h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

This is annoying. I'm supposed to be testing for anions and cations in solution and to 'prove' I can work at 'level 3' include my equations.

And I feel hopelessly ill-prepared for this task for example.

I have sodium chloride in solution NaCl (aq)

I add silver nitrate AgNO3(aq)

and I *think* I get NaNo3(aq) + AgCl(s)


Now here is where I get *REALLY* confused. //




I add in some dilute ammonium hydroxide NH4OH

The effect is supposed to disolve the solid precipitate from the last round.
Therefore I'm going to ASSUME that the NaNO3 isn't part of this equation and passes through unaffected.

NaNo3 + AgCl + NH4OH ----> ? ? NaNO3


Now if my valencys are correct AG is +1 charge and Cl is -1 charge, as NH4 is +1 so OH is -1

So therefore I should get:


NaNo3(aq) + AgCl(s) + NH4OH(aq) ----> NH4Cl(aq) AgOH(aq) NaNO3(aq)


My question: IS THAT RIGHT???

You shouldn't normally include your solvent (ammonium hydroxide) in an overall chemical reaction, unless you are writing ionic equations.

So, yes, NaCl(aq) + AgNO3(aq) -> NaNO3(aq) + AgCl(s) is correct.

You DISSOLVE the AgCl in ammonium hydroxide - no chemical reaction has taken place, so no equation is appropriate (unless you want to show the dissolution of ions).

The question now is why have you dissolved it in ammonium hydroxide? What is the next step?

B

Oh, unless you've just dissolved it to prove that you have made a silver nitrate precipitate. In which case, job done.

B

Sorry MB , was away having dinner.

Yes, these are ionic equations (anions and cations), I should have made that clear.

The background is we're trying to solve a poisoning at a restaurant, and the contaminant is in (conveniently) the salt-cellars.

So the tests we are doing are qualitative, attempting to establish the presence of sodium chloride and an unknown contaminant (using flame solution tests)

Flame test found both cations but the anions are giving me grief.

Some of the anion solution tests are dismissible by colour, however at least 3 produce a white precipitate. (Cl, Co3 , So4) which *all* dissolve in dilute Ammonium Hydroxide, except S04 which eliminates it. (so yes, I think that's what you mean proving we have a silver nitrate precipitate)

The next step is to add Barium chloride to a new sample solution which will eliminate one of the other options (chlorine or carbonate- it won't react with the chlorine)

Yes, the barium chloride will identify sulphates, and you can test for carbonates by reacting with some 2M HCl, producing carbon dioxide gas.

I don't see why it would be necessary for you to write out the equation for the dissolving in NH3OH. The process is the formation of a silver-ammonium complex molecule, which I can't really see as being relevant.

Incidentally, the best way to test for silver chloride is to leave it on a sunny windowsill for half an hour and watch it change colour.

OK, it's not the best way, but it is the most interesting.

B

now it's been some time since I've seen all this but ..

Ag forms a soluble complex ion with ammonia, doesn't it? so Silver Chloride would dissolve in ammonia? right?

don't make me search for my chemistry books .. they've been resting peacefully in the attic for well over thirty years

and lead chloride doesn't dissolve in ammonia if I remember correctly

No, but you test for lead chloride the same way, by reacting with silver nitrate to make silver chloride.

It may be best to clarify, Clive, but I would consider a detailed knowledge the mechanics of the dissolving to be completely superfluous to an understanding of this sort of work. Complex formation is only briefly touched on at Level 3 these days.

B

Thanks guys.

>>It may be best to clarify, Clive, but I would consider a detailed knowledge the mechanics of the dissolving to be completely superfluous to an understanding of this sort of work. <<

The teacher in this instance has been very explicit, not equations = no pass.

I've five more experiments like to this to grapple with ( )


just to reiterate, the experiment is find the contaminent in the salt sellar.

Used a flame test to identify sodium, also identified potassium (lilac flame)

Then added another quantity of the sample to deionised water to make solution.

