chem question

[medical doctor]

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given that you have this rxn in equilibrium:

co2 + H20 ----> H2CO3 ---> proton + HCO3

if you have that rxn in equilibrium, if you add HCL...how does it affect the concentration of H2CO3 ??

 

[lixxx669]

Add HCl means more H+ will present in the solution, so equilibrium shift left, so [H2CO3] will increase, then shift again to left to yield CO2. So my guess is [H2CO3] will be constant for a while till all HCO3- convert to H2CO3 then if you keep adding HCl, then [H2CO3] begin to drop

 

[futuredoctor10]

I would guess: as you add H+, it initially increases [H2CO3] (equilibrium shifts). As H2CO3 concentration rises, the eq will shift again, to form CO2 and H20 - decreasing the [H2CO3].

Basing this off the fact that [H+] increasing will ultimately cause an increase in [CO2].

 

[medical doctor]

I would guess: as you add H+, it initially increases [H2CO3] (equilibrium shifts). As H2CO3 concentration rises, the eq will shift again, to form CO2 and H20 - decreasing the [H2CO3].

Basing this off the fact that [H+] increasing will ultimately cause an increase in [CO2].

Its a tricky question... if you had to answer with a simple increase or decrease, which would it be??

 

[spyderracing32]

Its a tricky question... if you had to answer with a simple increase or decrease, which would it be??

You can't answer like that without specifying what is increasing or decreasing. This is a pretty basic LeChatlier's principle problem. Adding HCl increases [H+] and shifts the reaction to the left.

 

[medical doctor]

You can't answer like that without specifying what is increasing or decreasing. This is a pretty basic LeChatlier's principle problem. Adding HCl increases [H+] and shifts the reaction to the left.

hmmm..all the question says is what happens to the concentration of H2C03 with addition of HCL ...maybe its just a vague question..its from an old Princeton review workbook i have, and the answer sheet was torn out..i just really wanted to know the answer haha

 

[medical doctor]

it just wants to know if the concentration of H2CO3 increases or decreases upon addition, thats it....it doesnt specify any certain amounts...its pretty obvious the equilibrium will shift left, but what exactly would be the solution to the question 😕

 

[spyderracing32]

it just wants to know if the concentration of H2CO3 increases or decreases upon addition, thats it....it doesnt specify any certain amounts...its pretty obvious the equilibrium will shift left, but what exactly would be the solution to the question 😕

Sorry, I didn't read that it asked about the H2CO3. It should however increase.

 

[medical doctor]

Sorry, I didn't read that it asked about the H2CO3. It should however increase.

thanks man..is that just an educated guess? or is their a specific reason?... because on one hand, i thought it increases because more protons are present to create more of hte H2CO3, but then again, all of the created H2CO3 would just turn into carbon dioxide and water

 

[lixxx669]

My first response is wrong... sorry about that.
So now i'm sure that H2CO3 increases.
An easy way to think: since the whole reaction will shift to the left, in the end there would be more CO2 and H2O, in order to keep the product-reactant ratio unchanged for the first half of the reaction, there should be more H2CO3 too

I also did some math to calculate the concentration of H2CO3 (suppose the amount of C is constant), and found out that H2CO3 and H+ are positively correlated

 

[medical doctor]

My first response is wrong... sorry about that.
So now i'm sure that H2CO3 increases.
An easy way to think: since the whole reaction will shift to the left, in the end there would be more CO2 and H2O, in order to keep the product-reactant ratio unchanged for the first half of the reaction, there should be more H2CO3

thanks, i kinda new the answer was increase..but i couldnt figure out a specified reason

 

[futuredoctor10]

Immediately it increases.

but if (later) CO2 increases doesn't that mean for the equilibrium to shift again it will have to DECREASE the H2CO3 conc?

 

[BerkReviewTeach]

An easy way to think: since the whole reaction will shift to the left, in the end there would be more CO2 and H2O, in order to keep the product-reactant ratio unchanged for the first half of the reaction, there should be more H2CO3 too

You are assuming the system to be closed, in which case there should be a buildup of both H2CO3 and CO2 exactly as you described. However, if CO2 is free to escape the container (or system) upon the shift, then CO2 levels remain in essence constant, which results in no net change to the [H2CO3]😛CO2 relationship.

The question is ambiguous (probably by design), which is great for learning, even if it doesn't simulate the MCAT perfectly.

 

[silverlion]

The person who thought of this question is insane...and if it shows up in the MCAT then the test maker will go to hell....

 

[TheBoondocks]

The person who thought of this question is insane...and if it shows up in the MCAT then the test maker will go to hell....

It's acutally not difficult. It's a biochem question. That equation represents how CO2 is carried in the body, carbonic anhydrase. The only difficulty is as BRT said whether it's open or not.