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Ch_322b_18.02 - Chapter 18.2 lecture note

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Ionization of Carboxylic Acids Most simple carboxylic acids have K a values between 10 -4 and 10 -5 (pK a = 4-5) as defined by the following equations. RCO 2 H + H 2 O K eq RCO 2 - + H 3 O + K eq = [RCO 2 - ] [H 3 O + ] [RCO 2 H] [H 2 O] The acidity constant is defined as K a = K eq [H 2 O] = [RCO 2 - ] [H 3 O + ] [RCO 2 H] where for dilute solutions [H 2 O] is a constant, so the activity of water is a ~ 1 . ~
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example The pH of a 1 M solution of acetic acid (K a = 1.75 x 10 -5 ) is estimated as follows. For the equilibrium, CH 3 COOH + H 2 O CH 3 CO 2 - + H 3 O + [CH 3 CO 2 - ] = [H 3 O + ] and [CH 3 CO 2 H] ~ [CH 3 CO 2 H] initial ~ Therefore, K a = [H 3 O + ] 2 1.0 M = 1.75 x 10 -5 and [H 3 O + ] = 4.2 x 10 -3 and the pH of the solution is pH = -Log(4.2 x 10 -3 ) = 2.4.
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Carboxylic acids are sufficiently acidic to react with an aqueous solution of sodium bicarbonate to produce their carboxylate salts: COOH benzoic acid K a = 6.5 x 10 -5 stronger acid + NaHCO 3 stronger base CO 2 Na + [H 2 CO 3 ] carbonic acid K a = 4 x 10 -7 weaker base weaker acid sodium benzoate A classic way to distinguish between carboxylic acids (pK a ~ 4-5) and phenols (pK
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