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Old 12th April 2010, 08:19 PM   #1
davefoc
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Question about ammonia in water to solution

I ammonia an ion when it is dissolved in water?


I was the principle writer of the Wikipedia article on Jellyfish Lake and two of the sentences I wrote were these:
Quote:
The anoxic layer also contains high concentrations of ammonia and phosphate. These chemicals are almost completely absent from the upper layer.
The word chemicals was changed to ions by somebody who edited the page after I did. I have several questions about this:

1. Is it technically correct to call ammonia (NH3?) an ion when it is dissolved in water. I understand that salt (NaCl) disassociates when dissolved in water. Does ammonia do something similar?

2. If ammonia did disassociate in water what does it disassociate into?

3. Does dissolution in water generally involve some kind of disassociation? Would the disassociated components generally be called ions? For instance does glucose disassociate when dissolved in water or for the most part are there just complete glucose molecules floating in water in a glucose solution.

I have more questions but for the most part I don't know enough to ask good questions on this. I am really looking for just a little general information about the nature of solutes and in particular how should the sentences in the Jellyfish Lake article linked to above be worded to be correct.
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Old 12th April 2010, 08:49 PM   #2
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Originally Posted by davefoc View Post
I ammonia an ion when it is dissolved in water?


I was the principle writer of the Wikipedia article on Jellyfish Lake and two of the sentences I wrote were these:


The word chemicals was changed to ions by somebody who edited the page after I did. I have several questions about this:

1. Is it technically correct to call ammonia (NH3?) an ion when it is dissolved in water. I understand that salt (NaCl) disassociates when dissolved in water. Does ammonia do something similar?
It is incorrect to call ammonia an ion. It is a molecule.

Ammonia does not dissociate, but it does ionize in solution to a small degree. NH3 + H2O <-> NH4+ + OH-. The degree depends on conditions, but is unlikely to be above 5%.

Quote:
3. Does dissolution in water generally involve some kind of disassociation? Would the disassociated components generally be called ions? For instance does glucose disassociate when dissolved in water or for the most part are there just complete glucose molecules floating in water in a glucose solution.
Only for ionic compounds (and acids). Glucose and ammonia are neither.
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Old 12th April 2010, 11:25 PM   #3
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Maldach,
Thanks for the response. Could you confirm that I understand all this correctly?

1. Only a small percentage of the ammonia dissolved in water turns into ions so the use of the term "ammonia ions" is at least misleading if not flat out wrong in the sentence discussed in this thread.

2. Ammonia is a molecule and can not be an ion. At the point it ionized in water, neither individual part would be an ammonia ion. (What is the NH4+ called?)

3. The original sentence that used chemical was correct and the current sentence that uses the word ion is incorrect. (Is there some wording of the sentence that would be better?)

And a general question: When the concentration of a substance reaches the saturation point what is going on with the individual molecules? What causes the substance to come out of solution? Is the situation very different for a salt than something like glucose?
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Old 12th April 2010, 11:31 PM   #4
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Originally Posted by Madalch View Post
It is incorrect to call ammonia an ion. It is a molecule.
True. The ion is Ammonium

Originally Posted by Madalch View Post
Ammonia does not dissociate, but it does ionize in solution to a small degree. NH3 + H2O <-> NH4+ + OH-. The degree depends on conditions, but is unlikely to be above 5%.
Is this correct? I thought at neutral pH (like Jellyfish Lake) Ammonium was the predominant species?
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Old 12th April 2010, 11:36 PM   #5
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Originally Posted by davefoc View Post
3. The original sentence that used chemical was correct and the current sentence that uses the word ion is incorrect. (Is there some wording of the sentence that would be better?)
Chemical is correct. I'd use Madalch's word (Molecule) but that's just personal preference.
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Old 12th April 2010, 11:44 PM   #6
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Originally Posted by DogB View Post
True. The ion is Ammonium



Is this correct? I thought at neutral pH (like Jellyfish Lake) Ammonium was the predominant species?
Hmmm....if you simply add ammonia to water, it won't dissociate much, but enough to make the solution basic. If the solution is neutral, due to the presence of acids that neutralize the ammonia, then yes, it will be mostly ammonium ion. But then, you won't get much phosphate ion (it will be the dihydrogen phosphate ion and hydrogen phosphate ion) at a pH of 7.
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Old 13th April 2010, 10:12 AM   #7
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There is some equilibrium NH3 + H2O <-> NH4+ + OH-

Saying that a solution of ammonia exists mostly in the form of ammonium ions is equivalent to saying that this equlibrium is pushed far to the right.

If this was the case for a solution of ammonia in pure water it would have very nearly the same pH as a solution of sodium hydroxide of equivalent molarity; but this is not true, ammonia is a much weaker base than sodium hydroxide.

The base dissociation constant is defined as:
Kb= [NH4+][OH-]/[NH3]

Because you're dealing with differences that can be many orders of magnitude, it is customary to give pKb = -log(Kb). For ammonia pKb = 4.75 .

If have a very low concentration [OH-], as if for instance if you add some acid to the mix, you will necessarily have to have a much larger [NH4+] and a smaller [NH3] in order for the base dissociation constant to really be a constant. Without knowing the pH it is difficult to say how much of the ammonia is dissociated in the solution.

(I can feel my high-school chemistry class starting to come back to me now).
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Last edited by soylent; 13th April 2010 at 10:22 AM. Reason: ETA
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Old 13th April 2010, 05:22 PM   #8
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The anoxic layer is pH 6.9.
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Old 13th April 2010, 07:23 PM   #9
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Originally Posted by DogB View Post
The anoxic layer is pH 6.9.
At standard temperature and pressure p[H+] and p[OH-] sum to approximately 14. If p[H+] = 6.9 => p[OH-] = 7.1.

