Le Chatieler's - Does it work on solids and liquids?

[theun4given]

I'm specifically looking at question 124 in gen chem in DAT destroyer. Perhaps I've never run into this before, but if we have a reaction in equilibrium that contains solids and pure liquids, do the concepts of le chatieler still apply? according to this problem, it doesn't look like it:

CaSO3 (solid) ----> CaO (solid) + SO2 (gas)

if this reaction is endothermic, which change will cause an increase in SO2?

a) removing CaO
b) adding CaSO3
c) decreasing the volume of the container
d) increasing the temperature
e) more than one of the above

Answer is D

I would think normally with le chatier that removing CaO, adding CaSO3 and increasing the temp would all shif the equilibrium to the right and increase SO2.

Also suppose alternatively we increased the volume of the container (thereby decreasing pressure). Normally that would shift to the side with more moles but would that not apply here too because of the solids?

thanks for the help. so much confusion about something i thought was so simple

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[drteeth]

Firstly, be careful posting copyrighted material word for word. Its against sdn policy to allow for that. Instead just cite the question number and refer to it within your question...

Le chatelier's principle does not apply to pure solids and liquids as you had guessed. The basic idea is this - everything in his principle states that the system will work to counteract change and keep balance. Thus, if the temperature is raised, it will push the rxn. toward the endothermic side (where heat is absorbed) so as to minimize the affect of the added temp. If the temp. is decreased it would push it to the exothermic side (where heat is released) to again keep things balanced.
Accordingly, if there is a decrease in volume, and therefore an increase in pressure, the balance will shift to the side with the least moles of gas - thus relieving the pressure. The opposite situation, with decreased P and increased V would push the rxn to the side with more moles of gas, and thus keep the pressure constant. Adding molecules that are solid or liquid would have absolutely no effect on the system.

Hope this helps!

 

[biocmp]

agreeing....

 

[theun4given]

Very helpful. Thank you very much!