/sci/ - Science & Math

bamp.

>>le chateliers principle
First 6 issues of FMA manga.
Law of equivalent exchange.

here is a question:

my textbooks states that the equilibrium constant K
never changes. fair enough. then it states that K depends on temperature.
so which is correct? if K(T), then why doesnt it change?
So if you want to move the equilibrium constant of a reaction, the only thing you can do is change the temperature? Changing anything else will only change the number of products and reactants, but not their ratio?

also, if changing the concentration / volume of something so that the concentration quotient Q is different from K, is it correct to say that the reaction will happen in the direction that makes Q=K?
I hope so or I am fucked.

>>2196688
This may be due to vague descriptions, I'll try to put some in context of other concepts you're learning.

K is definitely temperature dependent. For a given temperature though K is constant for a reaction, even in the presence of a catalyst (catalysts effect the delta G of activation, not between final and initial). K is determined by the change in Gibbs free energy of products and reactants, and no matter what the reaction curve looks like the start and end will be the same at a given temperature.

And if I understand your second statement the way you said it then I think you've got that right...

>>2196700

ok thanks. this really isnt that hard, but "simple" textbooks can sometimes be really vague.

>>2196700
one more thing ( pardon the bad notation here)

if we consider reactions that have a eq like:
A = C + D

CD/A = K

where A,B,C are concentrations ( mol / L)
it seems to be reasonable for me that if we double the volume, we have

(C/2)(D/2) / (A/2) = K /2 (half of K, not K)

but then i gather this is a mistake?

but i also read that lowering the pressure moves the equlibrium to the side with fewest molecules, which would be the "reactant" A.

So is K also dependent on the volume, or am i missing something?

>>2196733

bump for help from helpfulanon

>>2196700

also, when dealing with spontaneous reactions - It says they are spontaneus if deltaG is negative.
This implies that K is "very large"
how much is "very large" here?

and vice versa for very small..

>>2196733
Its' simple.
K is only constant when concentrations AND temperature are constant.
So, obviously, changing any of that changes K.
P.S. highschoolfag detected.

>>2196733
There is a simple explanation and complicated explanation. K is really defined as the ratio of activities (I won't go into that) but for many systems can be defined as the ratio of concentrations (liquid systems). In gas systems it's the ratio of partial pressure (approximately).

As for...
>>(C/2)(D/2) / (A/2) = K /2 (half of K, not K)
I forget the terminology, but K will be a constant ratio of those concentrations, always the same. You're calculating some quantity that you compare to K to determine if you're at equilibrium (some textbooks might call it Q, I don't remember). So more accurately you'd write (C/2)(D/2) / (A/2) = Q /2
To avoid any confusion about K changing.

>>2196783
ok. so then I am right? this is actually a university class, but the lecturer and book is piss-poor.
I wish this was high school actually. Never had chemistry before.

tl;dr IF CHANGING ANYTHING CHANGES IT THEN WHY THE FUCK CALL IT A CONSTANT??!

>>2196783
You are wrong

>>2196774
Spontaneous reaction is any negative deltaG, that's correct. Even a small -deltaG like -0.001 is still a spontaneous reaction, it just may not occur often.

deltaG=-RT*ln(K)
This equation may or may not be beyond your class, so don't get worked up if it's new. But any negative delta G here gives a K>1

>>2196796
Because it is constant under constant conditions. Lol.

>>2196793
no u

>Le Chatelier's Principle

Sounds high school chemistry.

Having a test on this shit + ideal law today. Lololol, gonna nab meself an A.

>>2196801
Yeah what he said. To be more accurate you would simply say K=K(T) (approximately) since that means it's just a function of T. The important concept is K is independent of concentrations, independent on activation barriers, etc.

High school chemistry == Introductory college chemistry (chem101 and such) for all of you arguing about this trivial crap.

>>2196788
ok, that it depends on other things and that concentrations were just approx. i have more or less assumed. but the naming and explanations here are confusing.

so K is constant under change of pressure, and my equation is incorrect?
You are very correct about the other parameter being called Q.

If K does not change with pressure, how can the amount of reactant to product differ with pressure?

>>2196817
Don't quote me on any of this, it's been a few years since I've had real thermo classes (I just took a final on applied thermo an hour ago...). But gibbs energy
G = H - TS
and H = U + PV
So gibbs DOES depend on pressure. In liquid systems I'm pretty sure G is basically independent of pressure (the effect is negligible). This may not be the case in gas systems but that's beyond the scope of your course.

>>2196817
To answer the question of reactant:product depending on pressures....

So in gas systems you can define...
A + B -> C as the reaction
and Q = (Pc)/(Pa*Pb)
where Pi is the partial pressure of component i
K is defined similarly with partial pressure in gas situations. So you can see how pressures will change to achieve the equilibrium relationship? Just how concentrations will change to achieve the equilibrium relationship.

>>2196800
thanks, this is part of the curriculum. i had arrived at the same conclusion, and was wondering where in the world 1 is "very large".

Thanks for the help, am feeling pretty thick with high level math & phys classes but struggeling with this shit

>>2196800

Bro, even if delta G is -9001 it's not a guarantee it will go any faster than dG of -0.001.

>he got his thermodynamics in my kinetics!
>he got his kinetics in my thermodynamics!