Does adding or removing pure liquids/solids affect equilibrium?

[MechRocket]

According to Zumdahl's textbook, it doesn't.

In the book, there is a question that asks which way the equilbrium will shift if additional UO2(s) is added to the system.

UO2(s) + 4HF (g) <-------> UF4(g) + 2H2O(g)

The answer is the equilibrium is not affected.

HOWEVER,

I stumbled upon a thread on studentdoctor in which the original poster claims he/she has seen a problem in which removal of H2O(l) would shift the reaction to the left.

For the following reaction at equilibrium

2NO2(g)+H2O(l) (it's l not g) <-> HNO2(aq)+HNO3(aq),

which will shift reaction to the left?

Why the discrepancy?

Link here: http://forums.studentdoctor.net/showthread.php?p=9828806#post9828806

 

[Ygggdrasil]

In the second case, adding more water to the system will dilute the HNO2 and HNO3 (since these are aqueous). Therefore, to re-establish equilibrium, the reaction will proceed to create more products.

In general, however, adding more pure solid or liquid to a system should not affect the equilibrium of a system, assuming the addition of the solid or liquid does not change the concentrations/partial pressures of the other components of the system.

 

[MechRocket]

Brilliant, thanks!

 

[MechRocket]

@Yggg: I have another question.

How would you write the equilibrium equation? Would you take into account H2O in the denominator or no?

 

[Ygggdrasil]

No, you would not need to include the concentration of water in the denominator. Assuming the solution is sufficiently dilute, the concentration of water will essentially be a constant ~55.6 M throughout the reaction.

The expression for the equilibrium constant would look something like:
K = [HNO2][HNO3]/(P_NO2)^2