Acid-Base Equilibria Chapter 16

HA (aq) + H 2 O (l) H 3 O + (aq) + A - (aq) Weak Acids (HA) and Acid Ionization Constants HA (aq) H + (aq) + A - (aq) K a = [H + ][A - ] [HA] K a ≡ acid dissociation (ionization) constant KaKa weak acid strength

Calculations with Weak Acids Calculate K a from pH (Use ICE table) Calculate pH from K a and given concentration (ICE table) Calculate percent ionization = Concentration ionized Original concentration x 100%

What is the pH of a 0.50 M HF solution (at 25°C)? HF (aq) H + (aq) + F - (aq) K a = [H + ][F - ] [HF] = 6.8 x HF (aq) H + (aq) + F - (aq) Initial (M) Change (M) Equilibrium (M) x-x+x+x x x+x xx K a = x2x x = 6.8 x Ka  Ka  x2x = 6.8 x – x  0.50 If [HF] > 100 K a x 2 = 3.40 x x = M [H + ] = [F - ] = M pH = -log [H + ] = 1.72 [HF] = 0.50 – x = 0.48 M

When can I use the approximation? Then 0.50 – x  0.50 x = M 0.50 M x 100% = 3.6% Less than 5% Approximation ok. What is the pH of a 0.05 M HF solution (at 25°C)? Ka  Ka  x2x = 6.8 x x = M M 0.05 M x 100% = 12% More than 5% Approximation not ok. Must solve for x exactly using quadratic equation... Let’s determine error introduced: If [HF] > 100 K a

What is the pH of a M monoprotic acid whose K a is 5.7 x ? HA (aq) H + (aq) + A - (aq) Initial (M) Change (M) Equilibrium (M) x-x+x+x x x+x xx K a = x2x x = 5.7 x NO!! Approximation not ok. Is [HF] > 100 K a ?

K a = x2x x = 5.7 x x X x – 6.95 X = 0 ax 2 + bx + c =0 -b ± b 2 – 4ac  2a2a x = x = x = HA (aq) H + (aq) + A - (aq) Initial (M) Change (M) Equilibrium (M) x-x+x+x x x+x xx [H + ] = x = M pH = -log[H + ] = 1.625

Percent ionization = For a monoprotic acid HA: Percent ionization = [H + ] [HA] 0 x 100% [HA] 0 = initial concentration Concentration ionized Original concentration x 100% Fig 16.9 The more dilute the acid, the greater the percent ionization:

Polyprotic Acids Have more than one ionizable proton If difference between the K a1 and subsequent K a values > 10 3, the pH generally depends only on the first dissociation.

NH 3 (aq) + H 2 O (l) NH 4 + (aq) + OH - (aq) Weak Bases and Base Ionization Constants K b = [NH 4 + ][OH - ] [NH 3 ] K b ≡ base ionization constant KbKb weak base strength Solve weak base problems like weak acid except solve for [OHˉ] instead of [H + ].

What is the pH of a 0.15 M solution of NH 3 whose K b is 1.8 x ? In class exercise Ans. = 11.20

Table 16.4 Some Weak Bases and Aqueous Soln Equilibria

Relationship Between K a and K b HA (aq) H + (aq) + A - (aq) A - (aq) + H 2 O (l) OH - (aq) + HA (aq) KaKa KbKb H 2 O (l) H + (aq) + OH - (aq) KwKw K a K b = K w Weak Acid and Its Conjugate Base Ka =Ka = KwKw KbKb Kb =Kb = KwKw KaKa

Let’s say we need K b for the acetate ion, CH 3 COOˉ. CH 3 COOˉ + H 2 O ⇌ CH 3 COOH + OHˉ From a Table of Ionization Constants of Some Weak Acids: Acetic acid CH 3 COOH K a = 1.8 X 10ˉ 5 Kb =Kb = KwKw KaKa

Table 16.5 Some Conjugate Acid-Base Pairs

Acid-Base Properties of Salt Solutions Neutral Solutions: Salts derived from a strong acid and a strong base NaCl (s) Na + (aq) + Cl - (aq) H2OH2O Basic Solutions: Salts derived from a strong base and a weak acid. NaCH 3 COOH (s) Na + (aq) + CH 3 COO - (aq) H2OH2O CH 3 COO - (aq) + H 2 O (l) CH 3 COOH (aq) + OH - (aq) Hydrolysis ≡ the reaction of an anion or a cation of a salt with water

Acid-Base Properties of Salt Solutions Acid Solutions: 1) Salts derived from a strong acid and a weak base. NH 4 Cl (s) NH 4 + (aq) + Cl - (aq) H2OH2O NH 4 + (aq) NH 3 (aq) + H + (aq) 2) Salts with small, highly charged metal cations (e.g. Al 3+, Cr 3+, and Be 2+ ) and the conjugate base of a strong acid: Al(H 2 O) 6 (aq) Al(OH)(H 2 O) 5 (aq) + H + (aq) 3+2+

Acid Hydrolysis of Al 3+

Acid-Base Properties of Salts Solutions in which both the cation and the anion hydrolyze: K b for the anion > K a for the cation, solution will be basic K b for the anion < K a for the cation, solution will be acidic K b for the anion  K a for the cation, solution will be neutral

Factors Affecting Acid Strength Fig Trends in acid-base properties of binary hydrides

Factors Affecting Acid Strength

Arrhenius acid - a substance that produces H + (H 3 O + ) in water Brønsted acid - a proton donor Lewis acid - a substance that can accept a pair of electrons Lewis base - a substance that can donate a pair of electrons Definition of An Acid H+H+ H O H + OH - acidbase N H H H H+H+ + acidbase N H H H H +

Lewis Acids and Bases N H H H acidbase F B F F + F F N H H H No protons donated or accepted! Coordinate covalent bond