ch302 notes ch8.1 - CH302 Chapter 8 notes part 1 Common Ion...

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CH302 Chapter 8 notes part 1 Common Ion Effects The common ion effect occurs in a solution in which the same ion is produced by two different compounds. It is a special case of Le Chatelier’s Principle. Recall, a weak electrolyte will partially ionize(dissociate) to form ions. AB A + + B - If another source of one of these ions is added to the solution, then this tendency of the electrolye to ionize is suppressed. Adding something containing A + OR B - to the solution will drive the equilibrium backwards. Two examples of this would be: 1. A weak acid plus a salt of this weak acid and a strong base . 2. A weak base plus a salt of this weak base and a strong acid . In water the weak acid or base only partly dissociates. But there are also now some EXTRA ions from the salt: Salt Anion (conjugate base of parent acid) + Cation (conjugate acid of parent base) In all cases, treat by assuming the SALT fully dissociates: then consider the effect on the equilibria for the weak acid or base. Example 1: Weak Acid PLUS Salt of Weak Acid and Strong Base : NaCH 3 COO Na + + CH 3 COO - CH 3 COOH H + + CH 3 COO - Extra anion from the salt drives the equilibrium to the left Example: We have a solution that is 0.10M CH 3 COOH and 0.2M NaCH 3 COO. We assume the salt breaks apart fully and the weak acid partially dissociates. Calculate pH. Then compare to a solution that is 0.10M CH 3 COOH. Example 2: Weak Base PLUS Salt of Weak Base and Strong Acid : NH 4 Cl NH 4 + + Cl - NH 3 + H 2 O NH 4 + + OH - Extra cation from the salt drives the equilibrium to the left Example: We have a solution that is 0.20M NH 3 and 0.1M NH 3 Cl. We assume the salt breaks apart fully and the weak base partially dissociates. Calculate pH. Then compare to a solution that is 0.20M NH 3.
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ch302 notes ch8.1 - CH302 Chapter 8 notes part 1 Common Ion...

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