water and solids in the constant

[mano91]

we dont include these do we?
is there any limitations to this wen working out the equilibrium constant?

[TheJosh]

I know that when looking at a weak acid ionisation reaction eg.   HA   +   H2O   <--> H3O+   +   A-

In the constant you dont include the water as its concentration is virtually constant in weak ionisations

[linny]

you include it when water is in gaseous state apparently
but when water is in liquid form you dont include it

[StringFever]

Oh, I thought we were to include them in the constant expression, but NOT in the calculations of the value?

[linny]

tsfx says that to put it in the expression

i dont know. personally i'd leave it out; thats the way ive been taught.....

[StringFever]

tsfx says that to put it in the expression

i dont know. personally i'd leave it out; thats the way ive been taught.....

Hmm...that makes me more iffy about it. While they're geniuses no doubt, some of their things don't quite gel with VCELand Chemistry :S

[linny]

im pretty sure you leave it out though ..

look through your past papers solutions.

[arthurk]

tsfx is bs anyway

[TheJosh]

tsfx is bs anyway

true dat

[rajah21]

I heard that, because solids are assumed to have a value of 1, you should leave them out in order to indicate that you realise that solids don't alter the K value?

Although, don't quote me on this as I may be wrong - I'm not sure how credible my source is. 

[arthurk]

I heard that, because solids are assumed to have a value of 1, you should leave them out in order to indicate that you realise that solids don't alter the K value?

Although, don't quote me on this as I may be wrong - I'm not sure how credible my source is. 

i heard this too from my tutor so it should be right

[TheJosh]

can you give example of solid in a equilibrium equation? and where you would omit it

[Potter]

can you give example of solid in a equilibrium equation? and where you would omit it

Decomposition of Calcium carboonate
CaCO3(s) <--> CaO(s) + CO2(g)

K = [CO2](all solids have a value of 1). If it's the reverse reaction it's K = 1/[CO2]

I don't think the vce course goes into heterogeneous equilibria(different states)

Thinking about it now... this post is pretty useless since exams are over xD

[linny]

lol im sure it'll come in handy in ............uni........

[biggzee]

you cant change the concentration of a solid or liquid, only gaseous and aqueous species have changable concentrations, and hence are the only ones that will have any effect on K.

while we assume that the concentration of a solid or liquid is 1, it is likely not, as it all depends on the structure,but it doesnt matter for the purposes of any calculations anyway. water for example has a concentraion of 55.6M at 4C (1 litre -> 1000g -> 55.6mol)