Ebbing17 - John A. Schreifels Chemistry 212 Chapter 17-1 81...

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Unformatted text preview: John A. Schreifels Chemistry 212 Chapter 17-1 81 Chapter 17 Acid-Base Equilibria John A. Schreifels Chemistry 212 Chapter 17-2 82 Overview Solutions of a Weak Acid or Base Acid ionization equilibria Polyprotic acids Base ionization equilbria Acid-Base properties of Salts Solutions of a Weak Acid or Base with Another Solute Common Ion Effect Buffers Acid-Base Titration Curves John A. Schreifels Chemistry 212 Chapter 17-3 83 Acid Ionization Equilibria W eak acids and weak bases only partially dissociate ; their strengths are experimentally determined in the same way as strong acids and bases by determining the electrical conductivity. The reaction of a weak acid (or base) with water is the same as discussed in previous section. Consider the reaction: Hydronium ion concentration must be determined from the equilibrium expression. Relative strengths of weak acids can be determined from the value of the equilibrium constant. Large equilibrium constant means strong acid Small equilibrium constant means weak acid E.g. determine which acid is the strongest and which the weakest. Acid K a HCN 4.9x10- 10 HCOOH 1.8x10- 4 CH 3 COOH 1.8x10- 5 HF 3.5x10- 4 ] HA [ ] A ][ O H [ K ) aq ( A ) aq ( O H ) l ( O H ) aq ( HA 3 a 3 2- +- + = + + John A. Schreifels Chemistry 212 Chapter 17-4 84 Determining K from pH K a determined if pH and C HA known. Use the equilibrium expression for the acid. E.g. Determine the equilibrium constant of acetic acid if the pH of a 0.260 M solution was 2.68. Determine [H 3 O + ]; [HA]; and [A- ]. Strategy Calculate the [H 3 O + ] from pH; this is x in the table above. The rest of the quantities are obtained from the bottom row. HA(aq) + H 2 O(l) H 3 O + (aq) + A- (aq Initial conc 0.100 M to Equil.- x +x +x Equil. 0.100 M - x + x +x John A. Schreifels Chemistry 212 Chapter 17-5 85 Calculating Equilibrium Concentrations in Weak acid Solutions pH determined if K a and C a known; for the dissociation of acetic acid: [H 3 O + ] total = [H 3 O + ] CH3COOH + [H 3 O + ] H2O . [H 3 O + ] total [H 3 O + ] CH3COOH . The total hydronium ion concentration is often equal to the contribution from the weak acid which is usually a lot stronger acid than water. The total hydronium ion concentration is needed for the equilibrium calculation. ) ( ) ( l O H aq COOH CH 2 3 + ) aq ( COO CH ) aq ( O H 3 3- + + 5 3 3 3 a 10 x 8 . 1 ] COOH CH [ ] COO CH ][ O H [ K-- + = = ) l ( O H 2 2 ) aq ( OH ) aq ( O H 3- + + 14 3 w 10 x 00 . 1 ] OH ][ O H [ K-- + = = John A. Schreifels Chemistry 212 Chapter 17-6 86 pH from Ka and Ca E.g. Calculate the pH of 0.100M acetic acid. Given pKa = 4.76 Method I : Substitute into equilibrium equation to get x 2 + 1.75x10- 5 x - 1.75x10- 6 = 0....
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Ebbing17 - John A. Schreifels Chemistry 212 Chapter 17-1 81...

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