Applications of Chemical Equilibrium

I. Common Ions
A. When a weak acid solution has common ions added by a salt the equilibrium will shift by Le Chatelier's principle. This shift in equilibrium position that occurs because of the addition of an ion already involved in the equilibrium reaction is called the common ion effect.
B. Buffered solution is one that resists a change in its pH when either hydroxide ions or protons are added. Buffered solutions are simply solutions of weak acids or bases containing a common ion. When hydroxide ions are added to a buffer solution they react with the acid and are replaced by the Anions.
C. If you know the ration of acid to base and both amounts are large the proton concentration can easily be found using the equation: [H+]=Ka*[HA]/[A-]. To find the pH easily the Henderson-Hasselbalch equation can be used: pH=pKa+log([A-]/[HA].
D. Buffer capacity of a buffered solution represents the amount of protons or hydroxide ions the buffer can absorb without a significant change in pH. Most effective buffer solutions contain large amounts of acid and conjugate base.
E. Titrations are commonly used to determine the amount of acid or base in a solution. The progress of an acid-base titration is often monitored by plotting the pH of the solution being analyzed as a function of the amount of titrant added. This is called a pH curve. The optimum area for buffering in a pH curve is in the vicinity where pKa equals the pH (or where the ration of A- to HA equals 1). The equivalence point is where the amount of protons is equal to the amount of hydroxide ions.
F. Indicators are actually weak acids themselves that change color when H+ is added or taken away from the structure. The ratio that must be reached for a color change to occur is 1 to 10 of In- to HIn or vice versa. The best indicator for a titration is given by a indicator with pKa or endpoint closest to the pH of the equivalence point.

II. Solubility Equilibria
A. When salts are added to water they dissociate into their ions. The solubility product constant or Ksp gives the amount of ions that are dissociated.
B. The common ion effect works much the same way for salts as it does for acids and bases.
C. The ion product or Qsp can predict whether a precipitate will form from the ions present in a solution. If the Qsp is bigger than the Ksp a precipitate will form.

III. Complex Ion Equilibria
A. A complex ion is a charged species consisting of a metal ion surrounded by ligands. The number of ligands attached to a metal ion is called the coordination number. Metal ions add ligands one at a time in steps characterized by equilibrium constants called formation constants or stability constants.