Group 7 (Halogens) — A-Level Chemistry Revision

Topic has curated content entry with explanation, mistakes, and worked example. [auto-gate:promote; score=75.25]

What is Group 7 (Halogens)?

The Group 7 elements, the halogens, are reactive non-metals with seven outer electrons. This topic covers the trends in their physical properties, such as boiling points (increase down the group due to stronger van der Waals forces) and electronegativity (decreases down the group). A key chemical property is their ability to act as oxidising agents, with their oxidising power decreasing down the group. This leads to displacement reactions where a more reactive halogen will displace a less reactive halide ion from solution. The reactions of halide ions with concentrated sulfuric acid and with silver nitrate solution are also important identifying tests.

Board notes: All boards cover the characteristic trends and displacement reactions of the halogens. AQA often includes questions on the reactions of halides with concentrated sulfuric acid, testing the different products formed. Edexcel places emphasis on the use of silver nitrate and ammonia solution to distinguish between halide ions. OCR may ask more about the industrial importance of halogens, such as the use of chlorine in water treatment.

Worked example

Common mistakes

• 1Confusing the trend in reactivity of the halogens with that of the halides. Halogen reactivity decreases down the group, whereas the reducing power of the halide ions increases down the group.
• 2Incorrectly identifying the products of the reaction between halide ions and concentrated sulfuric acid. For example, with bromide ions, sulfuric acid is reduced to sulfur dioxide, not hydrogen sulfide.
• 3Mixing up the colours of the silver halide precipitates. Silver chloride (AgCl) is white, silver bromide (AgBr) is cream, and silver iodide (AgI) is yellow.

Group 7 (Halogens) exam questions

Exam-style questions for Group 7 (Halogens) with mark-scheme style solutions and timing practice. Aligned to AQA, Edexcel and OCR specifications.

Frequently asked questions

• Why do boiling points increase down Group 7?

  As you go down the group, the number of electrons in the halogen molecules increases. This leads to stronger temporary dipole-induced dipole forces (van der Waals forces) between the molecules, which require more energy to overcome, resulting in higher boiling points.

• What is disproportionation?

  Disproportionation is a redox reaction where the same element is both oxidised and reduced. An example is the reaction of chlorine with cold, dilute sodium hydroxide: Cl2(aq) + 2NaOH(aq) -> NaCl(aq) + NaClO(aq) + H2O(l), where chlorine is reduced to -1 in NaCl and oxidised to +1 in NaClO.