Carbonic acid

[monokekie]

Carbonic acid is a diprotic acid. The equations for each of its ionisations are shown below

H2CO3 + H2O <-> H3O+  +   HCO3-    Ka=4.3x10^-7

HCO3- + H2O <-> H3O+ + CO3 ^2-   Ka = 4.8 x 10^-11

i worked out its Kb value be 2.3 x 10^-8



question: if some sodium hydrogen carbonate is dissolved in pure water, will the solution formed be acidic or basic? please explain.

[lukeperry91]

"Sodium bicarbonate is an amphoteric compound. Aqueous solutions are mildly alkaline due to the formation of carbonic acid and hydroxide ion:

    HCO−3 + H2O → H2CO3 + OH− "
Therefore, as carbonic acid is very weak, it would be basic

[monokekie]

that's a really good reasoning, thanks lukeperry!

however, the answer states that the Kb value is higher than the Ka value and suggests this being the reason of its basic nature.

i don't see the logic behind this. and could anyone pls give me some hint?

[lukeperry91]

All that means is that it is more willing to accept a proton than it is to donate one (it is amphoteric), which is basically what I said when I mentioned that carbonic is a weak acid.

[monokekie]

mmmm... i see.. tkx

but excuse my confusion, how do we tell whether the Kb value's bigger or smaller?

which values is the answer referring to? (i know i m referring to BOB too much here, but i am really quite intrigued to find out why)

[lukeperry91]

Kb just means the degree of the substances willingness to accept a proton, if that's what you're asking? Can you formulate your question for clarity? lol sorry

[Mao]

Carbonic acid is a diprotic acid. The equations for each of its ionisations are shown below

H2CO3 + H2O <-> H3O+  +   HCO3-    Ka=4.3x10^-7

HCO3- + H2O <-> H3O+ + CO3 ^2-   Ka = 4.8 x 10^-11

i worked out its Kb value be 2.3 x 10^-8

You can hence set out the question like this:

HCO3- + H2O <-> H3O+ + CO3 ^2-   Ka = 4.8 x 10^-11
HCO3- + H2O <-> OH- + H2CO3   Kb = 2.3 x 10^-8

Competing equilibriums, :. the system with higher K will be preferred.
Hence the solution will be basic, since Kb>>Ka (by 500 times)

[monokekie]

ooooooooooooooooooooooh yuan lai ru ci. tkx so much