Overview
This lecture covers the physical and chemical properties of Group 7 (the halogens), methods for testing halide ions, and the uses and key reactions of chlorine.
Physical Properties of Group 7
- Halogens are diatomic non-metals with distinct colors that darken down the group.
- Halogen melting and boiling points increase down the group, making them less volatile lower in the group.
- Covalent bond strength in halogen molecules decreases down the group due to increasing atomic size and electron repulsion (especially in fluorine).
- Halogens have simple molecular structures with weak van der Waals forces; these forces get stronger with more electrons, increasing melting/boiling points down the group.
Chemical Properties of Group 7
- Halogens act as oxidising agents, gaining electrons from metals to form negative ions.
- Oxidising power and electronegativity decrease down the group due to increasing atomic radius and nuclear shielding.
- More reactive halogens can displace less reactive halogens from halide solutions.
Reducing Agents: Halide Ions
- Halide ions can act as reducing agents, losing electrons to other substances.
- Reducing power of halide ions increases down the group as their ionic size and shielding increase, making it easier to lose electrons.
Testing for Halide Ions
- Halide ions are detected by adding nitric acid and silver nitrate, forming precipitates: AgCl (white), AgBr (cream), AgI (yellow).
- Addition of ammonia distinguishes the precipitates: AgCl dissolves in dilute ammonia, AgBr in concentrated ammonia, AgI in neither.
- Halide ions react with concentrated sulfuric acid: Cl⁻ gives HCl (white fumes), Br⁻ gives Br₂ (reddish-brown gas, SO₂), I⁻ gives I₂ (purple vapour), S (yellow solid), H₂S (bad egg smell).
Uses & Reactions of Chlorine
- Chlorine undergoes disproportionation reactions in water and alkali, being both oxidised and reduced.
- Chlorine purifies water by forming HClO, a sterilising agent that kills bacteria.
- Sunlight decomposes chlorine in water, leading to the loss of effectiveness in pools.
Key Terms & Definitions
- Halogen — Group 7 element; non-metal; forms salts with metals.
- Volatility — Tendency of a substance to vaporize; decreases down the group.
- Van der Waals forces — Weak intermolecular forces; increase down the group.
- Oxidising agent — Substance that gains electrons in a reaction.
- Reducing agent — Substance that donates electrons in a reaction.
- Disproportionation — Reaction where the same species is both oxidised and reduced.
- Precipitate — Insoluble solid formed in a chemical reaction.
Action Items / Next Steps
- Review the trends in physical/chemical properties of halogens and halides.
- Practice writing ionic equations for displacement and disproportionation reactions.
- Memorize color changes and solubility in halide ion tests with silver nitrate and ammonia.
- Prepare for exam questions on halogen reactions and testing methods.