[Twickel]

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.

[Borek]

how do I express Ka2 in terms of Ka1. This of the hydrolysis of carbonic acid.

You don't. K_a1 is K_a1 and K_a2 is K_a2. All we know is that K_a1 > K_a2, but there is no way of calculating one knowing the other.

Are you sure you are not confusing K_a2 with the overall dissociation constant?

http://www.chembuddy.com/?left=pH-calculation&right=polyprotic-dissociation-constants

[Twickel]

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?

[Borek]

Both H⁺ and OH^- are always present in water due to autodissociation:

H₂O 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.

[Twickel]

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.

[Borek]

Yes.

Not that when the reaction is not acid/base related [H⁺]=[OH^-], so they can be both ignored in the charge balance.

[Twickel]

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

[Twickel]

Are there any sites that explain speciation and all the stuff that goes with it.... I am lost.

[Twickel]

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.

[Borek]

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] (K_a1, K_a2 and K_w 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 C_a to be a full (analytical) concentration of CO₂, full equation will be that of 4th degree, as derived here.

[Borek]

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).

[Twickel]

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

[Twickel]

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-?

[Twickel]

When using theeCO2 to co32- ( including all the steps in between) what does ph=0.5(pka1+pka2 tell  me?