Chap 19 - 19 Ionic Equilibria: Part II Buffers and...

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1 19 Ionic Equilibria: Part II Buffers and Titration Curves
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2 The Common Ion Effect and Buffer Solutions If a solution is made in which the same ion is produced by two different compounds the common ion effect is exhibited. Buffer solutions are solutions that resist changes in pH when acids or bases are added to them. Buffering is due to the common ion effect.
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3 The Common Ion Effect and Buffer Solutions There are two common kinds of buffer solutions: 1 Solutions made from a weak acid plus a soluble ionic salt of the weak acid. 2 Solutions made from a weak base plus a soluble ionic salt of the weak base.
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4 The Common Ion Effect and Buffer Solutions 1. Solutions made of weak acids plus a soluble ionic salt of the weak acid that contains it conjugate base One example of this type of buffer system is CH 3 COOH – weak acid NaCH 3 COO – contains conjugate base CH 3 COO -
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5 The Common Ion Effect and Buffer Solutions CH 3 COOH H + + CH 3 COO - weak acid reacts with base NaCH 3 COO Na + + CH 3 COO - 100% conj. base reacts with acid
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6 The Common Ion Effect and Buffer Solutions Example 19-1: Calculate the concentration of H + and the pH of a solution that is 0.15 M in acetic acid and 0.15 M in sodium acetate.
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7 The Common Ion Effect and Buffer Solutions CH 3 COOH H + + CH 3 COO - NaCH 3 COO Na + + CH 3 COO - 0.15 M 0.15 M
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8 The Common Ion Effect and Buffer Solutions CH 3 COOH H + + CH 3 COO - NaCH 3 COO Na + + CH 3 COO - 0.15 M - x x M x M 0 0.15 M 0.15 M
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9 The Common Ion Effect and Buffer Solutions CH 3 COOH H + + CH 3 COO - 0.15 M - x x M x + 0.15 M
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10 The Common Ion Effect and Buffer Solutions [ ][ ] [ ] COOH CH COO CH H K 3 3 a + = x - 0.15 0.15) (x (x) 10 8 . 1 5 + = ×
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11 The Common Ion Effect and Buffer Solutions 0.15 (0.15) (x) 10 8 . 1 0.15 x - 0.15 and 0.15 x 5 = × = + M ] [H 10 8 . 1 x 5 + = × = M 4.74 ] [H log - pH = = +
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12 The Common Ion Effect and Buffer Solutions This is a comparison of the acidity of a pure acetic acid solution and the buffer described in Example 19-1.
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13 The Common Ion Effect and Buffer Solutions Compare the acidity of a pure acetic acid solution and the buffer described in Example 19-1. Solution [H + ] pH 0.15 M CH 3 COOH 1.6 x 10 -3 2.80 0.15 M CH 3 COOH & 0.15 M NaCH 3 COO buffer 1.8 x 10 -5 4.74 [H + ] is 89 times greater in pure acetic acid than in buffer solution.
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14 The Common Ion Effect and Buffer Solutions HA H + + A - The generalized ionization constant expression for a weak acid is: The general expression for the ionization of a weak monoprotic acid is:
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15 The Common Ion Effect and Buffer Solutions If we solve the expression for [H + ], this relationship results: ] [A ] HA [ K ] H [ - a + = +
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16 The Common Ion Effect and Buffer Solutions If the salt’s cation is not univalent the relationship changes to: ion on charge n ] [A n ] HA [ K ] H [ - a = + = +
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This note was uploaded on 09/18/2010 for the course CHEM 1212 taught by Professor Suggs during the Fall '08 term at UGA.

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Chap 19 - 19 Ionic Equilibria: Part II Buffers and...

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