Sciencemadness Discussion Board - NH3 (aq) or NH4OH

NH3 (aq) or NH4OH

Mirage - 5-2-2012 at 09:22

So right now, I am trying to figure out weather aqueous ammonia is just an auqeous solution of it, like it should be, or weather it is actually ammonium hydroxide ( which doesn't make much sense). I read in an ancient chem text, that ammonia (household) is really a week solution of NH4OH. I think that that information may be outdated. So aqueous household ammonia is a base, so that means that it must contain an -OH ion. Correct? (I know...Lewis acid blah blah blah...let's
keep it basic

) But isn't gaseous ammonia NH3 a base?

Chemically yours
Mirage

[Edited on 5-2-2012 by Mirage]

Hexavalent - 5-2-2012 at 12:43

When ammonia dissolves in water it forms ammonium hydroxide, NH4OH. As a gas, ammonia has the formula NH3.

Pulverulescent - 5-2-2012 at 13:19

Some partial ionisation occurs so it is both solution and base . . .

P

Mirage - 5-2-2012 at 16:09

Ammonia isn't ionic. Correct? The equation would look a bit like an acid dissasociating...correct? What would it be...
NH3 + H2O <---> NH4+ + OH-

What side would be more favored, I'm expecting the aqueous ammonia to be more favored.

Thanks
Mirage

[Edited on 6-2-2012 by Mirage]

Engager - 5-2-2012 at 18:26

Ammonia in solution is a weak base, and mostly present in undissociated form. Equilibrium constant for reaction NH3 + H2O → NH4(+) + OH(−

is 1,8×10−5 at normal conditions, meaning equilibrium is shifted to the left side of eq. above. Aqueous solution of NH3 contains both dissolved NH3 (most part) as well as dissociated species (weak base). Gaseous ammonia isn't the base because terms of acid and base are determined only for solvated environment where ions of H+ or OH- can be formed by iteraction of substance with solvent (solvatation), gas phase at normal conditions can not contain ions so term base can't be used for ammonia in gas phase.

Mirage - 5-2-2012 at 20:19

I see. well thank you everyone for your help.

Quote:

Ammonia in solution is a weak base, and mostly present in undissociated form. Equilibrium constant for reaction NH3 + H2O → NH4(+) + OH(− is 1,8×10−5 at normal conditions, meaning equilibrium is shifted to the left side of eq. above. Aqueous solution of NH3 contains both dissolved NH3 (most part) as well as dissociated species (weak base). Gaseous ammonia isn't the base because terms of acid and base are determined only for solvated environment where ions of H+ or OH- can be formed by iteraction of substance with solvent (solvatation), gas phase at normal conditions can not contain ions so term base can't be used for ammonia in gas phase.

unionised - 6-2-2012 at 11:19

Quote: Originally posted by Hexavalent

When ammonia dissolves in water it forms ammonium hydroxide, NH4OH. As a gas, ammonia has the formula NH3.

Not really, no.
Most of the ammonia is just dissolved.
Some proton transfer happens so there are a few (about 10 in a million) NH4+ ions, but these are not associated with any particular OH- ions so there's no real NH4OH present.

Mirage - 6-2-2012 at 12:01

Okay, but if then if any NH4+ forms then the corresponding OH- must form. But what your saying is, that it forms in such small amounts that it's like trace compounds.

Mirage

Quote: Originally posted by unionised

Quote: Originally posted by Hexavalent

When ammonia dissolves in water it forms ammonium hydroxide, NH4OH. As a gas, ammonia has the formula NH3.

Nicodem - 6-2-2012 at 12:20

Quote: Originally posted by Mirage

Okay, but if then if any NH4+ forms then the corresponding OH- must form. But what your saying is, that it forms in such small amounts that it's like trace compounds.

What he is saying that there is no such thing as NH4OH. The pKa of the ammonium cation in water is about 8.9, so there is a very, very unfavorable equilibrium for its formation. Yet, that little that forms can only exist as solvated NH4+ cations and not as ammonium hydroxide which is an unknown compound anyway. The name "ammonium hydroxide" is an old trivial name that was used in some countries for aqueous solutions of ammonia - it is not a name for a compound corresponding to the actual empirical formula of NH4OH. I hope this is clear enough.

Mirage - 6-2-2012 at 16:49

Crystal clear! Finally! Thanks for everyone's help!

Mirage

Quote: Originally posted by Nicodem

Quote: Originally posted by Mirage

Okay, but if then if any NH4+ forms then the corresponding OH- must form. But what your saying is, that it forms in such small amounts that it's like trace compounds.

GreenD - 6-2-2012 at 17:01

Wow I really thought NH4OH was the main constituent of NH3+H2O...

Interesting.

bahamuth - 6-2-2012 at 17:47

Was told by one of my chemistry teachers that ammonium hydroxide does not exist, and is just a simplification (as mentioned by Nicodem).

Ammonium hydroxide does not exist (Read the letter/abstract)
Ammonia and "ammonium hydroxide" (I have no acces, this is just the article referred to in the above letter/abstract)

AirCowPeaCock - 6-2-2012 at 17:49

Why the hell would NH4+ form but not OH-?! Where is it getting its hydrogen? Other ammonia? Like NH4+ NH2-?

turd - 7-2-2012 at 01:26

spam reported.

