Chemistry › Ions in aqueous solution
Ions in aqueous solution
Unit C15 · 2 lessons published so far. The unit name describes the whole of this area of chemistry, and most of it is still to be written.
Iron(III) chloride solution has a pH near 2. In water a metal ion is surrounded by six water molecules, and the 3+ ions are more acidic than the 2+ ions because a smaller, more highly charged ion polarises its water ligands more strongly. The reactions with hydroxide, ammonia and carbonate then give precipitates and colours that identify each ion, and these test-tube results are examined in detail.
Carbonate is the trap. A 2+ ion such as iron(II) gives a metal carbonate precipitate, but a 3+ ion is acidic enough to release carbon dioxide and gives the hydroxide instead, so effervescence is part of the observation and the product is not a carbonate.
Questions on it can come up on AQA Papers 1 and 3, OCR A Papers 1 and 3, Edexcel Papers 1 and 3 and Cambridge Papers 1, 2 and 4; the past-paper pages say what else each of those papers covers.
Specification sections: AQA 3.2.6 · OCR A 5.3.1, 5.3.2 · Edexcel 12.14, 15.24 to 15.26 · CAIE 9.2, 28.2
Notes
Illustrated notes for each topic in this unit, with worked examples and a checklist of objectives.
- Metal-aqua ions and the acidity of their solutions
The hexaaqua ions of iron(II), copper(II), iron(III) and aluminium and the colours of their solutions, hydrolysis of an aqua ion as a Brønsted–Lowry acid with the equilibrium written, charge/size ratio and why 3+ aqua ions are more acidic than 2+ aqua ions, the pH of metal salt solutions with Ka calculations, the pH of the Period 3 chlorides in water, the effect of dilution on pH, and sodium carbonate as the test that tells 2+ ions from 3+ ions.
- Reactions of aqua ions with hydroxide, ammonia and carbonate
The reactions of the iron(II), copper(II), iron(III) and aluminium aqua ions with sodium hydroxide, aqueous ammonia and sodium carbonate written as acid–base reactions with state symbols, the colours of the precipitates, aluminium hydroxide as an amphoteric hydroxide, copper(II) hydroxide dissolving in excess ammonia by ligand substitution, the full identification table, chromium(III), manganese(II) and cobalt(II) with hydroxide and ammonia as OCR A and Edexcel set them, and the method for the test-tube practical (AQA required practical 11).
Key definitions for this unit
Practical work
The practical capabilities this unit's work belongs to, each with its techniques, its calculations and the errors that cost marks.
Worked in full, from the apparatus to the evaluation:
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