Homework Help: Chemistry

Posted by Riley on Friday, September 10, 2010 at 3:12am.

I'm doing chemical equilibrium, and there's a question in my lecture notes. Can someone check my answers and tell me if i'm right or wrong? Thanks!

The Kc expression for this reversible reaction:

3Fe(s) + 4H2O (g) --><-- Fe3O4(s) + 4H2 (g)

is [H2]^4 / [H2O]^4

Am I correct? My lecture notes apparently said that the reaction has a Kp expression but it didnt say anything about Kc expression. I just thought it can also have a Kc expression, not sure if i'm right.

Chemistry - Riley, Friday, September 10, 2010 at 3:17am
And this one too! I thought it should also have a Kc expression but my notes only mentioned the Kp expression:
( that weird arrow is the forward backward arrow for a reversible reaction)

H2(g) + Br2(l) --><-- 2HBr2(g)

Kc = [HBr]^2 / [Br][H2]

Please tell me if im right! I reasoned that there should be a Kc expression since for Kc expressions you should exclude water (when used as a solvent) and solids, so i thought that's applicable for these equations...

please help thanks!
Chemistry - Dr Russ, Friday, September 10, 2010 at 4:57am
What is included in the Kp expression - partial pressures of the components.

The partial pressure of a solid does not change, so does not appear in Kp. This is the same with Kc as solids do not appear in Kc.

Liquids do not appear in Kp either.
Chemistry - Riley, Friday, September 10, 2010 at 5:08am
Um. that does not answer my question of whether liquids, aqueous, and gases appear in Kc expressions or not.
Chemistry - Dr Russ, Friday, September 10, 2010 at 6:36am
OK

Gases appear in Kc expressions as concentrations.

In aqueous and other solvent systems the solute appears as a concentration in Kc expressions.

Pure materials such as solids and solvents (including water) do not appear in the equilibrium expression.

A substance such as water will of course appear if they are one of the products or reactants. For example in

H2(g)+O2(g)<=> H2O(g)

note the gas phase.

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