CCEA A Level Chemistry 1.8 Halogens Exam Questions

About CCEA A Level Chemistry 1.8 Halogens

Section 1.8 of Unit AS 1 is the main descriptive inorganic chemistry on the AS course. You need the colours, states and boiling point trend of the halogens, their colours in water and in hexane, and their reactions with cold dilute and hot concentrated sodium hydroxide, both examples of disproportionation. You must explain the trend in oxidising ability with displacement reactions, and the reactions of solid halides with concentrated sulfuric acid and phosphoric acid, which show the increasing reducing power of the hydrogen halides. CCEA also asks you to compare chlorine and ozone for treating water. Accurate observations and balanced equations earn most of the marks here.

What you need to know

• Physical properties Fluorine is a pale yellow gas, chlorine a green-yellow gas, bromine a red-brown liquid and iodine a grey-black solid that gives a purple vapour. Boiling point increases down the group as van der Waals forces increase.
• Colours in solution In water: chlorine is pale green, bromine orange and iodine brown. In hexane: chlorine is pale green, bromine orange-red and iodine violet (purple).
• Reactions with alkali Cold dilute: Cl2 + 2NaOH gives NaCl + NaClO + H2O. Hot concentrated: 3Cl2 + 6NaOH gives 5NaCl + NaClO3 + 3H2O. Chlorine with water: Cl2 + H2O gives HCl + HClO (reversible).
• Oxidising ability Oxidising power decreases down the group, so a halogen displaces a less reactive halide from solution, for example Cl2 + 2KBr gives 2KCl + Br2.
• Halides with sulfuric acid NaCl gives HCl (steamy fumes) only. NaBr gives HBr, which reduces H2SO4 to SO2 and forms orange-brown Br2. NaI gives HI, which reduces H2SO4 to SO2, S and H2S (bad egg smell) and forms purple/black I2. Reducing power of HX increases down the group. Phosphoric acid is not an oxidising agent, so it gives HBr and HI without further reaction.
• Water treatment Chlorine kills microorganisms and leaves a residual effect but can form chlorinated organic compounds. Ozone is a powerful disinfectant with no taste, but it has no residual effect and is more expensive to produce.

Common mistakes in 1.8 Halogens

• Saying iodine solution is purple in water. It is brown in water and violet in hexane.
• Saying sulfuric acid oxidises chloride ions. HCl is not a strong enough reducing agent; only Br- and I- are oxidised.
• Forgetting the oxidation state changes in the hot alkali reaction. Chlorine goes from 0 to -1 in NaCl and +5 in NaClO3.
• Giving bromine water as colourless after a displacement. Bromine formed makes the solution orange or yellow.
• Saying ozone leaves residual protection. It decomposes quickly, so there is no residual disinfecting effect in the pipes.

Try three 1.8 Halogens exam-style questions