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Offline onefear

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Lewis acids
« on: August 27, 2009, 05:43:27 PM »
right so im doing Advanced Higher Chemistry (equivelent to first year course in university)
and for my project i want to create an acid independant of water.
So my question is, is it possible to isolate a lewis acid from solution so that i can compare it with a normal acid?.
Thanks for the help

Offline Rudi

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Re: Lewis acids
« Reply #1 on: August 27, 2009, 05:57:31 PM »
an acid independant of water.
What do mean exactly?

Quote
So my question is, is it possible to isolate a lewis acid from solution so that i can compare it with a normal acid?.
What is 'a normal acid' in this context?

Offline onefear

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Re: Lewis acids
« Reply #2 on: August 27, 2009, 06:21:14 PM »
well a normal acid requires H2O and can be measured using the pH scale
so for instance
        -->
H2O          H+ + OH-
      <----

you then add say Na+ which removes  the OH- shifts the equlibrium to the foward reaction and leaves an excess of H+ ions, you have your acid

but lewis acid is based on the principle that an acid is an electron pair donator or acceptor

so you have say a reaction like this
       -->
NH4        NH3- + H+
     <----

where adding say Na+ will remove the ammonia ions to leave an excess of H+ ions, and there you also have an acid
the only problem is NH3- + H+ are in solution
and since my project is to create an acid independent of water i need to find out if i can extract the lewis acid from solution once it's ready..?

Note that the pH scale doesn't apply to a lewis acid.
however i intend to create a theoretical pH scale for my lewis acid during my project.

Offline zxt

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Re: Lewis acids
« Reply #3 on: August 28, 2009, 03:08:05 AM »
In pure liquid ammonia, ammonia can ionize like this:
                               2NH3 ::equil::NH4+ + NH2-
And you can distil acid solutions but you can't get 100% pure acid because acid molecules form hydrates with H2O

Offline onefear

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Re: Lewis acids
« Reply #4 on: August 28, 2009, 12:58:32 PM »
thank you but how to i get the ammonia to ionize?

Offline Borek

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Re: Lewis acids
« Reply #5 on: August 28, 2009, 01:49:19 PM »
thank you but how to i get the ammonia to ionize?

You don't have to - it ionizes when pure and liquid by itself, just like water does.
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Offline onefear

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Re: Lewis acids
« Reply #6 on: August 28, 2009, 03:11:32 PM »
so how much pressure would i have to apply to achieve this?.

And what do you mean by distil acid solutions as in solutions that use the pH scale?. if thats the case wouldn't that stop it being an acid as there's no water to form H+ ions?.

Offline Borek

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Re: Lewis acids
« Reply #7 on: August 28, 2009, 03:21:01 PM »
so how much pressure would i have to apply to achieve this?

To make liquid ammonia? Depends on the temperature.
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Offline Arctic-Nation

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Re: Lewis acids
« Reply #8 on: August 28, 2009, 03:41:52 PM »
thank you but how to i get the ammonia to ionize?

You don't have to - it ionizes when pure and liquid by itself, just like water does.
Except that the dissociation constant is almost 20 orders of magnitude smaller.

Offline Borek

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Re: Lewis acids
« Reply #9 on: August 28, 2009, 04:41:24 PM »
Except that the dissociation constant is almost 20 orders of magnitude smaller.

Yep, [NH2-][NH4+]=10-32.

Sulfuric acid will be much better example:

[H3SO4+][HSO4-]=10-2.9
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Offline onefear

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Re: Lewis acids
« Reply #10 on: August 28, 2009, 05:24:47 PM »
hmm well could you please give me a reasonable pressure and temperature that i could achieve liquid ammonia in a school lab.
unfortunately sulphuric acid requires water

Offline Borek

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Re: Lewis acids
« Reply #11 on: August 28, 2009, 05:42:02 PM »
I doubt anyone will allow use of liquid ammonia in the school lab.

Sulfuric acid reaction - the one I wrote - occurs in anhydrous acid.
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Offline onefear

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Re: Lewis acids
« Reply #12 on: August 28, 2009, 06:08:08 PM »
dont worry they do your american version of school is different from our scottish version
so any idea roughly what pressure and temperature i could do it at?.

Offline onefear

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Re: Lewis acids
« Reply #13 on: August 28, 2009, 06:16:46 PM »
although if that sulphuric acid one is a lewis acid, independant of water and easier to make then please do tell. :)

Offline Borek

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Re: Lewis acids
« Reply #14 on: August 28, 2009, 06:21:09 PM »
your american version of school

No american schools in the 6000 km radius from here ;)

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so any idea roughly what pressure and temperature i could do it at?

-34 deg C and atmospheric pressure will do. Still, it is not a chemical that you wan't to work with. Concentrated sulfuric acid is much more easy to handle and much less dangerous - it is not volatile, it won't boil at room temperature, it doesn't emit poisonous vapor and so on.
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