ATAR Notes: Forum
VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Chemistry => Topic started by: monokekie on August 19, 2009, 09:18:34 pm
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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.
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"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
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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?
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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.
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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)
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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
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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)
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ooooooooooooooooooooooh yuan lai ru ci. tkx so much