Self ionisation of water (1 Viewer)

[Aerlinn]

Just needed a little explanation of concepts. Is the ionisation constant/product of water (Kw) like K in that it tells you the extent the reaction progresses? So... this reaction:
H20(l) + H2O(l) <---> H3O+(aq) + OH-(aq) ionises very little? (Kw= 1.0*10^-14 at 25C)
When you get water like above and you increase and decrease the temp, the pH decreases and increases, but the solution basically stays neutral still doesn't it? 'Cause when you get net forward/reverse reactions because of the increases and decreases in temp, the H3O+ and OH- concentration both decrease and increase by the same amounts?

Just need clarification, that's all.

Also, don't exactly understand what the units of Kw refer to: 1.0* 10^-14 mol^2 dm^-6... =S And sometimes, it's written as mol dm^-3 ... is there a difference? :wave:

 

[Ea22.007]

with the dissociation of water its tru that the ionization is very little at 25C which is seen in how small Kw is, but since its in dynamic equilibrium if u raise of lower the temp the amount of ionization changes and also Kw changes.

if the amount of ionization is different at different temps then u would have different amounts of H30+ ions in the water at any one moment so ure pH would be different to 7 but the solution would still be neutral as u also have the same amount of OH- ions in there so a pH of 7 is not the only neutral one

the units of Kw are from [H+]*[OH-] which are each measured in mol dm^-3 so if u multiply them u get mol^2 dm^-3
but if u consider [H+]*[OH-] to be the same as 2*[H+] as in water [H+]=[OH-]
then it could be just mol dm^-3

 

[xiao1985]

ea22.007 is right in saying that it depends what you definte as neutral... if pH 7 = neutral, then yes, at different temperature, you will not get a neutral solution...

equilibrium constant is the only thing i am aware of which does not have a fixed unit... in this case, the equi constant is:

kw = [H3O+][OH-]/[H2O][H2O]

since concentration of water is more or less constant (55M), we usually just write:

kw = [H3O+][OH-]
and since both have the units of mol/L (or mols / 1000 cm^3; or mols / dm ^3 ) (note, 1dm = 10 cm), therefore unit for kw should be (mols/dm) * (mols/dm) = mols^2 * dm^-6


@ea22.007:

quote: but if u consider [H+]*[OH-] to be the same as 2*[H+] as in water [H+]=[OH-]

not 2*[H+], [H+]^2, which means the unit stays the same...
and no, it should not be mols dm^-3... the only way i can think of why it should be mol/dm^3 is when you have:

H2O <---> H+ + OH-
kw = [H+][OH-]/[H2O]
unit for all three quantitiy above are mol/dm^3
so the unit for kw would be: (mol/dm^3)*(mol/dm^3)/(mol/dm^3) = mol/dm^3

 

[Aerlinn]

with the dissociation of water its tru that the ionization is very little at 25C which is seen in how small Kw is, but since its in dynamic equilibrium if u raise of lower the temp the amount of ionization changes and also Kw changes.

if the amount of ionization is different at different temps then u would have different amounts of H30+ ions in the water at any one moment so ure pH would be different to 7 but the solution would still be neutral as u also have the same amount of OH- ions in there so a pH of 7 is not the only neutral one

Thanks for clarifying that bit, ea22.07... so my understanding is right, yes?

quote: but if u consider [H+]*[OH-] to be the same as 2*[H+] as in water [H+]=[OH-]

not 2*[H+], [H+]^2, which means the unit stays the same...
and no, it should not be mols dm^-3... the only way i can think of why it should be mol/dm^3 is when you have:

H2O <---> H+ + OH-
kw = [H+][OH-]/[H2O]
unit for all three quantitiy above are mol/dm^3
so the unit for kw would be: (mol/dm^3)*(mol/dm^3)/(mol/dm^3) = mol/dm^3

Xiao, you have successfully confused me with the units thingy now ... I dun know how you get from one step to the next... (but maybe because I've neva worked with mol/dm before...)

ea22.007 is right in saying that it depends what you definte as neutral... if pH 7 = neutral, then yes, at different temperature, you will not get a neutral solution...

I think he interpreted neutral as being [H3O+] = [OH-], which is what I see 'neutral' as, too.
As I see it, if you get some pure neutral water, and heat it up or cool it... the pH would change... but you would still have a neutral solution...

 

[xiao1985]

ok... when we are doing calculation to numbers, we can, actually, do calculation to units (actually we ARE doing calculation to units)

say for eg, you find mass of 2 moles of Na (MW = 22.99 g/mole)
what you do, instinctively, is 2 x 22.99 = 45.08 g...

so we multipled the two NUMBERS, and at the same time, we also multiplied the UNITS:

g/ mole x mole = g

because the moles cancel out... it works in alot of places... try it.... say d = s * t, molar volume things as well... try it out...

re: interpretation of neutral: yes you are absolutely right...

 

[Aerlinn]

I'm following... I think, except up to this bit:

g/ mole x mole = g

I'm not sure why, when you multiply the units, they cancel out. I'd understand if you had division, eg. g/mole divided by mole, maybe the moles would cancel... (if that's at all correct)... but when its being multiplied... huh?

 

[xiao1985]

lol look at it this way:

if you have 2/3 x 3 = 2

but 2/3 divide by 3 = 2/9... so when you multiply them, they cancel out...

same goes with units