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Chemistry Forums for Students => Inorganic Chemistry Forum => Topic started by: shorae_chen on September 21, 2006, 12:07:28 PM

Title: AgNO3 + NH3 = ??
Post by: shorae_chen on September 21, 2006, 12:07:28 PM
when i add drop-wise 2mL of very dilute NH3 solution to 1mL of AgNO3 solution. i got a dirty brown precipitate when i add one drop of dilute NH3 solution. but when i add the 2nd drop, the ppt dissolved. what is the compound formed ? which is the acid and base involved in this reaction?? thanks alot :)
Title: Re: AgNO3 + NH3 = ??
Post by: Alberto_Kravina on September 21, 2006, 01:21:16 PM
when i add drop-wise 2mL of very dilute NH3 solution to 1mL of AgNO3 solution. i got a dirty brown precipitate when i add one drop of dilute NH3 solution. but when i add the 2nd drop, the ppt dissolved. what is the compound formed ? which is the acid and base involved in this reaction?? thanks alot :)
Since you did the reaction by yourself and as I see you made very good observations and described them perfectly I think that it is VERY easy to write the equation starting with the reagents.
Title: Re: AgNO3 + NH3 = ??
Post by: Albert on September 22, 2006, 04:15:02 AM
which is the acid and base involved in this reaction?? thanks alot :)

Title: Re: AgNO3 + NH3 = ??
Post by: hmx9123 on September 27, 2006, 01:20:17 AM
Here's a hint: how else can you write NH3 when it's in aqueous solution?
Title: Re: AgNO3 + NH3 = ??
Post by: woelen on September 29, 2006, 03:37:57 AM
This reaction  is less trivial than one would expect.

At first, you get a precipitate. The NH3 forms NH4(+) and OH(-). The OH(-) reacts with the Ag(+).

When more NH3 is added, then a complex is formed. Ag(+) ions form a fairly stable complex with NH3, this complex is [Ag(NH3)2](+). So, any precipitate formed redissolves again, but now the Ag(+) is locked up in the complex.

When the solution is allowed to stand for a long time, and when the concentration is sufficiently high (or the pH is suffciently high), then the complex [Ag(NH3)2](+) decomposes, giving highly complicated macroscopic structures, with Ag(+), coordinated to the nitrogens of the ammonia, but with hydrogen atoms replaced by Ag. A complicated non-stoichiometirc compound is formed, which sometimes is called "silver nitride", but that name by no means precisely describes the nature of this compound. Anyways, the compound formed in this solution is VERY explosive and for this reason one should not store solutions with the [Ag(NH3)2](+) ion in it.
Title: Re: AgNO3 + NH3 = ??
Post by: Borek on September 29, 2006, 04:21:17 AM
At first, you get a precipitate. The NH3 forms NH4(+) and OH(-). The OH(-) reacts with the Ag(+).

AFAIR to add to the system complexicity this AgOH precipitate is not stable and decomposes, so when you add only some NH3 and let it stand for some time, adding excess NH3 later may not dissolve the initial precipitate completely.

Someone check me on this.
Title: Re: AgNO3 + NH3 = ??
Post by: hmx9123 on October 01, 2006, 02:23:47 PM
Woelen, aren't you thinking about fulminating silver (i.e., like what you get from Tollen's Test)?

That is:

"A black crystalline substance, Ag2O.(NH3)2. When dry it explodes violently on the slightest percussion."

Not to be confused with:

"Silver fulminate, a white crystalline substance, Ag2C2N2O2; also called fulminate of silver. When dry it is violently explosive."

Both compounds are extremely unstable--the former can explode simply upon standing and is extremely dangerous.  The latter is also extremely dangerous although it is less likely to explode upon standing.
Title: Re: AgNO3 + NH3 = ??
Post by: woelen on October 04, 2006, 04:29:10 AM
I indeed was thinking of fulminating silver. That formula, which you give, however, certainly is not the precise composition of this stuff. In fact, the precise composition of this compound cannot be easily written down in a single formula, it is a complex mess of silver(I) ions, oxide, hydroxide, ammonia, amide, nitride, and water groups.

I have also seen references like Ag3N.xH2O and Ag3N.yAgNH2.zH2O. One thing we at least agree upon, and that is the extremely explosive nature of this stuff  ;D.
Title: Re: AgNO3 + NH3 = ??
Post by: hmx9123 on October 04, 2006, 04:40:03 AM
As far as I know, the exact formula is undetermined due to the extreme reactivity (i.e., explosive character) of the material.  I didn't write the stuff in quotes, I just did a quick google search because I was too lazy to type anything.  The best idea of what it's comprised of would include a synthesis for the material, but I don't really think that's responsible to post in this discussion.
Title: Re: AgNO3 + NH3 = ??
Post by: AWK on October 04, 2006, 05:07:56 AM
see:
Jenkins, Ian D.
Tollens's test, fulminating silver, and silver fulminate
JCE, 1987, 164

Title: Re: AgNO3 + NH3 = ??
Post by: hmx9123 on October 05, 2006, 01:53:53 AM
That's a nice little article.  Seems that it's Ag3N and Ag[sub2[/sub]NH.  Davis states in his 1943 text:

"Fulminating gold, silver, and platinum (Latin, fulmen, lightning flash, thunderbolt) are formed by [ ... ].  They are perhaps nitrides or hydrated nitrides, or perhaps contain hydrogen as well as nitrogen and water of composition, but they contain no carbon and must not be confused with the fulminates which are salts of fulminic acid, HONC.  They are dangerously sensitive, and are not suited to practical use."