Chem_111_unit_5 - Dr. M. Zewail UNIT V-ACID-BASE...

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Dr. M. Zewail UNIT V--ACID-BASE EQUILIBRIUM A) IONIZATION OF WEAK ACIDS The extent to which an acid ionizes in aqueous medium can be expressed by the equilibrium constant, K a , for the ionization reaction: HC 2 H 3 O 2 (aq) + H 2 O (l) H 3 O + (aq) + C 2 H 3 O 2 - (aq) K a = [H 3 O + ] [C 2 H 3 O 2 - ] [HC 2 H 3 O 2 ] The smaller the value of K a , the weaker the acid. From the value of K a , it is possible to calculate the concentration of the hydronium ion , [H 3 O + ] , in a solution of a weak acid. Example: K a for acetic acid is 1.8 x 10 -5 . Find the concentration of the [H 3 O + ] in a 0.10 M 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) Initial concentration Change in concentration Equilibrium concentration [H 3 O + ] = __________M Because the value of K a is small, we might guess that x is quite small. If we solve the problem using a quadratic formula, we find that x = 1.3 x 10 -3 M. We can make the approximation by ignoring x in the denominator. This type of approximation is used whenever a small fraction of acid ionizes. If the quantity ‘x’ which is subtracted from the original concentration is more than 5% of the initial value, it is best to use quadratic formula. In our example, x = 1.3 x 10 -3 M. % ionization = x ( 100 ) = 1.3 x 10 -3 (100) = 1.3 % C 0.10 B) IONIZATION OF WEAK BASES (NITROGENOUS BASES) Nitrogenous bases have a nonbonding pair of electrons on a nitrogen atom much like that found in ammonia, NH 3 . Like ammonia, these nitrogenous bases can react with water to produce OH - ions. NH 3 (aq) + H 2 O(l) NH 4 + (aq) + OH - (aq) K b = [NH 4 + ] [OH - ] (K b is the base ionization constant.) [NH 3 ] Example:
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K b for NH 3 (aq) is 1.8 x 10 -5 . Calculate the concentration of OH - in a 0.15 M NH 3 solution. NH 3 (aq) + H 2 O(l) NH 4 + (aq) + OH - (aq) Initial concentration Change in concentration Equilibrium concentration [OH - ] = ____ M C) THE EFFECT OF DILUTION ON THE PERCENT IONIZATION OF A WEAK ACID The extent of ionization depends on the initial concentration of the acid. The more dilute the solution, the higher the percent ionization. A qualitative explanation: When the acid is diluted, the number of particles per unit volume is reduced. According to Le Chatelier’s principle, to counteract this stress (that is the dilution) the equilibrium is shifted to the right to produce more particles. Although the percent ionization of the acid increases , yet the H + ion concentration becomes lower by dilution. Example: Compare the percent ionization of HF(aq) at the following concentrations: i) 0.10 M ii) 0.010 M K a for HF (aq) i s 7.1 x 10 -4 . i)
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Chem_111_unit_5 - Dr. M. Zewail UNIT V-ACID-BASE...

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