Taking the cologarithm of both sides of Kb = [NH4+][OH-]/[NH3] gives: pKb - p[OH-] = p([NH4+]/[NH3]).

Putting pKb = 4.75 and p[OH-] = 7.1 into the left side gives p([NH4+]/[NH3]) = -2.4 => [NH4+]/[NH3] = 10^2.4

A little over 200 ammonium ions per molecule of ammonia at STP, if I haven't done anything hideously wrong in the above.
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Old 13th April 2010, 07:43 PM   #10
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Originally Posted by soylent View Post
A little over 200 ammonium ions per molecule of ammonia at STP, if I haven't done anything hideously wrong in the above.
That jibes with my remembered ratios for a neutral environment.
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Old 16th April 2010, 02:06 PM   #11
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Thanks to everybody that responded. I changed the Jellyfish Lake article to read as follows:

Quote:
The anoxic layer also contains high concentrations of ammonia and phosphate which are almost completely absent from the upper layer.
It seems like it is technically correct now, but if somebody would like to comment on the grammar or anything else, your input would be appreciated. The word, which, and the word, that, are almost interchangeable but one requires a comma and one doesn't? I don't know but if anybody has any thoughts, I enjoy pedantic grammar discussions.

I added a section in the discussion section about the change:
http://en.wikipedia.org/wiki/Talk:Jellyfish_Lake

-Dave
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Old 19th April 2010, 02:00 AM   #12
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Would anybody mind answering a few more questions about water solutions?

I get the idea from some of the posts that with regard to dissolution in water there are five kinds of substances:
1. Salts
2. Bases
3. Acids
4. Substances that dissolve in water that aren't one of the above
5. Substances that don't dissolve in water

Is this right?

Could somebody talk about what it is about a molecule that causes it to be in one of the groups?

I know that as sugar dissolves in water the solution becomes more viscous. Is that true of all solutes in group 4?
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Old 19th April 2010, 03:44 AM   #13
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Originally Posted by davefoc View Post
Would anybody mind answering a few more questions about water solutions?

I get the idea from some of the posts that with regard to dissolution in water there are five kinds of substances:
1. Salts
2. Bases
3. Acids
There's an overlap between these three.

A salt is any substance that is not made up of molecules but of ions: positively charged cations and negatively charged anions. When dissolving in water, a salt dissolves into the individual ions. An example of a salt is table salt NaCl, which consists of Na+ and Cl- ions. In its pure form, the ions form a crystal structure; dissolved in water, each ion is individually surrounded by water molecules.

A base is any substance which, when dissolved in water, has OH- ions. In general, a base is a salt with such ions. Example: sodium hydroxide, NaOH, consisting of Na+ and OH- ions both in its pure form and when dissolved in water.

An acid is any substance which, when dissolved in water, produces H+ or rather H3O+ ions. In general, an acid in its pure form is not a salt but forms a molecule. Example: hydrogen chloride HCl, which when pure consists of HCl molecules but when dissolved in water breaks down completely into H+ and Cl- ions.
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Last edited by ddt; 19th April 2010 at 04:29 AM. Reason: see Evilgiraffe's post
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Old 19th April 2010, 03:47 AM   #14
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Originally Posted by ddt View Post
negatively charged cations and positively charged anions.
Pssst. It's the other way round.
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Old 19th April 2010, 04:30 AM   #15
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Originally Posted by Evilgiraffe View Post
Pssst. It's the other way round.
What do you mean? I did write it correctly.

Yes, I edited it after your post. Thanks for correcting, BTW!
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Old 19th April 2010, 04:54 AM   #16
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No problem.

Like most things in electrochemistry, it's all backasswards. Cations are so named because they migrate to the cathode when current is passed through the solution. This obviously means that they're positively charged otherwise they wouldn't go to the negative cathode. However the etymology of the name causes confusion.
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Old 19th April 2010, 10:46 AM   #17
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Originally Posted by davefoc View Post
Would anybody mind answering a few more questions about water solutions?

I get the idea from some of the posts that with regard to dissolution in water there are five kinds of substances:
1. Salts
2. Bases
3. Acids
4. Substances that dissolve in water that aren't one of the above
5. Substances that don't dissolve in water

Is this right?

Could somebody talk about what it is about a molecule that causes it to be in one of the groups?

I know that as sugar dissolves in water the solution becomes more viscous. Is that true of all solutes in group 4?
There are two main kinds of compound- ionic compounds ("salts") and molecular ones. Some of these will dissolve in water, and some of them won't.

Salts can be acidic, basic, or neutral. If they contain the ammonium cation (NH4+), a highly-charged, small cation (such as Zn2+, Fe3+, or Al3+), or an acid anion like HSO4-, they will be acidic. If they contain hydroxide or oxide ions, or any anion that is the conjugate base of a weak acid (carbonate, bicarbonate, sulphide, fluoride, etc), they will be basic. If they contain neither (such as NaCl, KNO3, MgI2, etc), they will be neutral.

Molecular compounds can also be acidic, basic or neutral. Ammonia is a basic molecule- it can react with water to give ammonium ion and hydroxide ion (in small concentrations). Hydrogen fluoride and acetic acid are molecular compounds which are weak acids. Things like ethanol and sucrose are neutral.

The change in viscosity of a solution depends on the solute. If the solute itself is viscous when liquid (such as sugar or glycerol), it will increase the viscosity of the solution. If it isn't, it will decrease it (ethanol and methanol, for example). I don't know about ionic compounds (although these are quite viscous in extremely concentrated solutions).
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