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Chemistry Forums for Students => Inorganic Chemistry Forum => Topic started by: Twickel on May 01, 2012, 04:43:27 AM

Title: Expressing acid dissociation constants in terms of another.
Post by: Twickel on May 01, 2012, 04:43:27 AM
Hi

Having a lot of trouble with this, how do I express Ka2 in terms of Ka1. This of the hydrolysis of carbonic acid.

Lecturer is not making sense, I cannot find any information on the internet and have no text book for this unit. starting to stress big time.
Title: Re: Expressing acid dissociation constants in terms of another.
Post by: Borek on May 01, 2012, 05:13:04 AM
Quote from: Twickel on May 01, 2012, 04:43:27 AM
how do I express Ka2 in terms of Ka1. This of the hydrolysis of carbonic acid.

You don't. Ka1 is Ka1 and Ka2 is Ka2. All we know is that Ka1 > Ka2, but there is no way of calculating one knowing the other.

Are you sure you are not confusing Ka2 with the overall dissociation constant?

http://www.chembuddy.com/?left=pH-calculation&right=polyprotic-dissociation-constants
Title: Re: Expressing acid dissociation constants in terms of another.
Post by: Twickel on May 01, 2012, 05:28:47 AM
Maybe, I am not sure at all, what is going on.

In slide 3 of the file attached. K2=  [ cabronate][hydronium ion]^2/Ka1 [h2Co3]

ok figured that out, but I am not sure of the unknowns on slide four.. where do the hydroxides come from?
Title: Re: Expressing acid dissociation constants in terms of another.
Post by: Borek on May 01, 2012, 06:05:43 AM
Both H+ and OH- are always present in water due to autodissociation:

H2O :lequil: H+ + OH-

[tex]K_w = [H^+][OH^-][/tex]

So you can always calculate [H+] when you know [OH-] and vice versa. In the charge balance you need both.
Title: Re: Expressing acid dissociation constants in terms of another.
Post by: Twickel on May 01, 2012, 07:23:50 AM
ok, so whenever I figure out charge balance, and see a H2O, I must always think of the auto hydrolysis of water, even is in the respective reaction the Ka has no OH- formation.
Title: Re: Expressing acid dissociation constants in terms of another.
Post by: Borek on May 01, 2012, 08:07:37 AM
Yes.

Not that when the reaction is not acid/base related [H+]=[OH-], so they can be both ignored in the charge balance.
Title: Re: Expressing acid dissociation constants in terms of another.
Post by: Twickel on May 01, 2012, 08:09:34 AM
Some thing is not clicking, I get t expression in slide 4, not the cubic equation the bit in blue.

How do I calculate pH from that?

More importantly how is the cubic equation derived in this case
Title: Re: Expressing acid dissociation constants in terms of another.
Post by: Twickel on May 01, 2012, 08:59:20 AM
Are there any sites that explain speciation and all the stuff that goes with it.... I am lost.
Title: Re: Expressing acid dissociation constants in terms of another.
Post by: Twickel on May 01, 2012, 11:16:55 AM
On slide 9, I do not understand how they reach the alpha values of all the species ( the bottom line).

I am able to get the mass balance as F= [H2A] + k1/[H+] x [H2a] + k1k2/[H+]^2x H2A

and I know that alpha = species we are interested in over F, I can not figure out the denominators for each one.
Title: Re: Expressing acid dissociation constants in terms of another.
Post by: Borek on May 01, 2012, 11:23:40 AM
Quote from: Twickel on May 01, 2012, 08:09:34 AM
Some thing is not clicking, I get t expression in slide 4, not the cubic equation the bit in blue.

How do I calculate pH from that?

More importantly how is the cubic equation derived in this case

Slide four shows how to get the 3rd degree equation. You start with a charge balance and replace everything on the RHS with expressions that are functions of [H+] and [H2CO3] (Ka1, Ka2 and Kw are all known constants), no problems with the derivation (note that LHS is just [H+], as defined at the top of the slide).

That being said equation still contains two unknowns, so it can be solved only if you assume known concentration of [H2CO3]. Your teacher assumes concentration of the H2CO3 is known from the Henry's law which is IMHO incorrect - Henry's law constant is determined experimentally, so doesn't say anything about concentration of H2CO3, it rather tells us how much CO2 was dissolved in total - so it gives us a sum of concentrations of all forms of dissolved CO2.

Assuming Ca to be a full (analytical) concentration of CO2, full equation will be that of 4th degree, as derived here (http://www.chembuddy.com/?left=pH-calculation&right=pH-polyprotic-acid-base).
Title: Re: Expressing acid dissociation constants in terms of another.
Post by: Borek on May 01, 2012, 11:26:48 AM
Quote from: Twickel on May 01, 2012, 11:16:55 AM
On slide 9, I do not understand how they reach the alpha values of all the species ( the bottom line).

These are identical formulas as the ones derived on the polyprotic acid pH page I linked to (eqs 9.11, 9.12 and 9.13).
Title: Re: Expressing acid dissociation constants in terms of another.
Post by: Twickel on May 01, 2012, 11:32:33 AM
Oh yeah, there is a slide that says it is common practice to not wirte out CO2(aq) eq H2Co3, rather its just CO2(g) + H20 eq H2co3
Title: Re: Expressing acid dissociation constants in terms of another.
Post by: Twickel on May 01, 2012, 11:36:31 AM
Quote from: Borek on May 01, 2012, 11:26:48 AM
Quote from: Twickel on May 01, 2012, 11:16:55 AM
On slide 9, I do not understand how they reach the alpha values of all the species ( the bottom line).

These are identical formulas as the ones derived on the polyprotic acid pH page I linked to (eqs 9.11, 9.12 and 9.13).

One mole snack a day for you for a week. Thank you so much.

One more problem.. slide 13, please explain the top 2 equations

in polyprotic acids, is HA- considered the intermediate form ?

How did they get to the equilibrium between H2CO3 and CO32-?


Title: Re: Expressing acid dissociation constants in terms of another.
Post by: Twickel on May 02, 2012, 04:20:58 AM
When using theeCO2 to co32- ( including all the steps in between) what does ph=0.5(pka1+pka2 tell  me?