CHM1311 - Lecture(12)

CHM1311 - Lecture(12) - 1 More Acid-Base Chemistry: More...

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Unformatted text preview: 1 More Acid-Base Chemistry: More Acid-Base Chemistry: Common Ion Effect Common Ion Effect Buffer Solutions Buffer Solutions Acid-Base Titrations Acid-Base Titrations Petrucci, Ch. 17 Petrucci, Ch. 17 2 Common Ion Effect Common Ion Effect The common ion effect is a special case of Le Chateliers Principle. CH 3 COOH (aq) + H 2 O (l) H 3 O + (aq) + CH 3 COO- (aq) What happens if more CH 3 COO- is added to the solution? 3 Common Ion Effect Common Ion Effect Common Ion Effect : A shift in equilibrium caused by addition of a compound having an ion in common with the dissolved substance. Addition of sodium acetate (a strong electrolyte) to a solution of acetic acid will shift the equilibrium to the left due to the common ion effect. CH 3 COO- is the common ion: 4 Common Ion Effect and pH Common Ion Effect and pH CH 3 COOH (aq) + H 2 O (l) H 3 O + (aq) + CH 3 COO- (aq) K a = [H 3 O + ][CH 3 COO - ] [CH 3 COOH] We can rearrange this equation to derive an equation that relates the pH to the concentrations of acid and conjugate base: [H 3 O + ] = K a [CH 3 COOH] [CH 3 COO- ]- log [H 3 O + ] = - log K a + (- log [CH 3 COOH] [CH 3 COO- ] ) pH pK a + log [CH 3 COO- ] [CH 3 COOH] 5 Henderson-Hasselbalch Equation Henderson-Hasselbalch Equation pH = pK a + log [conjugate base] [acid] For an equilibrium involving a weak acid and its conjugate base: HA (aq) + H 2 O (l) H 3 O + (aq) + A- (aq) weak acid weak acid conjugate conjugate base base Equilibrium Equilibrium concentrations concentrations 6 Calculate the pH of a solution containing 0.20 M acetic acid (K a = 1.8 x 10-5 ) and 0.30 M sodium acetate. CH 3 COOH (aq) + H 2 O (l) H 3 O + (aq) + CH 3 COO- (aq) CH 3 COONa (aq) CH 3 COO- (aq) + Na + (aq) (Dissociates Completely) 0.30 M 7 Calculate the pH of a solution containing 0.20 M acetic acid (K a = 1.8 x 10-5 ) and 0.30 M sodium acetate. 8 Henderson-Hasselbalch Equation Henderson-Hasselbalch Equation In solutions containing a common ion, the initial concentrations of acid and conjugate base can be used in the Henderson- Hasselbalch Equation to calculate pH if the following is true: 1) The initial concentration of acid is large relative to the K a (K a x 100 < [acid]) 2) The initial concentration of the common ion (conjugate base) is large relative to the K a (K a x 100 < [common ion]) Initial concentrations can be used under these conditions because ionization of the acid and hydrolysis of the conjugate base are negligible . 9 What is the pH of a buffer solution composed of equimolar amounts of dimethylamine and dimethylammonium chloride? dimethylamine, K b = 5.4x10-4 [ ] [ ] a base pH = pK + log acid 1. 3.27 2. 4.73 3. 7.00 4. 10.73 5. 14.00 10 Buffer Solutions Buffer Solutions A buffer solution is a solution that contains relatively large concentrations of either: 1) a weak acid and its conjugate base or 2) A weak base and its conjugate acid Buffer solutions resist changes in pH. (Well see how!) Examples of buffer solutions: A solution containing 1.0 M acetic acid (CHA solution containing 1....
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CHM1311 - Lecture(12) - 1 More Acid-Base Chemistry: More...

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