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Why do some form precipitate?

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Guest The X

If I add regular table salt to water, it dissolves in the water. However, if I add Silver Chloride to water, is just precipitates. Why the difference?
I've been discussing this with my chemistry teacher, and it seems that she can't remember, having only just accepted that "that's the way it is". But we're both curious now, and that's why I'm asking. We've considered both electronegativities and ionic radius, but cannot seem to find an answer.

An other interesting question I've got for you, is why HCl dissociates completely. Why should not CH[sub]3[/sub]COOH do so?
Does CH[sub]3[/sub]COOCl (Chlorine Acetate/Ethanoate) do so?

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[quote name='The X' post='16091' date='May 5 2008, 02:26 PM']If I add regular table salt to water, it dissolves in the water. However, if I add Silver Chloride to water, is just precipitates. Why the difference?
I've been discussing this with my chemistry teacher, and it seems that she can't remember, having only just accepted that "that's the way it is". But we're both curious now, and that's why I'm asking. We've considered both electronegativities and ionic radius, but cannot seem to find an answer.

An other interesting question I've got for you, is why HCl dissociates completely. Why should not CH[sub]3[/sub]COOH do so?
Does CH[sub]3[/sub]COOCl (Chlorine Acetate/Ethanoate) do so?[/quote]

Ok...your CHEMISTRY teacher doesn't know why Silver chloride precipitates in water? Don't you know your solubility rules?

"...3. Salts containing Cl -, Br -, I - are generally soluble. Important exceptions to this rule are halide salts of Ag+, Pb2+, and (Hg2)2+. Thus, AgCl, PbBr2, and Hg2Cl2 are all insoluble. "

This is a simple equation:

AgCl (s) + H2O (l) yields AgCl (s) + H2O (l)...that's it.

As far as the why does HCl dissociate completely, there's two answers:

One is because the solubility rules say so...but that's kind of a shallow answer. The real answer is because of the Ksp (solubility equilibrium constant). For HCl it so happens to be very high when put in water. Obviously this was done experimentally so if you want to know why HCl dissociates completely just grab some and do an equilibrium test, you'll see that it'll give you a Ksp of at least over 1.0 which generally means it dissolves completely.

As to why those organic elements don't dissolve is just a matter of shape. As you might know, shape and size are everything in the world of organic chem. So it's just how those molecules work...

Hope this helped :yes:

PS: sorry about the sarcastic tone, I just took a very hard AP test and have 2 IB tests and 2 APs comin up this week.

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Guest The X
I know, and I'm quite sure my teacher knows, what you just mentioned :D It's the real "why" behind it that I'm looking for. Even if a rule says so, [i]why[/i] does it say so? And experimental evidence isn't enough. :yes: What is it about Silver which makes it insoluble? Is it the shape, the electronegativity, the larger radius, the increased mass, the extra electrons, when compared to Na? :)

Same thing actually goes for the dissociation of HCl. Yes, the solubility equilibrium constant is over 1, which is calculated from experimental data. But by saying that HCl dissociates completely, we are already stating that. I guess I'm looking for the "higher answer" as to [i]why[/i] this occurs. Let's say that [i]Ammonia[/i] solution has never been found before now, and that some spacecraft is bringing it back from the moon. [You can tell I'm a physicist, can't you?]
It's been really hard to find, and there's very little of it, so we are trying to calculate everything we can about the substance. We do know everything else about Hydrogen and Nitrogen, and we understand how NH[sub]3[/sub] bonds. How could we say, without looking at the substance first that NH[sub]3[/sub] only partially dissociates?

Btw, I had my English P1 yesterday, and I'm having Math HL P1 tomorrow, P2 and Chem HL P1, and P2 on Friday ^^ It's totally allowed to be sarcastic :)

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[quote name='The X' post='16091' date='May 6 2008, 04:26 AM']If I add regular table salt to water, it dissolves in the water. However, if I add Silver Chloride to water, is just precipitates. Why the difference?
I've been discussing this with my chemistry teacher, and it seems that she can't remember, having only just accepted that "that's the way it is". But we're both curious now, and that's why I'm asking. We've considered both electronegativities and ionic radius, but cannot seem to find an answer.

