carbon dioxide equillibrium (1 Viewer)

[mei_ling03]

ok, i've got written in my notes that water does not affect the equillibrium ( CO2 + H2O <-> H2CO3 ) but i can't rememebr where i got the info...

so firstly could someone pls tell me if this is right.. and if so, why?

 

[phenol]

well if you look at your equilibrium constant for this reaction

K = [H2CO3]/[CO2] (better to use partial pressure for CO2, same thing different smell)

adding water dilutes both the top and bottom of the K (assume dilution factor is g then)

so we can write

K = g[H2CO3]/g[CO2] = K as the g on top and bottom cancels out

ur still at equilibrium position

so adding water doesn't affect this eqm.

 

[phenol]

But this is not the whole story

Consider this equilibrium

H2CO3 <=> H+ + HCO3-

adding water will push this equilibrium to the RHS, more H2CO3 will dissociate

Therefore for this eqm

CO2 + H2O <=> H2CO3

it will be pushed to the right as well, as more CO2 dissolves.

Know that adding water doesn't push the above eqm to the right just because water appears on the LHS but rather water causes H2CO3 to dissociate more

 

[Foong]

Originally posted by phenol
well if you look at your equilibrium constant for this reaction

K = [H2CO3]/[CO2] (better to use partial pressure for CO2, same thing different smell)

adding water dilutes both the top and bottom of the K (assume dilution factor is g then)

so we can write

K = g[H2CO3]/g[CO2] = K as the g on top and bottom cancels out

ur still at equilibrium position

so adding water doesn't affect this eqm.

Wow..... I didn't know bout this constant thngy.... it isn't in the syllabus, right? Hopefully it isn't...... could you explain it more to me? Could you also explain to my why is it that when u add water it will casue more H2CO3 to dissociate?

 

[abdooooo!!!]

Originally posted by Foong
Wow..... I didn't know bout this constant thngy.... it isn't in the syllabus, right? Hopefully it isn't...... could you explain it more to me? Could you also explain to my why is it that when u add water it will casue more H2CO3 to dissociate?

its not in the core... but read industrial chemistry option if you wish to learn the basics of k constant

 

[Foong]

oh ok.... phew! thanks.... i was a bit worried there...

 

[phenol]

There are two ways to explain why adding water cause more H2CO3 to dissociate

one is to again look at K (a1) - i.e. the K for the first dissociation step.

H2CO3 <=> H+ + HCO3 -

K = [H+][HCO3-]/[H2CO3]

as you can see if you add water top and bottom are diluted by the same amount. again let dilution factor be g (notice g < 1) then

Q = g^2 [H+][HCO3-] / g[H2CO3] = g * K

therefore ur Q, reaction quotient, is smaller, therefore eqm shifts to the RHS to make up for this decrease in K.

another way to think about is

H2CO3 is a weak acid, it doesn't dissociate 100%, but if you add more water, surely more will dissociate. Just like something wont dissolve 100%, if you add enough water, surely all will dissolve

this is a crude method though but easy to understand

 

[mei_ling03]

Originally posted by phenol
But this is not the whole story

Consider this equilibrium

H2CO3 <=> H+ + HCO3-

adding water will push this equilibrium to the RHS, more H2CO3 will dissociate

Therefore for this eqm

CO2 + H2O <=> H2CO3

it will be pushed to the right as well, as more CO2 dissolves.

Know that adding water doesn't push the above eqm to the right just because water appears on the LHS but rather water causes H2CO3 to dissociate more

ohh.. i didn't do the industrial chemistry option.. was getting scared there. is it possible that you could explain in brief terms what the equilibrium constant is?
also i'd still like to know why water causes H2CO3 to dissociate more (sorry i'm a lil confused by the explanation). i don't understand why water wouldn't affect the equilibrium seeing that it is a reactant (le chatelier's principle). is it because water itself dissasociates to from hydrogen ions and hydroxide ions, which affects both forward and reverse reactions?
eek

 

[phenol]

it is impossible to say more about equilibrium constant without going into first year stuff - law of mass action and thermodynamics

but one think i can say is

K is a CONSTANT that means this ratio of concentrations

[H+][HCO3-]/[H2CO3] is a constant at some specific temperature.

