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
Group 7 holds fluorine, chlorine, bromine and iodine. Each atom is one electron short of a full outer shell, so each pulls hard on electrons from other atoms. That pull makes halogens strong oxidising agents, and fluorine pulls hardest of all.
Oxidising power falls as you go down the group. Atoms get bigger and the inner shells shield the nucleus, so an incoming electron joins a shell further from a well shielded centre. The pull weakens, which is why iodine oxidises far less readily than chlorine.
Chlorine displaces bromine from a bromide solution, and bromine displaces iodine from an iodide solution, but never the other way round. To predict a displacement, ask which halogen is the stronger oxidiser. The stronger one takes the electrons and forces the weaker one out as the free element, so bromide ions are oxidised to bromine while chlorine is reduced to chloride.
Concentrated sulfuric acid tells the halides apart. With sodium chloride it just makes steamy hydrogen chloride gas. With sodium bromide it also makes bromine and sulfur dioxide. With sodium iodide it goes further still, making iodine, sulfur dioxide, hydrogen sulfide and sulfur. In the lab, confirm with silver nitrate and ammonia as one package: white that vanishes in dilute ammonia means chloride, cream that only clears in concentrated ammonia means bromide, and yellow that stays put means iodide.
Oxidising power falls down Group 7, so stronger halogens displace weaker ones and the acid reduces further with heavier halides.
2 · Watch
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Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.