Past exam of the medicine course of the University of Oxford / 1865 / First examination / Chemistry / 5 / Solution

Iron in nonoxidizing hydrochloric acid principally gives an iron(II) solution; tin in oxidizing aqua regia principally gives a tin(IV) solution. These are the two preparations paired most directly by “respectively”. Their contrast is clearer when the other acid treatment of each metal is considered as well. “Nitrohydrochloric acid” is the historical name for aqua regia, containing nitric and hydrochloric acids.

For iron in dilute hydrochloric acid,The fresh solution is typically pale green when sufficiently concentrated. Hydroxide gives a pale green precipitate of iron(II) hydroxide, which darkens and turns brown on exposure to air as oxidation proceeds. Ammonia can similarly precipitate a ferrous hydroxide rather than forming a stable soluble excess-ammonia complex under the usual test conditions. Ferricyanide gives a deep blue product; the modern structural account of this blue material should not be replaced by an overly literal formula inferred solely from the reagent names. Ferrous solution also reduces a suitable oxidant such as acidic permanganate:

If instead oxidizing acid treatment produces iron(III), the solution exhibits ferric reactions: hydroxide or Ammonia gives red-brown iron(III) hydroxide; thiocyanate gives a strong red colour; and ferrocyanide gives Prussian blue. Chloride complex formation affects colour and some equilibria, so “yellow” is not a universal iron(III) identity test. Oxidizing aqua regia supplies this ferric comparison rather than merely generating more of the same ferrous salt.

Tin in ordinary hydrochloric acid gives a tin(II) solution with hydrogen evolution:Its characteristic property is reducing power. With mercury(II) chloride, controlled addition can first form white mercury(I) chloride and then, with further reductant, grey metallic Mercury:The chloride formulae are useful net bookkeeping, not a complete statement of all aqueous chloro-complexes.

In aqua regia, tin is oxidized predominantly to tin(IV). A representative balance, choosing nitrogen monoxide as the nitrate-reduction product, isActual gas products depend on the acid conditions. Sufficient chloride and acidity retain tin in solution; dilution can cause hydrolysis and precipitation of hydrated tin(IV) oxide. This differs from the simple expectation that every dilution leaves unchanged molecular ${\mathrm{SnCl}}_4$ in water. Hydrogen sulfide in suitable acidic conditions gives yellow tin(IV) sulfide, compared with brown tin(II) sulfide from a stannous solution. Tin(IV) does not reproduce the characteristic reducing reaction of fresh tin(II) with mercuric chloride; reduction to tin(II) is needed first. Oxidation during storage can therefore change the reactions of a nominally stannous preparation.