We continued to discuss solubility by analyzing the effect pH has on solubility. Acids and bases neutralize each other and in the process the concentration of [H+] and [OH-] will either increase or decrease causing the equilibrium to shift. A shift in this equilibrium influences the solubility.
In order for an acid to neutralize a base or for a base to neutralize an acid, a species in the solution must be either acidic or basic. After writing out the equilibrium expression for the slightly soluble salt of interest, we need to determine if the cations/anions in solution are acidic or basic.
The conjugate acid of a weak base is considered to be acidic.
The conjugate base of a weak acid is considered to be basic.
The conjugate acid of a strong base is considered to be neutral.
The conjugate base of a strong acid is considered to be neutral.
In order to determine if the cation/anion of interest is acidic or basic, you need to be able to rattle off all of the strong acids and strong bases off the top of your head.
Strong Acids: HCl, HBr, HI, HClO3, HClO4, HNO3, and H2SO4.
Strong Bases: LiOH, NaOH, KOH, RbOH, CsOH, Ca(OH)2, Sr(OH)2, Ba(OH)2
I went through a few examples of how varying the pH can influence the solubility. Be sure to write out your solubility equilibrium expression as the first step to each problem.
We found that 8.27 x 10-7 grams of Fe(OH)2 will dissolve in 4 mL of water at a pH = 7, but when a solution is buffered to a pH = 6, the solubility dramatically increases to 0.175 grams in 4 mL.
Based on what we discussed up to this point, when we add a base, such as ammonia, to Zn(OH)2 we would expect its solubility to decrease. But experimentally we find that when concentrated ammonia is added the solubility increases. We now need to come up with a reason for why this happens. Chemists in the early 1900's were able to determine this increase in solubility was due to complex ion formation.
A complex ion contains a central metal ion surrounded by a number of ligands which are held together by coordinate covalent bonds. A ligand is a surrounding anion or molecule containing an unshared pair of electrons. Common ligands are NH3, Cl-, H2O, OH- among others
An example problem was performed showing how the formation of the Zn(NH3)42+ increases the solubility of Zn(OH)2.
On Monday we will pick up with this example and discuss a special case of the effect of complex ion formation referred to as amphoterism.
Quote of the Day:
"Failure to change is often just stubbornness that comes from an unwillingness to learn, an inability to realize you’re not perfect. There cannot be progress without change – even though not all change is progress."
Songs Played Before Class:
Tom Petty- Free Falling
Kenney Chesney - This is Our Time
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