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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: Diamonds on January 13, 2013, 01:00:22 AM

Title: Solubilities in solvents other than water
Post by: Diamonds on January 13, 2013, 01:00:22 AM
Which solid is much more soluble in 1M HCl than in water?

a) CaHPO4
b) CaCl2
c) BaBr2
d) BaSO4

I was thinking of the reactions that each would make with the HCl

a) CaHPO4 + 2HCl  ::equil:: CaCl2 + H3PO4
b) CaCl2 + 2HCl ::equil:: CaCl42- + H2
c) BaBr2 + 2HCl ::equil:: BaCl2 + 2HBr
d) BaSO4 + 2HCl ::equil:: BaCl 2 + H2SO4

I am confused on how to use these equations to determine which is more soluble in HCl than in water.
Title: Re: Solubilities in solvents other than water
Post by: Borek on January 13, 2013, 04:57:05 AM
a) CaHPO4 + 2HCl  ::equil:: CaCl2 + H3PO4

Makes sense.

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b) CaCl2 + 2HCl ::equil:: CaCl42- + H2

Not balanced (and impossible to balance - you have charge on the RHS only).

Quote
c) BaBr2 + 2HCl ::equil:: BaCl2 + 2HBr

HBr is a stronger acid than HCl, so the protonation is unlikely.

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d) BaSO4 + 2HCl ::equil:: BaCl 2 + H2SO4

H2SO4 is a strong acid, so protonation won't go that far, but HSO4- is quite likely...

In general this is just about acid/base equilibrium - you are trying to dissolve salt of a weak (or relatively weak) acid in the strong acid, so protonating the anion can shift the dissolution equilibrium to the right.
Title: Re: Solubilities in solvents other than water
Post by: Diamonds on January 13, 2013, 02:07:46 PM
So then:

a) CaHPO4 + 2HCl  ::equil:: CaCl2 + H3PO4
b) CaCl2 + 2HCl ::equil:: CaCl42- + 2H+
c) BaBr2 + 2HCl ::equil:: BaCl2 + 2H+ + 2Br-
d) BaSO4 + 2HCl ::equil:: BaCl2 + H+ + HSO4-

So now that I have these (hopefully) correct chemical equations, I need to know which of the equations will proceed the most to the right in order to find which of the four compounds will dissolve the most in HCl (as opposed to their dissolutions into water).
I am still confused on how to figure out which equation will proceed the furthest to the right.
Title: Re: Solubilities in solvents other than water
Post by: Borek on January 13, 2013, 03:07:35 PM
b) CaCl2 + 2HCl ::equil:: CaCl42- + 2H+

Problem with balancing were not the only problem with this reaction. There is no CaCl42- complex.

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c) BaBr2 + 2HCl ::equil:: BaCl2 + 2H+ + 2Br-

This is wrong - but it is at least partially a problem with all your reactions. What is BaBr2? Is it solid? Or a dissolved, undissociated molecule? And what is BaCl2? And why there is HCl on the left, but H++Br- on the right? Make all these reaction net ionic, and mark what is solid, what is not.

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d) BaSO4 + 2HCl ::equil:: BaCl2 + H+ + HSO4-

So now that I have these (hopefully) correct chemical equations, I need to know which of the equations will proceed the most to the right in order to find which of the four compounds will dissolve the most in HCl (as opposed to their dissolutions into water).
I am still confused on how to figure out which equation will proceed the furthest to the right.


