1991 Chem MC

2008-04-10 12:47 am

Iron(II) sulphate solution is mixed with chlorine water. Excess aqueous ammonia is then added to the mixture. What is the color of the precipitate formed?

A.white
B.yellow
C.green
D.brown

Please explain briefly.

回答 (1)

Uncle Michael

2008-04-10 1:07 am

✔ 最佳答案

The answer is D.

Firstly, chlorine gas is a strong oxidizing agent, which oxidizes the iron(II) ions in the iron(II) sulphate solution to iron(III) ions.
2Fe2+(aq) + Cl2(aq) → 2Fe3+(aq) + 2Cl-(aq) ...... (*)

Ammonia is a weak base. Ammonia is partly ionized in water to give OH-(aq) ions.
NH3(aq) + H2O(l) ≒ NH4+(aq) + OH-(aq) ...... (**)

The iron(III) ions formed in reaction (*) and the hydroxide ions formed in reaction (**) would then react to give a brown precipitate of iron(III) hydroxide.
Fe3+(aq) + 3OH-(aq) → Fe(OH)3(s)

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