Lecture #5, Wednesday, January 13th

Lecture started off by continuing with the influence of pH on the solubility of slightly soluble salts. By adjusting the pH, chemists have an excellent way to manipulate the solubility.

If you are asked if HCl (or any other reagent) would increase, decrease, or have no effect on the solubility of say, zinc hydroxide, the very first and most important thing you want to do is WRITE OUT THE SOLUBILITY EQUILIBRIUM. Remember, we are trying to manipulate the equilibrium, so it makes sense to write out the balanced equation first.

The second step is to identify if any of the cations/anions dissolved are acidic or basic. This requires a knowledge of the strong acids and strong bases. Remember from Chem 122 that the conjugate acid of a strong base is neutral and the conjugate base of a strong acid is also neutral. The general rule of thumb is: "The stronger the acid, the weaker the conjugate base and the stronger the base, the weaker the conjugate acid."

To a first approximation we can use Le Chatlier's principle to determine how an equilibrium will shift upon the addition of an acid or base. Acids neutralize bases (lower the hydroxide concentration) and bases neutralize acids (lower the hydrogen ion concentration). This can have a dramatic effect on the molar solubility. The calculations in class showed that at a pH = 7.0, the solubility of zinc hydroxide is 8.9 x 10^-7 grams in 4 mL of solution. But if the pH is adjusted to 6.0 then we can dissolve 0.0179 grams of zinc hydroxide in 4 mL of this solution. The take home message here is that pH plays a huge role in the solubility of slightly soluble salts and with this knowledge at our disposal, we can manipulate the solubility equilibrium by adding acidic or basic reagents.

Notice that in the previous paragraph I used the phrase "to a first approximation." Sometimes, based on fundamental principles, things happen that we do not expect. For example if you add concentrated ammonia (a base) to zinc hydroxide, Le Chatlier's principle would predict that the equilibrium will shift to make the zinc hydroxide less soluble. Experimentally, when ammonia is added to zinc hydroxide we observe a large increase in solubility.

Initially this was frustrating to chemists as they tried to come up with a theory for why this was happening. The theory we can use to explain this observation is "complex ion formation." If you have been paying attention you will have heard this term mentioned several times this quarter, but now we will investigate how and why complex ions form.

This opened up a whole new branch of chemistry as complex ions are sometimes referred to as coordination compounds, which involve a transition metal center attached to ligands (surrounding atoms/ions/molecules). This branch of chemistry dealing with transition metal, or coordination complexes, is inorganic chemistry (which is my favorite of all the chemistry subdivisions if you happen to be scoring at home).

We will go over several examples in lecture talking about coordination complexes and how complex ion formation shifts the resulting equilibrium expressions. Then we will discuss more on selective precipitation, which wraps up the solubility "unit." We will then transition to coordination compounds and investigate how and why a transition metal bonds to form a complex ion or compound.

Songs played before class:
KE$HA - TiK ToK (Apparently Dr. Zellmer is not very find of this song)
The Killers - All These Things That I've Done