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What is responsible for converting H2CO3 to HCO3- and H+ ...

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What is responsible for converting H2CO3 to HCO3- and H+ ...

Postby NeedsAPbiohelp » Sat May 07, 2011 1:33 am

Hi, this is my first time here! I'm preparing for the AP Bio test May 9th.

What is responsible for dissociation of carbonic acid (H2CO3) to a bicarbonate ion (HCO3-) and H+ in response to the rise in pH levels? This question is about the bicarbonate buffering system in pH. I understand how HCO3- and H+ can react together again to form carbonic acid (H2CO3),but how exactly does carbonic acid (H2CO3) separate into a bicarbonate ion (HCO3-) and H+? Does it just spontaneously happen? Does a enzyme facilitate their dissociation?
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Postby NeedsAPbiohelp » Sat May 07, 2011 1:43 am

How exactly does carbonic acid dissociate to yield a bicarbonate ion and a hydrogen ion? Sorry if it's a dumb question, but I have no chemistry background.


My textbook says they just dissociate, but it doesn't explain how or why.
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Postby Babybel56 » Sat May 07, 2011 3:13 pm

When you have an ionically bonded compound (such as H2CO3) it isn't a single molecule, but is two separate ions that happen to form a lattice when they're solidified. If they's dissolved in solution the lattice disintegrates and the ions become separate. So long as the compound exists they're never actually a single molecule however.
In the case of H2CO3 it's soluble in water because of the polar nature of water. It has slightly negative and positive ends (won't go into it if you don't like chemistry but they're called dipoles if you want to look them up) that are attracted to either the positive or negative ion. This pulls them apart and destroys the lattice, like when salt or sugar dissolves in water.

Hope this helps :)
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Postby JackBean » Sat May 07, 2011 4:33 pm

you're mixing several thigs. H2CO3 is not ionic compound! And sugars are not ionic :)
http://www.biolib.cz/en/main/

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Re: What is responsible for converting H2CO3 to HCO3- and H+ ...

Postby NeedsAPbiohelp » Sat May 07, 2011 5:05 pm

Could you help me jack bean? I don't know think Babybel56 has provided an accurate answer.
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Postby JackBean » Sat May 07, 2011 5:48 pm

H2CO3 is weak acid, thus its dissociation is pH dependent, but doesn't require any enzyme, if you meant that. However, there is the carbonate anhydrase, which accelerates the reaction CO2 + H2O <-> H2CO3 (but that doesn't relate to the dissociation)
http://www.biolib.cz/en/main/

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Re: What is responsible for converting H2CO3 to HCO3- and H+ ...

Postby NeedsAPbiohelp » Sun May 08, 2011 7:29 pm

I haven't take AP chemistry yet, but am still curious about this dissociation. How exactly is H2CO3 pH dependent? If a solution becomes more alkaline, what would instigate the dissociation of H2CO3 into H+ and H2CO3-?
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Postby JackBean » Wed May 11, 2011 6:41 am

nothing, it's just equilibrium.
The thing is, that both reactions (H2CO3 -> HCO3 + H; H + HCO3 -> H2CO3; + and - omitted) run at the same time and their speed is dependent on the concentrations of the reactants, thus if you add H+, you increase the association, but that will increase concentration of H2CO3 and thus the speed of dissociation etc. untill new equilibrium is reached.
http://www.biolib.cz/en/main/

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Postby kwisatz » Fri Oct 12, 2012 7:36 am

Can I ask one question which relates to this topic?
What actually happens when I add CO2 to the water constantly? As I have read the carbon is in the water in 3 forms: as CO2 (H2CO3), HCO3, and CO3. Under pH 8 nearly all carbon should be in the form of HCO3 (97%), and just a small fraction in the form of H2CO3 (3%). What if I add CO2 into the water? What reactions (and how quickly) will take place?

I know that the following reactions should took place in the water:
CO2 + H2O <=> H2CO3 <=> HCO3 + H

But if I add CO2 into the water, the KH (alkality, which consist mainly of HCO3) does not change. So it means that CO2 is not changing into HCO3 actually. Why?

Can anyone try to explain it to me?
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