3.1.3 (g) Characteristic reactions of Halide ions
Syllabus
(g) the precipitation reactions, including ionic equations, of the aqueous anions Cl –, Br– and I– with aqueous Silver ions, followed by aqueous Ammonia, and their use as a test for different Halide ions.
{Complexes with ammonia are not required other than observations.}
What does this mean?
Precipitates and Ionic Equations
The test for Halide ions is another GCSE topic but at A level it goes a little deeper.
You may recall that Chloride, Bromide and Iodide ions all form a precipitate (insoluble solid) when Silver Nitrate is added.
You may even recall that the ppts are white, cream and yellow respectively.
And the Fluoride does not precipitate.
There's some chance that you recall the ionic equations for these precipitation reactions.
Although GCSE students often object to the idea of ionic equations.
The point is that it doesn't matter if you test Sodium Chloride, Potassium Chloride, Lithium Chloride or any other soluble Chloride.
Only the Chloride part of the substance ends up in the ppt.
The rest remains dissolved and the same is true of the Nitrate part of the Silver Nitrate.
They are spectator ions.
We could write a test for Chloride ions in Sodium chloride as:
NaCl(aq) + AgNO3(aq) → NaNO3(aq) + AgCl(s)
But actually both reactants are dissolved so they are not actually bonded to each other. It would be better to write:
Na+(aq) + Cl-(aq) + Ag+(aq)+ NO3-(aq) → Na+(aq)+ AgCl(s)+ NO3-(aq)
And if we remove ions that didn't change:
Na+(aq) + Cl-(aq) + Ag+(aq)+ NO3-(aq) → Na+(aq)+ AgCl(s)+ NO3-(aq)
We get :
Cl-(aq) + Ag+(aq)→ AgCl(s)
And obviously, Br-(aq) + Ag+(aq)→ AgBr(s) and l-(aq) + Ag+(aq)→ Agl(s)
Distinguishing between precipitates.
Usually, if you do all three tests you can easily tell the difference between the yellow Iodide precipitate and the other two.
It can be harder to distinguish between cream and white.
Especially if you have only tested one or the samples.
And what would you do with a sample that contained a mixture of halide ions?
We need a follow up tests that confirms our suspicions.
So, if you think your precipitate is white (but could be cream) you should add dilute Ammonia solution.
If it is really pure Chloride then it will all dissolve.
Or if it's a mixture at least some of it will dissolve and some will not no matter how much dilute Ammonia you add.
If it is a pure Bromide then it should look cream - that could be a mixture of white and yellow (Chloride and Iodide).
If you added dilute Ammonia and there was no change then you at least know it doesn't contain Chloride.
But maybe your cream contains a small amount of yellow Iodide.
If you add concentrated Ammonia then the Bromide should dissolve but any Iodide would not.
Be able to reproduce something like this in an exam.
Acidifying the solution
It doesn't mention this on the syllabus but the Halide tests are carried out after acidifying with Nitric Acid.
This is because solutions can absorb Carbon Dioxide to form Carbonate ions over time.
CO2(g) + H2O(l) → CO32-(aq) + 2H+(aq)
And the Silver ions we use to test for Halides also precipitate with Carbonate ions
2Ag+(aq) + CO32-(aq) → AgCO3(s)
We couldn't use Hydrochloric Acid to acidify due to the extra Chloride ions.
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