Whenever you remove one of the dissolution products from the reaction mixture, you shift the equilibrium, don't you? For example, calcium carbonate dissolves producing two ions:

CaCO3(s)   :lequil: Ca2+ + CO32-

In the presence of any acid strong enough CO32- gets protonated:

CO32- + H+  :requil: HCO3-

and the overall reaction of CaCO3 dissolution in the presence of another acid is:

CaCO3 + H+  :requil: Ca2+ + HCO3-

with the equilibrium shifted (when compared to the first CaCO3 dissolution reaction) to the right, as protonation removes CO32- from the solution.
Title: Re: Solubilities in solvents other than water
Post by: Ben Cohen on January 13, 2013, 03:18:54 PM
So, for example choice a)

Equilibrium 1: CaHPO4 + 2HCl  ::equil:: CaCl2 + H3PO4

So does this mean that if the H3PO4 will protonize to give:

Equilibrium 2: H3PO4  ::equil:: H+ + H2PO42-

So the net equation will be

CaHPO4 + 2HCl ::equil:: H+ + H2PO42-

Meaning the dissolving it in HCl acid will increase the [HCl] on the right, driving the reaction to the right and increasing the solubility of CaHPO4 ?
Title: Re: Solubilities in solvents other than water
Post by: Borek on January 13, 2013, 03:41:06 PM
So, for example choice a)

Equilibrium 1: CaHPO4 + 2HCl  ::equil:: CaCl2 + H3PO4

So does this mean that if the H3PO4 will protonize to give:

Equilibrium 2: H3PO4  ::equil:: H+ + H2PO42-

So the net equation will be

CaHPO4 + 2HCl ::equil:: H+ + H2PO42-

Meaning the dissolving it in HCl acid will increase the [HCl] on the right, driving the reaction to the right and increasing the solubility of CaHPO4 ?

Where did you got H3PO4 in your second reaction from - that's not what you start with. What are the products of CaHPO4 dissolution?

Note: you are still ignoring fact some of the substances involved are always dissociated, it doesn't help you.
Title: Re: Solubilities in solvents other than water
Post by: Ben Cohen on January 13, 2013, 03:46:42 PM
So, for example choice a)

Equilibrium 1: CaHPO4 + 2HCl  ::equil:: CaCl2 + H3PO4

So does this mean that if the H3PO4 will protonize to give:

Equilibrium 2: H3PO4  ::equil:: H+ + H2PO42-

So the net equation will be

CaHPO4 + 2HCl ::equil:: H+ + H2PO42-

Meaning the dissolving it in HCl acid will increase the [HCl] on the right, driving the reaction to the right and increasing the solubility of CaHPO4 ?

Where did you got H3PO4 in your second reaction from - that's not what you start with. What are the products of CaHPO4 dissolution?

Note: you are still ignoring fact some of the substances involved are always dissociated, it doesn't help you.

The products of CaHPO4 dissolution are

CaHPO4  ::equil:: Ca2+ + HPO42-

The HPO42- is a very weak acid and practically does not protonize.
Title: Re: Solubilities in solvents other than water
Post by: Borek on January 13, 2013, 04:34:30 PM
The HPO42- is a very weak acid and practically does not protonize.

Never heard a word protonize, what you mean is HPO42- doesn't dissociate (well... it does, to some extent, but we can assume it doesn't). But it doesn't matter much here, as you got it all reversed. In acidic solution HPO4- will get PROTONATED - that is, it will accept a proton.
Title: Re: Solubilities in solvents other than water
Post by: Ben Cohen on January 13, 2013, 09:38:10 PM
The HPO42- is a very weak acid and practically does not protonize.

Never heard a word protonize, what you mean is HPO42- doesn't dissociate (well... it does, to some extent, but we can assume it doesn't). But it doesn't matter much here, as you got it all reversed. In acidic solution HPO4- will get PROTONATED - that is, it will accept a proton.
http://www.chemicalforums.com/Themes/Borek/images/bbc/sub.gif
OH!

So you mean:

CaHPO4 ::equil:: Ca2+ + HPO42-
And then it is in an acidic solution, so the HPO42- will

HPO42- + H+ ::equil:: H2PO4-
Which will take the HPO42- out of solution.
And that means that, according to Le Chatelier's, more CaHPO4 will dissolve to accomodate the loss of the HPO42- ion?

Is this reasoning correct?
Title: Re: Solubilities in solvents other than water
Post by: Borek on January 14, 2013, 03:44:53 AM
You got it now.