Basic, Acidic, or Neutral

You are definitely thinking backwards, except your "neutral" reasoning. If you need more help, just ask.

 

You should consider hydrolysis of the salts .You are way off the mark
regards
poolwin2001

 

[tex]Ch_3COONa[/tex]Think again
Logically,[tex]CH_3COOH[/tex]is a weak acid and [tex]NaOH [/tex]is a strong base.SO How should their salt behave?
regards
poolwin2001

 

Hello

Tetramethylammonium chloride is produced from tetramethylammonium hydroxide and hydrochloric acid. The base is way weak than sodium hydroxide, so the salt should be acidic when hydrolyzed.

In sodium acetate, it converts to sodium hydroxide and acetic acid upon hydrolysis.

 

I think that the tetramethylammonium chloride one will be neutral. Cl- is too weak a base to significantly deprotonate water and tetramethylammonium ions are hardly acidic at all.

 

I don't think so, dear Movies. Please look at the hydrolysis equation: An acid/base pair occurs, and please compare the relative powers. Tetramethylammonium hydroxide is a powerful base for non-aquatic systems, that's correct. But we are in an aqueous solution, so the base should not be that powerful. Am I thinking correctly?

[tex]Me_4N^+Cl^- + H_2O \leftrightharpoons Me_4N^+OH^- + HCl[/tex]

 

Yes, I think so, since hydrolysis causes smaller inorganic molecules to go out. It is the preferred way already.

 

When in doubt, try it.

I found a bottle of tetramethylammonium chloride and made up a solution of it. The pH of the deionized water in our lab is 6 and the pH of the tetramethylammoium chloride solution was 6 as well.

In aqueous solutions ions like Cl- and Me4N+ will be completely dissociated, but there is no impetus for the dissociation of water in this case. Tetramethylammonium hydroxide would be a strong base in aqueous solution as it would be in an organic solution. The same is true for HCl. The equilibrium that you drew above is certainly a valid one, but the equilibrium is almost certainly shifted almost entirely to the left because on the right you have a strong acid (HCl) with a strong base (OH-).

Does this make sense?

 

Yes, this makes sense. If you keep the solution for a period there might be some differences in pH. You are right about the beginning; but since hydrochloric acid is an inorganic molecule, and all systems favor inorganic molecules to go out so that the overall yield can be greater, I think the overall pH would be -at least- slightly lower than 6. Hydrolysis is inevitable; old solutions of lead(II) nitrate always contain some precipitates of lead(II) hydroxide, so we put some nitric acid to avoid precipitations.

To sum up, until the equilibrium belonging to tetramethylammonium chloride in aueous solution is reached, the pH will seem to be neutral, after that, I presume that it will be acidic.

Thank you for your feedback, chem_tr

 

Nautica - yes, those are correct.

chem_tr - Although HCl is an inorganic molecule, it won't precipitate from an aqueous solution because it has extremely high solubility. This, therefore, can't be a driving force for the equilibrium you drew. Lead(II) is a very different ion than H+!

 

added a link

Dear Movies, I think there is a misunderstanding here. Nautica's question has already answered, tetramethylammonium chloride (TMAC) alone may be a neutral substance due to the facts you explained, but when subjected to water, hydrolysis will make some changes to this compound, therefore an acid and base will be obtained at the end. Precipitates are the easiest way to show this change, if any.

Note that I added a link reachable by clicking TMAH above, and there reads that 1) pH is changeable in the range 4-10, and 2) the compound is very hygroscopic. That's why it shows an irregular pH change and reactivity to water.

In my opinion, the higher ionic the products, the more the reaction tends to shift to that side. As water is not ionic in normal circumstances, I presume that the reaction will, eventually, shift to the other side in which four ions are present. That's my reasoning, but please report the result of your pH measurement, I mean, after waiting for a long time to see if any change occurred.

 

Chem_tr, adding TMAC to water will cause dissociation into tetramethylammonium ions and chloride ions, agreed? (Note that the link you provided indicates that this substance is soluble in water). In order for an equilibrium to be driven by precipitation one of the material has to be either insoluble or only slightly soluble in the solvent. However, both tetramethylammonium hydroxide and HCl are completely soluble in water (see their respective MSDS for verification:
http://www.jtbaker.com/msds/englishhtml/T1814.htm
http://www.jtbaker.com/msds/englishhtml/H3880.htm )

So there is no driving force in terms of precipitation of ionic products. Your example with lead(II) hydroxide precipitation is different since lead hydroxide is only slightly soluble in water (Ksp = 1.2 * 10^-5, according to http://www.csudh.edu/oliver/chemdata/data-ksp.htm ). When the ionic products are highly soluble it's like saying that an aqueous solution of NaCl will eventually form NaOH and HCl because they are ionic, whereas anyone who has mixed aqueous HCl and aqueous NaOH knows that the opposite is true.

I hope that this clears up any confusion.

 

Thank you for your response, it is clear now. I thought that TMAC is an acidic salt like ammonium chloride. My example on precipitation was only to show that hydrolysis changes some compounds after a long time, it is invalid for TMAC-TMAH conversion. Thank you.

 

So finally why is (Ch3)4NCl neutral
Won't hydrolysis take place like tr said:
[tex]Me_4N^+Cl^- + H_2O \leftrightharpoons Me_4N^+OH^- + HCl[/tex]

Why would you consider pptation ?
Please Explain clearly.
I am still in high shool.

 

If you can precipitate a salt then you can drive the equilibrium because ionic salts tend to be very stable. An example would be the lead(II) hydroxide precipitate that chem_tr described. A precipitate will be much less reactive than a species in solution. Since tetramethylammonium hydroxide and HCl are both completely soluble in water, there will never be a solid precipitate and the right hand side of the equilibrium will be better represented as four ions, a tetramethylammonium cataion, a hydroxide anion, an H+, and a Cl-. Whenever you have a significant amount of H+ and OH- in the same aqueous solution it will almost instantly neutralize to form water, which is very stable.

I'm not sure that I have answered you question to your satisfaction, I hope that I have. Perhaps it would help to say that the (CH3)4NCl is completely dissociated in aqueous solution to form tetramethylammonium cations and chloride anions which are both very well solvated by water molecules.

In the case of NH4Cl there is a proton available from the ammonium cation which makes the solution acidic; there is no such proton available with (CH3)4NCl.

Let me know if this still doesn't make sense.


chem_tr, my thanks for not turning this discussion into a flame war. It is very refreshing to have a cordial discourse.

 

No, you are not wrong, Nautica. As movies has explained, the basicity is as strong as NaOH, KOH, etc.

I can add some organic-chem-based approach; which involves hyperconjugation. Since alkyl groups do a positive inductive effect on the central atom (here, ammonium nitrogen), there is a further electron flow through the central atom, so hydroxide ion does not want to be in the "neighborhood" of the electronically-enhanced-cation, making it very basic.

You can even increase the basicity of these kind of compounds by using higher homologues, such as butyl; tetrabutylammonium hydroxide is a well known non-aqueous base, very soluble in water and organic mediam, thus a phase transfer catalyst at the same time.