Equilibrium at Saturation Point

[BlueMoon22]

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Hi all,

After taking the EK Chem. Lecture 4 30 min. exam, I think I have pinpointed a concept that I do not fully understand. If you guys could help me with this, it would be much appreciated. Let's consider the simple reaction:

NaCl(s) <==> Na+(aq) + Cl-(aq)

Scenario 1: Let's say that we keep adding NaCl(s) and it keeps dissolving until a point that it no longer dissolves anymore. In other words, it is in equilibrium, and it has become "saturated." At this point, if we add more solid NaCl, no more will dissolve. But, this is contrary to what you would think would happen if you consider Le Chatelier's Principle. According to that Principle, it should keep dissolving. Is it because the concentration of the ions has reached the Ksp and therefore no more will dissolve once you reach the solubility currently equals Ksp?

Scenario 2: Again, let's have the solution in equilibrium (saturated). This time, what would happen if we remove ALL of the NaCl solid that was in equilibrium with the ions? According to Le Chatlier's principle, it should shift left. This is what really confuses me because from what I read online (http://bouman.chem.georgetown.edu/S02/lect8/lect8.htm, paragraph under "Change in reactant or product concentration"), nothing would happen. Can someone please why no shift would occur? Is it again because the solubility currently equals Ksp?

I feel like I'm on the right track, but some help would be nice!

Thanks! 🙂

 

[V5RED]

LeChatlier's principle applies to concentration of solutes and pressure of gasses, not to liquids and solids.

The principle is that the ratio of products and reactants will shift so that K is reached. Solids and liquids don't get counted in K, so they are a non factor.

 

[shaboobly]

think about what happens in real life. if you dissolve salt in water to the point of saturation, and then remove the remaining solid crystals, would the dissolved salt particles precipitate? no. this is because of what was stated in the post above: solids and liquids do not play a part in the equilibrium constant.

 

[BlueMoon22]

Great, thanks for your responses. So, if you remove all of the solid in scenario 2 will you still be considered in equilibrium? I feel it could go both ways. One way of looking at is it that there is no more precipitate and therefore it is no longer saturated and thus not in equilibrium. However, another way of looking at is that the product of the ion concentrations still equal Ksp so therefore isn't it in equilibrium?