AQA A Level Chemistry
AQA A Level Chemistry 3.2.3 Group 7 (17), the Halogens Exam Questions
Free AQA A Level Chemistry exam questions for 3.2.3 Group 7 (17), the Halogens, with a mark scheme for every pack. Covers halogen trends, oxidising ability, displacement, halide reducing ability with sulfuric acid, silver nitrate tests and chlorine uses.
3.2.3 Group 7 (17), the Halogens exam questions
1 question pack of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
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About AQA A Level Chemistry 3.2.3 Group 7 (17), the Halogens
Group 7 is an AS inorganic topic on the AQA A Level course that combines trends, redox and practical tests. You need the trends in electronegativity, boiling point and oxidising ability of the halogens, displacement reactions, the increasing reducing ability of the halide ions shown by their reactions with concentrated sulfuric acid, and the silver nitrate tests for halide ions. You also study the reactions of chlorine with water and with cold dilute sodium hydroxide, and the benefits and risks of chlorinating water. This topic relies heavily on 3.1.7 Redox. Examiners frequently ask for observations, half equations, oxidation state changes and explanations of trends.
What you need to know
- Physical trends Down the group electronegativity decreases as atomic radius and shielding increase. Boiling point increases because larger molecules with more electrons have stronger van der Waals forces.
- Oxidising ability Oxidising ability decreases down the group. A halogen displaces a less reactive halide from solution, e.g. Cl2 + 2Br- → 2Cl- + Br2.
- Halides with concentrated sulfuric acid Reducing ability of halide ions increases down the group. NaCl gives only HCl (misty fumes). NaBr gives HBr, then Br2 and SO2. NaI gives HI, I2, SO2, S and H2S, as iodide reduces sulfur from +6 to +4, 0 and -2.
- Silver nitrate test Acidify with dilute nitric acid, then add silver nitrate. AgCl is white and dissolves in dilute ammonia, AgBr is cream and dissolves in concentrated ammonia, AgI is yellow and is insoluble in concentrated ammonia.
- Chlorine and water Cl2 + H2O ⇌ HCl + HClO, a disproportionation reaction. In sunlight, 2Cl2 + 2H2O → 4HCl + O2. Chlorine kills bacteria in drinking water, and the benefit outweighs the toxicity risk and the possible formation of chlorinated hydrocarbons.
- Chlorine and sodium hydroxide Cl2 + 2NaOH → NaClO + NaCl + H2O, made in cold dilute alkali, forms bleach. Chlorine is oxidised from 0 to +1 and reduced from 0 to -1.
Common mistakes in 3.2.3 Group 7 (17), the Halogens
- Acidifying with hydrochloric acid before the silver nitrate test. This adds chloride ions; use dilute nitric acid to remove carbonate and hydroxide ions.
- Explaining boiling point trends using bond strength within molecules. It is the van der Waals forces between molecules that increase.
- Describing the reaction of NaI with sulfuric acid only as giving purple fumes. Give the full set of products and observations such as a yellow solid (sulfur) and a bad egg smell (H2S).
- Calling the reaction of chlorine with water a displacement reaction. It is disproportionation because chlorine is both oxidised and reduced.
- Giving vague colours such as 'white' for silver bromide. Silver bromide is cream and silver iodide is yellow.
Try three 3.2.3 Group 7 (17), the Halogens exam-style questions
1. Write an equation for the reaction of chlorine with cold dilute sodium hydroxide and give the oxidation states of chlorine in each product. (3 marks)
Show the mark scheme answer
Cl2 + 2NaOH → NaClO + NaCl + H2O; chlorine in NaClO is +1; chlorine in NaCl is -1.
2. Explain why chlorine displaces bromine from potassium bromide solution but bromine does not displace chlorine. (2 marks)
Show the mark scheme answer
Chlorine is a stronger oxidising agent than bromine; chlorine atoms are smaller, so the nucleus attracts an incoming electron more strongly (less shielding), so chlorine gains electrons from bromide ions.
3. A solution is thought to contain bromide ions. Describe how you would confirm this using silver nitrate and ammonia, giving observations. (3 marks)
Show the mark scheme answer
Add dilute nitric acid then silver nitrate solution; a cream precipitate forms; the precipitate dissolves in concentrated ammonia but not in dilute ammonia.
3.2.3 Group 7 (17), the Halogens FAQs
What does AQA A Level Chemistry 3.2.3 Group 7 cover?
It covers trends in the properties of the halogens, oxidising ability and displacement, reducing ability of halide ions with concentrated sulfuric acid, silver nitrate tests, and reactions and uses of chlorine.
Which paper is 3.2.3 Group 7 examined in?
It is examined in Paper 1, and Paper 3 can also include it through Required practical 4.
How do I remember the sulfuric acid reactions of the halides?
Learn the products in order of increasing reducing power: chloride gives HCl, bromide adds Br2 and SO2, iodide adds I2, S and H2S. Link each sulfur product to its oxidation state (+4, 0, -2) and its observation.