An other interesting question I've got for you, is why HCl dissociates completely. Why should not CH[sub]3[/sub]COOH do so?
Does CH[sub]3[/sub]COOCl (Chlorine Acetate/Ethanoate) do so?[/quote]

Huh. Mmm... can't further explain the NaCl and AgCl thing- it's all to do with whether or not the solvent is polar or not, because polar molecules dissolve in polar solutions, yeah? And water is polar. :) NONONONO ignore what I just said, please. Would it have anything to do with the fact that Na and Cl atoms are mixable with OH atoms, whereas Ag and OH atoms just don't form bonds?

I am confusing myself. I think I'm waaaay out of my league :|

And I think CH3COOH doesn't dissociate completely because there are hydrogen bonds in action, which are hard to break...

I seriously hope I'm right, but don't trust me because I'm only in my first year of IB. Edited by Silviana

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[quote name='The X' post='16091' date='May 5 2008, 08:26 PM']If I add regular table salt to water, it dissolves in the water. However, if I add Silver Chloride to water, is just precipitates. Why the difference?
I've been discussing this with my chemistry teacher, and it seems that she can't remember, having only just accepted that "that's the way it is". But we're both curious now, and that's why I'm asking. We've considered both electronegativities and ionic radius, but cannot seem to find an answer.

An other interesting question I've got for you, is why HCl dissociates completely. Why should not CH[sub]3[/sub]COOH do so?
Does CH[sub]3[/sub]COOCl (Chlorine Acetate/Ethanoate) do so?[/quote]

To answer your question, refer to Topic 4: Bonds and Topic 11: Organic Chemistry from the IB Chemistry material.

- As a rule of thumb; to differentiate between ionic and covalent molecules, find the electronegativity values of both atoms involved in each molecule (e.g. NaCl and AgCl). To be considered an ionic molecule, the difference in electronegativity between the atoms must be higher than 1.8. That of NaCl is definitely higher, while that of AgCl is lower. As a result, NaCl is calssified as an ionic compound, whereas AgCl is a covalent one. From the ionic characteristics, all ionic compounds are soluble in water, that's why NaCl dissolves in water (water molecules split the positive ions of Na from the negative ions of Cl). Although AgCl is considered covalent and some covalent molecules are soluble in water, AgCl is not soluble because of the low difference in electronegativity between the atoms sharing electrones (Ag) and (Cl). That is why it forms a precipitate at the bottom of the beaker.

- HCl, CH[sub]3[/sub]COOH and CH[sub]3[/sub]COOCl are all soluble in water. HCl is a strong acid, so it completely dissociates in water to produce H+ ions. CH[sub]3[/sub]COOH is a carboxylic acid; it has the (OH) group, which is polar due to the difference in electronegativity between (O) and (H), and the molecule as a whole is able to form Hydrogen bonds with water molecule. It also has the slightly positive © pole and slightly negative (O) pole. Not only is it soluble, but it is also highly polar and highly soluble at the same time (who told you it's not? :) ). CH[sub]3[/sub]COOCl is also soluble (this is actually weird.. It may have emerged as a result of chlorine gas reacting with ethanoic acid, or maybe NaCl dissolving in a solution of ethanoic acid?) but anyways, it is highly soluble. Take the chain --COOCl, the C is slightly positive, the 2Os and the Cl are negative; thus it is a highly polar molecule which is soluble in water. I don't think the last molecule is part of the syllabus anyways (correct me if I'm wrong), as the ones included in the material are organic salts as a result of a reaction between an acid and a base (such as ethanoic acid and sodium hydroxide, giving sodium ethanoate and water), between an acid and a metal (such as ethanoic acid and sodium, giving sodium ethanoate and hydrogen gas) and between and acid and metal carbonate (such as ethanoic acid and sodium carbonate, giving sodium ethanoate, water and carbon dioxide).

Hope this has helped. Have a nice day! :) Edited by BIO-AQUA

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[quote name='BIO-AQUA' post='17039' date='May 23 2008, 06:16 AM']To answer your question, refer to Topic 4: Bonds and Topic 11: Organic Chemistry from the IB Chemistry material.

