Group 7: Oxidising Power, Displacement and Halide Tests
Down group 7 the halogens get worse at taking an electron, so oxidising power falls while the halide ions get better at giving one up. Displacement reactions and the reactions of solid halides with concentrated acid both follow from that.
What a learner can do afterwards
- Explains the fall in oxidising power down the group using atomic radius and shielding
- Predicts and writes the displacement reactions between halogens and halide solutions
- Explains the different products when concentrated sulfuric acid meets sodium chloride, sodium bromide and sodium iodide
- Identifies a halide with silver nitrate and ammonia and states the colours at each stage
1 · Read
Fluorine, chlorine, bromine and iodine each need one electron for a full outer shell. They act as oxidising agents by taking electrons. A species that loses electrons is oxidised, and one that gains them is reduced, so the halogen itself is reduced. Down Group 7, atoms become larger and inner shells shield the nucleus, so the pull on a new electron weakens.
A stronger halogen displaces a weaker halogen from a solution of its halide. Chlorine takes electrons from iodide ions: Cl2 + 2I- -> 2Cl- + I2. The iodide ions lose electrons, so they are oxidised. Chlorine gains them, so it is reduced. The freed halogen colours the solution, and here iodine turns it brown. A weaker halogen cannot take electrons from the halide of a stronger one, so iodine added to bromide solution leaves it unchanged.
Concentrated sulfuric acid first reacts with a solid halide to form a hydrogen halide. Chloride gives hydrogen chloride only. Bromide is oxidised to bromine while the acid is reduced mainly to sulfur dioxide. Iodide is oxidised most easily and gives iodine, sulfur dioxide, hydrogen sulfide and sulfur.
For the silver nitrate test, chloride gives a white precipitate that dissolves in dilute ammonia. Bromide gives a cream precipitate that needs concentrated ammonia. Iodide gives a yellow precipitate that stays undissolved. Use the colour and the ammonia result together.
Oxidising power falls down Group 7. A stronger halogen displaces a weaker one from its halide solution, and the heavier halides reduce concentrated sulfuric acid further.
Original chemistry lesson and assessment text authored for os-taxonomy.
All four readings on this page are original writing for os-taxonomy.
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