I took my final about a week ago, and I was curious about the answer to one of the questions. I haven't been able to ask my professor about it (since I'm no longer in that class), so I figured I'd try here.

 

Consider the following balanced equation. The reaction is endothermic as written.

BaSO4(s) + H(+)(aq) in equilibrium with Ba(2+)(aq) + HSO4(-)(aq)

(I was going to use the LaTeX, but when I looked at the guide all the appeared were errors, sorry).

 

Which of the following effects will favor a shift to the right side?

 

(A) Decrease temperature (B) Decrease pH © Add a soluble barium salt (D) Increase the pressure in the container (E) Add more BaSO4 (s)

 

I put E right away and moved on, but the correct answer is B. I understand why B is correct (lower pH, more H+), but why isn't E correct as well?

I'm pretty sure because it's a solid, and so doesn't play a role in the equilibrium. Only liquids and aqueous solutions do.

Pure solids and liquids have an activity (or in this case a concentration) of 1, so it doesn't matter how much of the solid you add because its coresponding value in the equilibrium constant equation will always be 1. H(+)aq on the other hand is not a pure solid or liquid, thus an increase or decrease in its concentration can cause a shift.

Pure solids and liquids have an activity (or in this case a concentration) of 1, so it doesn't matter how much of the solid you add because its coresponding value in the equilibrium constant equation will always be 1. H(+)aq on the other hand is not a pure solid or liquid, thus an increase or decrease in its concentration can cause a shift.

 

Ok, so I didn't get it quite right... I think I remember from something about my Gen Chem course that gases contribute to the equilibrium as well. Is that correct?

Ok, so I didn't get it quite right... I think I remember from something about my Gen Chem course that gases contribute to the equilibrium as well. Is that correct?

 

I know some gases, if not most, do. Like HCl bubbled through water gives HCl(aq), which isn't a pure solid or liquid, thus its addition would cause the reaction to shift. If I recall right, I believe some gases are more soluble than others. Something to do with the polarity of the gas molecules.

I thought all gases did, but it had to do with their pressure since most gases are very insoluble due to entropy?

 

Thanks, btw. I typed it out, and somehow I still failed to realize that it was a solid. I guess I should slow down just a little bit in the future.

I thought all gases did, but it had to do with their pressure since most gases are very insoluble due to entropy?

 

Thanks, btw. I typed it out, and somehow I still failed to realize that it was a solid. I guess I should slow down just a little bit in the future.

 

Yeah you're right. Now I remember you can use the equilibrium equation for gases as long as their concentrations are in atm's.