Effect of solids and liquids on equilibrium?

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Jan 27, 2013
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Okay, so these two questions from BR chapter 3 (Equilibria) are kind of confusing me.

Passage 2, q. 15:

H2 (g) + Br2 (l) ←→ 2 HBr (g)

"Addition of which of the following to the equilibrium mixture will NOT affect the partial pressure of H2 (g)?

A. NaOH (aq)
B. Br2 (l)
C. H2 (g)
D. HBr (g)"

With the answer being Br2, given the explanation:

"If the reaction is already at equilibrium, then neither a solid nor a liquid affects the equilibrium. ... A solid or liquid can only affect the net results (equilibrium conditions) if the system is not at equilibrium and the compound being added (the solid or liquid) is the limiting reagent. When added in, the system can then achieve equilibrium."

Passage 12, q. 81:

NO2 (g) + H2O (l) ←→ HNO2 (aq) + HNO3 (aq)
Reaction 4

MnCl2 + 2 NO2- (aq) ←→ Mn(NO2)2 + 2 Cl- (aq)
Reaction 5

MnCl2 + 2 NO3- (aq) ←→ Mn(NO3)2 + 2 Cl- (aq)

"Which will shift reaction 4 to the left?

A. Addition of sodium hydroxide to the solution
B. Addition of manganese(II) chloride to the solution
C. Removal of nitrate from the solution
D. Removal of water from the solution"

The answer given was D, with the explanation:

"Removal of water (a reactant) results in a shift in the reverse direction (left) to re-establish equilibrium."

I get that if water is the solvent, its concentration stays around 55.6 M and doesn't factor into the equilibrium constant, and nor would its addition shift equilibrium.

However, the first question tells me solids and liquids don't affect equilibrium, while the second is telling me it does.

This thread: http://forums.studentdoctor.net/thr...id-from-solution-displace-equilibrium.739815/

addresses the question, but not with a very satisfactory answer.

If we write out the equilibrium expressions for both, I come up with:

K = P(HBr)/P(H2), from which bromine is omitted because it is not a gas. As I understand, you can also write:
K = [HBr] / ([Br2] × [H2]) in which Br2 is included.

For Reaction 4:

K = ([HNO2] × [HNO3]) / ([NO2] × [Br2]); is that right?

I guess I'm just confused as to whether solids and liquids really do affect equilibrium or not, and if so, when. Thanks for the help!