Acids and Bases2

Acids and Bases2 - AdditionalAspectsof AqueousEquilibria...

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Additional Aspects of Aqueous Equilibria David A. Katz Department of Chemistry Pima Community College
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he Common n Effect The Common Ion Effect Consider a solution of acetic acid: HC 2 H 3 O 2( aq ) + H 2 O ( l ) H 3 O + ( aq ) + C 2 H 3 O 2 ( aq) If acetate ion is added to the solution, Le Châtelier’s rinciple says the equilibrium will shift to the left Principle says the equilibrium will shift to the left. Acetate ion can be added in the form of a strong lectrolyte such as sodium acetate Na electrolyte such as sodium acetate, Na C 2 H 3 O 2 Na C 2 H 3 O 2 (aq) Na + (aq) + C 2 H 3 O 2 (aq) Sodium acetate is 100% ionic in solution
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he Common n Effect The Common Ion Effect he extent of ionization of a weak electrolyte The extent of ionization of a weak electrolyte can be decreased by adding a strong lectrolyte to the solution that has an n in electrolyte to the solution that has an ion in common with the weak electrolyte.
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he Common n Effect The Common Ion Effect Example: Calculate the fluoride ion concentration and pH of a solution that is 0.20 M in HF and 0.10 M in HCl. K a for HF is 6.8 10 4 . Solution: HF (aq) + H 2 O (l) H 3 O + (aq) + F (aq) The equilibrium constant expression is q p [H 3 O + ] [F ] [HF] K a = = 6.8 10 4
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he Common n Effect The Common Ion Effect Because HCl, a strong acid, is also present, HCl (aq) + H 2 O (l) H 3 O + (aq) + Cl - (aq) e initial [H is not 0 but rather010 the initial [H 3 O + ] is not 0, but rather 0.10 M . [HF], M [H 3 O + ], M [F ], M Initial concentration 0.20 0.10 (from the HCl) 0 Change x + x + x Equilibrium oncentration 0.20 x 20 0.10 + x 10 x concentration 0.20 0.10 Remember, x is small and can be ignored
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he Common n Effect The Common Ion Effect (0.10) ( x ) (0.20) 6.8 10 4 = Solve for x. Rearrange to: = x (0.20) (6.8 10 4 ) (0.10) 1.4 10 3 M = x
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he Common n Effect The Common Ion Effect Therefore, [F ] = x = 1.4 10 3 M If we add x to 0.10 M [H 3 O + ] = 0.10 + x = 0.10 + (1.4 10 3 ) = 0.10 M and pH = log (0.10) pH = 1.00
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uffer Solutions Buffer Solutions A buffer is an application of the common ion effect th Because the common ion shifts the quilibrium in one equilibrium in one direction, a buffer is resistant to pH changes, even when small amounts of a strong acid or base is added.
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uffer Solutions Buffer Solutions If a small amount of hydroxide is added to an equimolar solution of HF in NaF, for example, the HF reacts with the OH to make F and water.
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uffer Solutions Buffer Solutions If acid is added, the F reacts to form HF and water.
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uffer Calculations Buffer Calculations Consider the equilibrium constant expression for the dissociation of a generic acid, HA: A + H + A [H 3 O + ] [A ] A] K = HA + H 2 O H 3 O + A [HA] a Rearrange this to separate out the [H O + ] [A ] A] K a = [H 3 O + ] [HA]
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uffer Calculations Buffer Calculations [A ] + [HA] K a = [H 3 O ] ke the negative log of both sides Take the negative log of both sides [A ] g = g [H + g [HA] log K a ( log [H 3 O ] ) + ( log p K a id base ) pH acid and [base] p K a = pH log [acid]
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