hydroxide_title.gif (15329 bytes)


 

You might think that, since the effects of aqueous sodium hydroxide and of aqueous ammonia on solutions of the common aqua ions of the first transition series are required by most A level syllabuses, the reactions and their products are well-understood. Not a bit of it.

There are several problems in getting solid information:

  • different books often don’t agree – usually on the formulae of the product. This might be because one book was written from another and there were transcription errors here, but more likely because:
  • there is genuine uncertainty about the nature of the compounds, and perhaps one author made an arbitrary choice (for the sake of simplicity) from several options and this has become perpetuated. Not many authors write books by doing all the experiments themselves or by going to the ‘original literature’. In any case much of this work is from the late 19th or early 20th century and is confused because:
  • techniques of analysis left something to be desired, or perhaps there really are several possibilities from a given aqua ion depending on the conditions under which the reactions are performed. The situation is unlikely to become much clearer because:
  • research is expensive since someone has to do it, and the facilities and the people have to be paid for, and:
  • the problem is pretty unimportant in terms of the progress of science.

But it is quite important for you if you have to answer a question on it! I have tried to assemble here all of the answers which are likely to be acceptable. Firstly, though, a word on the nature of the ‘hydroxide’ precipitates generally and how you should/might write equations.

 

Transition metal hydroxides and the equations for their formation.

The simplest way for the Universe to have been constructed would be for all aqua ions to give the corresponding hydroxides when sodium hydroxide is added:

Mn+(aq) + nOH- (aq) M(OH)n (s)

Alas, it is not that simple.

The transition metal ions are the hexaaquo species [M(H2O)6]n+, octahedrally co-ordinated. The following three questions (answers shown) arise:

  • Does one start from this hexaqua species in any equation? Yes.
  • Do the water molecules remain in the hydroxide that is precipitated? Not for long.
  • Should these be shown in the equation? Probably better not, but it wouldn’t be marked wrong.

The precipitation of the hydroxide is a sequential process of removing protons from the ligand water molecules in an acid-base reaction. The precipitation of copper(II) hydroxide from aqueous copper(II) solutions is as good an example as any:

[Cu(H2O)6]2+ (aq) + OH- (aq)     [CuOH(H2O)5] + (aq) + H2O (l)

[CuOH(H2O)5] + (aq) + OH- (aq) [Cu(OH)2(H2O)4] (s) + H2O (l)

This is an acceptable answer to a question asking for the equation for the reaction between aqueous copper(II) ions and aqueosu hydroxide ions. However, the precipitate is very unlikely to be [Cu(OH)2(H2O)4]. So what to do?

The nature of the precipitate depends on how the reaction is performed.

If sodium hydroxide solution is added to copper(II) sulphate solution, the precipitate is greenish-blue and is probably CuSO4.3Cu(OH)2, that is the basic sulphate rather than the true hydroxide Cu(OH)2. Precipitates from solutions of copper(II) nitrate or copper(II) chloride give Cu(NO3)2.3Cu(OH)2 or CuCl2.3Cu(OH)2. This is NOT EXPECTED as an answer to an A level question – but these are the compounds formed. It explains why precipitates which are often given the same formula in textbooks do not actually look the same when produced in the laboratory.

If copper(II) sulphate solution is added to sodium hydroxide solution, the precipitate is blue and is gelatinous, and is probably Cu(OH)2. Addition of ammonia to a boiling solution of copper(II) sulphate until the green precipitate becomes blue, followed by washing, and then warming with sodium hydroxide solution will give crystalline Cu(OH)2.

The composition of the precipitate is in any case variable; many oxides and hydroxides are not exactly stoichiometric, that is their formulae are approximate. In addition there is the probability that the solid will occlude (i.e. contain in pockets in the solid lattice) some of the precipitating reagent, sodium hydroxide or ammonia; this is very difficult or impossible to eliminate (Partington 1926).

Why, then, do we not tell the truth in A level texts? Lots of people think that it is easy for students to become overburdened with information. I think that, although you should not expect to have to reproduce it in an exam, you should know something of the background to a very complex problem. As landscape. Any other approach is patronising. Indeed, too little information makes a problem harder. Some will strongly disagree – but you don’t have to read on! But please do….

The simple lesson? That the equations that you use to represent the precipitation of metal hydroxides from alkaline solution are inevitably approximate, because the composition of the precipitate is also approximate. So what to do in your answer?

A simple solution is to write, for copper(II),

[Cu(H2O)6]2+ (aq) + 2 OH- (aq) Cu(OH)2 (s) + 6 H2O (l),

a representation which will get you the credit. If, that is, the question is about copper! Similar versions for other aquo ions apply.

 


Chemistry contents        Home Page