UKnowNotWatUDo - 7-2-2012 at 02:02

Entropy51, so many of your posts lately have been made up of snide, rude, and insulting remarks toward other members. I don't know what your problem is, but a little civility isn't too much to ask for. It's one thing to disagree with someone, but its a whole other thing to simply post for the sake of being a complete and utter asshole.

[Edited on 2/7/2012 by UKnowNotWatUDo]

woelen - 7-2-2012 at 02:57

@entropy51: :mad:

Stop digging up old and unrelated quotes! GreenD only wrote about the NH3+H2O in this thread. What is written in other threads by him is NOT relevant for this thread. I expected better from you :mad:

woelen - 7-2-2012 at 03:04

Quote: Originally posted by AirCowPeaCock

Why the hell would NH4+ form but not OH-?! Where is it getting its hydrogen? Other ammonia? Like NH4+ NH2-?

What unionised says is that each NH4(+) ion is not associated to a particular OH(-) ion. You cannot say THIS NH4(+) ion is bound to/associated with THAT OH(-) ion. Of course, for each NH4(+) there also is one OH(-). There will be no NH2(-) in solution.

Just look at it as when you dissolve NaCl in water. In the solid, each Na(+) ion is surrounded by a few Cl(-) ions, and these ions are fixed at their position and there is an association between ions. If you could label each ion (e.g. with a name), then you could say that ion with name A is next to ion with name B and this remains true as long as the solid exists. In solution, however, the Na(+)/Cl(-) lattice breaks up and the ions can move through the liquid independent of each other. Ion A can for instance move to one end of the beaker, while ion B moves to the other end of the beaker. You only can say that for each Na(+) ion there is one Cl(-) in solution, but you cannot associate couples of ions to each other.

GreenD - 7-2-2012 at 06:55

Thanks for the response on my behalf. The connections he is making in his post are far from reality.

It was not my choice to label a NH3 aq solution as sodium hydroxide.

So what would the pH of a 30-35% NH3 (aq) solution be?

ScienceSquirrel - 7-2-2012 at 07:12

From this data here, I would guess at just over 12

http://www.airgasspecialtyproducts.com/Libraries/Aqua_Ammoni...

woelen - 7-2-2012 at 07:32

Yes, pH will be around 12. Normal household ammonia has a pH around 11. This means that the concentration of OH(-) is really low, even in concentrated solutions, in the order of 0.001 M to 0.01 M. The concentration of NH3 is around 3 M for normal household ammonia and the concentrated stuff has around 15 M, or even higher. This nicely shows how weak a base ammonia is.

GreenD - 7-2-2012 at 07:33

I think I understand. We are not debating that there is a large portion of NH4(+) molecules in solution, right? (.01-.0001M)

We are debating that the molecule NH4OH exists.

NOW this makes sense. I thought everyone was trying to tell me that NH4+ is not being formed.

but it just the complex NH4OH ! Right?

This would be the same argument that H4O2 doesn't exist, right - H3O+/OH- molecule?

[Edited on 7-2-2012 by GreenD]

Vikascoder - 7-2-2012 at 08:26

Ph refers to power of hydronium ion even the diluted naoh have ph of 14 but its concentration is low so why is the difference of 12 to 11 in ammonia

ScienceSquirrel - 7-2-2012 at 09:15

Quote: Originally posted by Vikascoder

Ph refers to power of hydronium ion even the diluted naoh have ph of 14 but its concentration is low so why is the difference of 12 to 11 in ammonia

Sodium hydroxide is highly dissociated into solvated sodium and hydroxide ions in water so strong solutions are all around pH14 and even quite dilute solutions are still strong bases, it is only very dilute soltions that are substantially less than that.

Ammonia solutions are quite the opposite, dilute solutions are mainly dissolved ammonia with a small proportion of ammonium and hydroxide ions, concentrated ammonia solutions contain much more dissolved ammonia but not much more ammonium and hydroxide ions.

I have had a look at this Wikipedia article and I think it is a good explanation;

http://en.wikipedia.org/wiki/PH

arsphenamine - 11-2-2012 at 12:41

It looks like the hydrogen bond in un-dissociated NH4OH is longer than usual because of the weaker coulombic attraction by the nitrogen.

At the MP2/6-311G(3df2p) theory+basis level, the model also doesn't permit a straight line H-bond between the N and O, unlike in water.

Here is a model of the HOMO-2 molecular orbitals that show the essential connection and geometry.

Mirage - 11-2-2012 at 17:14

You lost me at "Columbia attraction"

...

Quote: Originally posted by arsphenamine

It looks like the hydrogen bond in un-dissociated NH4OH is longer than usual because of the weaker coulombic attraction by the nitrogen.

At the MP2/6-311G(3df2p) theory+basis level, the model also doesn't permit a straight line H-bond between the N and O, unlike in water.

Here is a model of the HOMO-2 molecular orbitals that show the essential connection and geometry.

arsphenamine - 11-2-2012 at 23:24

Quote: Originally posted by Mirage

You lost me at "Columbia attraction"

...

Oh well.

The main thing is that it gives us some more confirmation that
ammonia and water are happy to form a weak ionic bond,
and even produces an entertaining little image in the process.

Mirage - 12-2-2012 at 08:29

Cool, alright, thanks for all your effort! Nice pic.

Mirage

Quote: Originally posted by arsphenamine

Quote: Originally posted by Mirage

You lost me at "Columbia attraction"

...

Oh well.

The main thing is that it gives us some more confirmation that
ammonia and water are happy to form a weak ionic bond,
and even produces an entertaining little image in the process.