To this added the silver nitrate, produced a white precipitate.

Made up another solution added barium chloride, produced another white precipitate (finer, denser) establsihign sulphate.

The chlorine doesn't react with the barium so figured that meant we identified the chlorine the first time as well as the sulphate, and the fine white precipitate was the the sul[hate on it's own.

Conclusion: Sodium Chloride with Potassium Sulphate.



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Now as it happens I know subsequently that that answer is completely wrong and all my results are down to contamination (the lilac flame) and the imprecision of qualitative data (one kind of white precipitate turns out looks much like another) but I'm suppose to write up the results I have and say why I got the ones I did because apparently falsifying your results is like bad or something....

AgCl + 2NH3 ---> [Ag(NH3)2]Cl

or if they really do want it as ammonium hydroxide (which is just a solution of ammonia in water)...

AgCl + 2NH4OH ----> [Ag(NH3)2]Cl + 2H2O

As stated above by someone Ag+ forms a linear divalent complex with NH3 which is a stable species in solution, so an equation is appropriate.

I believe steps are afoot within the country to try and get school chemistry out of the 19th century but I see it's not there yet.*

Of course, in reality nowadays one would just stick the salt through a combination of spectrometers - probably a flame photometer and a mass spectrometer and the job would be done.


*Yes I do appreciate that this probably teaches you more about chemistry than that but this stuff really is out of the dark ages.

I think the point is to tech us on one level how unreliable diagnosis by sight is, but okay as a guide.

Just sorting out breakfast then onto round two.

A identifying a mixture of Potassium Chloride and Sulphate then Iron Sulphate and Copper (II) Sulphate.

I'll scream if I need help.

Still not entirely sure about the equation from last night Worry about that one later....

>>Still not entirely sure about the equation from last night Worry about that one later....<<

I should elaborate - because I'm dealing with unknown contaminants, how do I include those in the equation, or do I?

AgOH is not right. Silver will form silver oxide from that

2AgOH --> Ag2O + H2O


formation of the complex is the correct equation for when you add ammonium hydroxide to precipitated AgCl

Ah.


so instead of


NaNo3(aq) + AgCl(s) + NH4OH(aq) ----> NH4Cl(aq) + AgOH(aq) + NaNO3(aq)


hould it read:

NaNo3(aq) + AgCl(s) + NH4OH(aq) ----> NH4Cl(aq) + 2AgOH(aq) + NaNO3(aq)


then


NaNo3(aq) + AgCl(s) + NH4OH(aq) ----> NH4Cl(aq) + Ag2O(aq)+ H2O(aq) + NaNO3(aq)

No.

NaNO3(aq) + AgCl(s) + 2NH4OH(aq) ---> [Ag(NH3)2]Cl(aq) + 2H2O + NaNO3(aq)

But as you can see NaNO3 doesn't do anything in this reaction and so is what we call a spectator.

You can therefore knock it out of the equation and it becomes what I put in a previous post.

Sorry I missed post 12.

I'm not sure I follow it though - if contaminants are unknown, how can you include them in a quantifiable equation? At least until they become known (from your qualitative tests).

>>if contaminants are unknown, how can you include them in a quantifiable equation?<<

That was the question that was bugging me last night too. Does it appear on the right but not on the left. Is it some form of algebra?

Worse, the chemical I thought the ppt was - was wrong, so my equation will be chemical nonsense.

CSI!!!!!!

Mass Spec

job done

Just want to think through a problem of organisation.


Which of these is correct for describing solubility of the precipitate?


NaNO3(aq) + AgCl(s) + NH4OH(aq) -----> NaNo3(aq) + Ag(aq) + Cl(aq) + NH4OH(aq)


OR:

NaNO3(aq) + AgCl(s) + NH4OH(aq) -----> NaNo3(aq) + AgOH(aq) + NH4Cl(aq)


Thanks.