaCl + NaClO3 + 3H2O. Chlorine is used to kill bacteria in water supplies.

Common mistakes in Group 17

• Saying oxidising power decreases because electronegativity increases down the group; electronegativity decreases, and the explanation is about attraction for an incoming electron.
• Confusing the oxidation states in disproportionation; chlorine goes from 0 to -1 and +1 in cold alkali, and to -1 and +5 in hot alkali.
• Describing silver iodide as white; AgCl is white, AgBr cream and AgI yellow.
• Saying HI is the most stable hydrogen halide because iodine is largest; the H-I bond is the weakest, so HI is least stable.
• Forgetting to acidify with dilute nitric acid before adding silver nitrate, which removes carbonate that would also give a precipitate.

Try three Group 17 exam-style questions