- As a rule of thumb; to differentiate between ionic and covalent molecules, find the electronegativity values of both atoms involved in each molecule (e.g. NaCl and AgCl). To be considered an ionic molecule, the difference in electronegativity between the atoms must be higher than 1.8. That of NaCl is definitely higher, while that of AgCl is lower. As a result, NaCl is calssified as an ionic compound, whereas AgCl is a covalent one. From the ionic characteristics, all ionic compounds are soluble in water, that's why NaCl dissolves in water (water molecules split the positive ions of Na from the negative ions of Cl). Although AgCl is considered covalent and some covalent molecules are soluble in water, AgCl is not soluble because of the low difference in electronegativity between the atoms sharing electrones (Ag) and (Cl). That is why it forms a precipitate at the bottom of the beaker.

- HCl, CH[sub]3[/sub]COOH and CH[sub]3[/sub]COOCl are all soluble in water. HCl is a strong acid, so it completely dissociates in water to produce H+ ions. CH[sub]3[/sub]COOH is an alcohol; it has the (OH) group, which is polar due to the difference in electronegativity between (O) and (H), and the molecule as a whole is able to form Hydrogen bonds with water molecule. Not only is it soluble, it is also highly polar and highly soluble at the same time (who told you it's not? :mellow: ). CH[sub]3[/sub]COOCl is also soluble (this is actually weird.. It may have emerged as a result of chlorine gas reacting with ethanoic acid, or maybe NaCl dissolving in a solution of ethanoic acid?) but anyways, it is highly soluble. Take the chain --COOCl, the C is slightly positive, the 2Os and the Cl are negative; thus it is a highly polar molecule which is soluble in water. I don't think the last molecule is part of the syllabus anyways (correct me if I'm wrong), as the ones included in the material are organic salts as a result of a reaction between an acid and a base (such as ethanoic acid and sodium hydroxide, giving sodium ethanoate and water).

Hope this has helped. Have a nice day! :) [/quote]

I read what you wrote, and all of it sounds really plausible so well done! *But CH[sub]3[/sub]COOH is a carboxylic acid, not alcohol.

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[quote name='Silviana' post='17073' date='May 23 2008, 05:19 AM']I read what you wrote, and all of it sounds really plausible so well done! *But CH[sub]3[/sub]COOH is a carboxylic acid, not alcohol.[/quote]

RIGHT!! I can't believe I did that stupid mistake! Sorry.. :mellow: (edited my post above).

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[quote name='The X' post='16117' date='May 6 2008, 08:29 AM']I know, and I'm quite sure my teacher knows, what you just mentioned :help: It's the real "why" behind it that I'm looking for. Even if a rule says so, [i]why[/i] does it say so? And experimental evidence isn't enough. :dash: What is it about Silver which makes it insoluble? Is it the shape, the electronegativity, the larger radius, the increased mass, the extra electrons, when compared to Na? :)

Same thing actually goes for the dissociation of HCl. Yes, the solubility equilibrium constant is over 1, which is calculated from experimental data. But by saying that HCl dissociates completely, we are already stating that. I guess I'm looking for the "higher answer" as to [i]why[/i] this occurs. Let's say that [i]Ammonia[/i] solution has never been found before now, and that some spacecraft is bringing it back from the moon. [You can tell I'm a physicist, can't you?]
It's been really hard to find, and there's very little of it, so we are trying to calculate everything we can about the substance. We do know everything else about Hydrogen and Nitrogen, and we understand how NH[sub]3[/sub] bonds. How could we say, without looking at the substance first that NH[sub]3[/sub] only partially dissociates?

Btw, I had my English P1 yesterday, and I'm having Math HL P1 tomorrow, P2 and Chem HL P1, and P2 on Friday ^^ It's totally allowed to be sarcastic :good: [/quote]

The real reason why has to do with the molar solubility of each of the dissociated ionic species resulting from the dissolution of AgCl in H2O. More improtantly, it has to do with MO theory and the placement of the LUMO and HOMO orbitals within the Ag+ ion, and its relative crystal field stabilization for the coordination of water. Due to the electron configruation of Ag+ and Hund's Rule of Max. Multiplicity, the relative solubility of Ag+ is low, thereby rendering the solubility product constant (Ksp) of the dissolution reaction to also be very low. Technically to really understand the solubility rules, you need to take a course in physical chemistry :help:

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