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Topic: heat of formation and electrolytes  (Read 5599 times)

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Offline 113zami

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heat of formation and electrolytes
« on: July 25, 2008, 01:59:14 PM »
1)which of the following substances does not have a heat of formation equal to zero at standard conditions?
a)F2(g)
b)Cl2(g)
c)Br2(g)
d)I2(g)
 
I am stuck between C and D , I know that in standard conditions Bromine is Liquid and Iodine is solid not gas so both C and D should have a non zero heat of formation, but only one answer is correct, how can you eliminate one aswer ??

 2)which is the weakest electrolyte?
a) NH4I
b)LiF
c) AgBr
d)H2O2

the correct ansr is D but it also says in the explanation "All ionic compounds, whether soluble or not, are defined as strong electrolytes so choices A,B,C are eliminated.
it's this explanation that confuses me AgBr is insoluble, how can it be a strong electrolye?? electrolyts must produce ions in aquous solution and conduct electricity, without being soluble you can't produce ions or conduct electricity, why do they still call insoluble ionic compounds strong electrolytes???

please help thanks

Offline Borek

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Re: heat of formation and electrolytes
« Reply #1 on: July 25, 2008, 02:17:00 PM »
Once AgBr is dissolved it is fully dissociated, from this point of view it can be considered a strong electrolyte. But I would say this is a grey area of not precisely defined terms, I have seen long discussion on the subject between chemistry teachers (including University professors) this year; no final conclusion, rather agreement on disagrement.

No idea about the first question though.
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Offline 113zami

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Re: heat of formation and electrolytes
« Reply #2 on: July 25, 2008, 05:08:48 PM »
Once AgBr is dissolved it is fully dissociated, .

but how is that possible,? according to the solubility rules in my text book, Bromides are soluble except when with Ag, Pb, Hg

Offline azmanam

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Re: heat of formation and electrolytes
« Reply #3 on: July 25, 2008, 05:19:38 PM »
silver bromide has a solubility constant of 5.3x10(-13).

http://www.csudh.edu/oliver/chemdata/data-ksp.htm

That is effectively zero (we say silver bromide is insoluble), but still there will be at least one molecule that makes it into the aqueous solution.

Once there, that one molecule will fully dissociate.

Once fully dissociated, the solution becomes, technically, a strong electrolyte.

(edited to include link to source)
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Offline Borek

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Re: heat of formation and electrolytes
« Reply #4 on: July 25, 2008, 05:28:12 PM »
Once AgBr is dissolved it is fully dissociated, .

but how is that possible,? according to the solubility rules in my text book, Bromides are soluble except when with Ag, Pb, Hg

You are mixing two things. AgBr is almost insoluble, that's true. But it is soluble to some extent. This small amount that dissolves is completely dissociated. So if you take a solution of NaCl - undoubtedly strong electrolyte - and you dilute it so that it has the same concentration as saturated solution of AgBr - both solutions contain fuly dissociated salt.

At the same time, if you take something like acetic acid - it may be perfectly miscible with water (it has infinte solubilty), but it is in most cases dissociated only partially, for example in 0.01M water solution 96% of acetic acid molecules are not dissociated.
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Offline acidball

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Re: heat of formation and electrolytes
« Reply #5 on: July 25, 2008, 06:36:04 PM »
can you guys explain what exactly you mean by "fully dissociate" vs "partially dissociate"? i always assumed that saying something was "dissolved" was synonymous with saying it was "dissociated"? but it seems like you guys are making a distinction between being dissolved vs dissociated? doesn't an ionic compound need to essentially dissociate in order to be fully solvated by water (ie. dissolved)?

Offline Borek

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Re: heat of formation and electrolytes
« Reply #6 on: July 26, 2008, 03:28:52 AM »
I don't think I can add much more to what I wrote about acetic acid. It is dissolved but not dissociated. Dissolved doesn't mean dissociated - sucrose can be dissolved, but is never dissociated. NaCl is both dissolved and dissociated. There are plenty of substances that are dissolved, but dissociated only to some extent. In the case of ionic substances dissolution usually means dissociation - that's what is happenning with AgBr.
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Offline azmanam

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Re: heat of formation and electrolytes
« Reply #7 on: July 26, 2008, 07:05:06 AM »
Quote
i always assumed that saying something was "dissolved" was synonymous with saying it was "dissociated"?

Qualitatively, dissociation implies breaking chemical bonds (most usually ionic bonds).  When a molecule dissociates, it breaks into at least 2 ions.  A molecule can dissociate essentially completely (essentially every molecule in solution has broken into at least 2 ions), or a molecule can dissociate partially (Some of the molecules will break ionic bonds, but some of the molecules will remain as bonded, neutral molecules).

Dissolution refers only to a molecule going from the pure solid, liquid or gas state to the aqueous state (if the solvent is water).  No bonds are broken in dissolution.

http://en.wikipedia.org/wiki/Dissociation_(chemistry)
http://en.wikipedia.org/wiki/Solubility
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