Why doesnt pressure alter the equlibrium constant value?

T.r.i.n.i.t.y

Why doesnt pressure alter the equilibrium constant value?

Reply 1

hornblower

T.r.i.n.i.t.y

Why doesnt pressure alter the equilibrium constant value?

Because it's a constant.

The equilibrium constant stays constant at constant temperature.

Reply 2

vinny2256

Kc is only altered by temperature.

Reply 3

theoffender

eah this confused me a bit...

Say you've got the Haber process, N2(g) + 3 H2(g) ---> 2 NH3(g) , then surely if you increase the pressure, the equilibrium will shift to the right because there are less moles of gas on the right (in an attempt to equalise the pressure according to le chatelier's principle) . Therefore, there is now a greater proportion of products.

So, if Kc (the equilibrium constant) = [NH3]squared / [N2] x

cubed , then if there are more products (ie NH3) the equilibrium constant will increase?

My teacher told me why this is wrong but I can't remember what he said. Hmmm, this stuff gets confusing when you think too hard about it. I think i'll just accept that Kc is only changed by temperature!

Reply 4

ResidentEvil

theoffender

The equilibrium DOES shift to the right due to less gas moles, and yes there will be greater products. However, we are not working with Kc! Pressure is to do with Kp. If the pressure increases or decreases, then finding the partial pressure of each compound will remain entirely constant if u think about it (u multiply each mole fraction by its pressure, so pressure does not affect Kp).

If you want Kc in the haber process above, then u dont take into account pressures, its excluded from Kc, u take into account concentration as being equal to pressure in Kp. Consider increasing the concentration of N2, u would form more products, and is this balanced out? Yes. You've increased N2 and cause ammonia to increase, therefore its balanced and constant so does not affect Kc.

Reply 5

theoffender

ResidentEvil

Hmm..... I don't get it!

Reply 6

Ben.S.

ResidentEvil

Just because you 'multiply each mole fraction by its pressure' does not mean that pressure does not have an effect upon the equilibrium constant! That theory falls apart whenever Kp does not happen to be dimensionless (at A-level anyway: normally we make it dimensionless and define it in a slightly different way). The real reason (or, the next elaborate lie) has to to with the relationship, (delta)G(standard) = -RT(lnK). The 'standard' part implies conditions of constant pressure, indicating that Kp does not change value when the temperature changes. A difficult concept to grasp (I think!) is that Kp does not vary with actual pressure, but the composition of the equilibrium mixture does - this has to do with a parameter called the 'degree of dissociation'. It gets a bit complicated. At A-level, you don't even get taught why changing concentrations does not effect the equilibrium constant! Arguably, I've just stated some more waffle which looks more complex and doesn't explain it any better - that's chemistry for you!

Ben

Reply 7

T.r.i.n.i.t.y

THANKS EVERYONE.. erm... ill just stick to the fact the pressure doesnt affect the value of the equilibrium constant. LOLZ