i.e. concentration of H+ x concentration of HCO3- / concentration fo H2CO3 at equilibrium never changes.

if at any time [H+][HCO3-]/[H2CO3] <- this ratio is smaller than K then eqm goes to RHS
if at any time [H+][HCO3-]/[H2CO3] <- this ratio is larger than K then eqm goes to LHS

(this is what HSC calls Le Chatelier's principles)

so we consider our case

adding water dilutes the concentrations but notice it dilutes it by the same amount

so

[H+] new = g * [H+] old
g is some constant < 1 (because we are diluting so must be less)
similarly

[HCO3-] new = g * [HCO3-] old
[H2CO3] new = g * [H2CO3] old

because originally we are at equilibrium

[H+]old * [HCO3-] old / [H2CO3] old = K

but once we add water

[H+] new * [HCO3 -]new / [H2CO3] new = g^2 / g * K = g * K

as g is less than 1, the new value g * K is smaller

therefore equilibrium goes to the RHS

 

[Frigid]

phenol... even i dun get what you say....

 

[phenol]

Le Chatelier's principle works but it is dodgy

it is a fundamental way of summing up partially the laws of thermodynamics and mass action. When it comes to water, the principle kinda stuffs up a little

CO2 + H2O <=> H2CO3

this eqm doesn't get shifted by adding water, think of it this way

if we ignore the water

CO2 <=> H2CO3 - same number of moles on either side, diluting either side = same therefore water doesn't do anything

but for this one

H2CO3 + H2O <=> H3O+ + HCO3 -
ignoring water again

H2CO3 <=> H3O+ + HCO3-

different amounts of moles on either side, 1 mol on LHS 2 mols on RHS so adding water shifts it to the side where there's more mols


this is a method i just came up with, there are flaws, dont kill me for it. helps understanding

 

[phenol]

frigid, ur crap

u dont deserve to have selina pic under ur name

 

[Frigid]

with only hsc chemistry bro...
lemme try this one... bcoz basically carbon dioxide eqm is the natural buffer in human blood:

H2CO3 + H2O <=> H3O+ + HCO3-

but, depending on whose acid theory you choose to believe, H3O+ = H2O + H+

therefore, the eqm becomes,

H2CO3 + H2O <=> H+ + H2O + HCO3-

since H2O exists on both, it regarded as not a factor.

so, yeah, it doesn't matter how much H2O you're adding, it's the same for both sides, everything else stays the same.
nice explanation, aint it phenol? even if its totally invalid .

or think of it this way, mei_ling, human blood is slightly basic... just bcoz i drink more water, does that mean my blood's gonna change pH?

 

[phenol]

it is totally invalid dude

i wont disprove you on your birthday

HAPPY BIRTHDAY FRIGID

 

[mei_ling03]

lol nice explanation frigid...isn't that just cos water disassociates to form OH- and H+ ions?

thanks for the help from both of you but really now i'm just in a state of confusion. this won't be in the exam, will it?

 

[phenol]

nope, most unlikely

frigid is misleading, his explanation is nice but wrong

 

[mei_ling03]

yeah but easy to understand for people like me. it would have been nicer if i'd done physics and industrial chemistry, then i might have been able to understand whatever you wrote before lol

happy birthday frigid!

 

[Frigid]

to be on the same level as phenol, you need to at least have competed in chem olympiad or done first year chem...
haha, crazzzzzzzy!
thx mei_ling

 

[mei_ling03]

ohh i see. well, i have to say i'm not that into chem that i'd do it at uni... these exams are the end of my chem days!

Originally posted by phenol
Le Chatelier's principle works but it is dodgy


H2CO3 + H2O <=> H3O+ + HCO3 -
ignoring water again

H2CO3 <=> H3O+ + HCO3-

different amounts of moles on either side, 1 mol on LHS 2 mols on RHS so adding water shifts it to the side where there's more mols

i thought the movement to the side of less molecules only applied for gases?

 

[Frigid]

the movement to side of less molecules applies for increasing gas pressure...

but yeah, phenol's explanation is 'sort-of' putting it in terms that we can understand...

and yes, frigid's results are a bit fucked, but so is hsc chem course, wouldn't you agree phenol?