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Acid-base Equilibria 2.pdf

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University of Alberta
Sai Yiu

UNIVERSITY OF ALBERTA INTRODUCTORY UNIVERSITY CHEMISTRY II (CHEM 102) Ionic equilibria (Buffers& Titrations) The common-ion effect Common-ion effect occurs w hen a reactant containing a given ion is added to an equilibrium mixture that already containsthat ion. - For example: HA + A A is the common ion The equilibrium of acetic acid in water is: - + CH C3OH (aq) + H O (l) 2CH COO (aq) + 3 O (aq) 3 If a solution of sodium acetate is added, the acetate ion becomes the common ion. + - CH 3COONa (s) + H O (l2 → Na (aq) + CH COO (aq) 3 Common ion - acetate (3H COO ) Acetic acid Sodium acetate CH 3OO - CH 3OO - H3O+ Na+ CH COO - 3+ H3O Na+ Acetic acid + sodium acetate What is the effect of the common ion? - + CH C3OH (aq) + H O (l) 2CH COO (aq) + 3 O (aq) 3 1 Acid-base buffer solution A buffer is a solution that resists a change in its pH when either an acid or a base is added to it. - • A solution that contains an acidic component (reserve) – to react with OH and a basic component (reserve) – to react with H . + • The two components must not neutralize each other and so a strong acid and a strong base cannotact as a buffer. What constitutes a buffer? Most commonly, the components of a buffer are the conjugate acid-base pair (preferably with similar concentrations) of a weak acid / base Which of the following pair of compounds would form a buffer solution? H SO / NaHSO 2 4 4 Na 2CO /3NaHCO 3 NH C4 / NH 3 How does an acid-base buffer work? (Qualitative approach) For example: A buffer solution of acetic acid (CH 3COOH) and its conjugate base sodium acetate (CH COO Na ). + 3 The equilibrium of the buffer is: CH C3OH (aq) + H O (l)2 When a strong acid is added The conjugate base CH COO3 removes the proton form ing the conjugate acid - + CH C3O (aq) + H O (a3) → + The effect of the acid, H 3 is removed. When a strong base is added The conjugate acid CH CO3H removes the hydroxide ionforming the conjugate base - CH C3OH (aq) + OH (aq) → The effect of the hydroxide ion removed 2 Fig 19.3, p834 Buffers pH calculations (Quantitative approach) Calculation of the pH changein water : What is the change in pH when 1.00 mL of a solution of HCl (1.0M) is added to 1.0L of water? + - HCl (aq) + H 2O (l) → H O3(aq) + Cl (aq) Water is therefore a poor buffer – it does not resist pH changes. Homework What is the change in pH when 1.00 mL of NaOH (1.00 M) is added to 1.0 L ofwater? 3 Henderson-Hasselbalch Equation – An easy way for buffer calculations For any weak acid, HA, the equilibrium is: HA (aq) + H O 2l)  H O (a3) + A (aq) - Equation can be applied to a buffer as: As you can see, the Henderson [Base] pH = pK + aog Hasselbalch equation refers to [Acid ] the K af the conjugate acid, not the K bf the conjugate base This is called the Henderson-Hasselbalch Equation whichis best for calculating pH of a buffer solution. 4 The following is a typical example for buffe calculations. Example problem A buffer contains 0.22 M KHCO and 0.37 M of K CO . What is the pH of the buffer? If 3 2 3 0.020 mol of HCl is added to 0.25 L of this solution, what would be the resulting pH of this buffer? We now need to find the pH after the addition of the HCl. 5 Since an acid is added, the conjugate base is going to react with it as: CO 32-(aq) + H 3 (aq) → HCO + H O 3l) 2 Buffer Capacity The amount of protons or hydroxide ions a buffer can absorb without a significant change in pH is itsbuffer capacity. Two factors affect the buffer capacity: 1 The relative concentrations of the acid and its conjugate base A buffer whose pH is equal to or near the p K af its acid component has the highest capacity. + - HA (aq) + H O 2l)  H O (aq3 + A (aq) 6 [Base] pH = pK +alog [Acid ] Therefore a buffer with the highest capacity is when the concentration of its component is equal. Example: Given that we have a buffer system with HA / A , what is the percentage change in ratio of concentration by adding 0.010 mol of OH - to 1.00 L buffer of the following concentration: a) [HA] = [A-] = 1.000 M b) [HA] = 0.250 M, [A-] = 1.750 M 7 Conclusion: For a given addition of acid or base, the concentration ratio changes less for similar buffer component concentrations than it does for different concentrations. 2 The more concentrate the components of a buffer, the greater the buffer capacity. For example: A buffer with [HA] = [A -] = 1.0 M has a greater buffer capacity than a b uffer with [HA] = [A ] = 0.10M although both give the same pH. + - You must add more H O or3OH to a high-capacity buffer than to a low capacity buffer to obtain a given pH change. Buffer range Buffer range is the pH range over which the buffer acts effectively. - - When[A ] / [HA] > 10 and [A ] / [HA] < 1/10 , the buffer capacity is poor. The limit of pH in the buffer is therefore: pH = pK + 1 a or pH = pKa -1 Conclusion: The buffer range is: pH = pK a± 1 For example: A buffer with initial pH = 3.74 becomes useless when it shows a pH of 2.74 or 4.74 8 How to preparea buffer Thereare two ways by m
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