Acid-Base Equilibria Acid-Base Properties of Water Water is an amphoteric substance. It is a weak electrolyte and is a poor conductor of electricity. However, it undergoes dissociation to a small extent: H2O(l) H+(aq) + OH-(aq) 2 H2O(l) H3O+(aq) + OH-(aq) Which are the conjugate acid-base pairs? Autoionization of water The equilibrium constant expression for the autoionization of water is given by

Kc =

[H+][OH-] [H2O]

The concentration of water in aqueous solutions is considered constant.

Kc [H2O] = [H+][OH-] Kw = [H+][OH-] Kw is the ion-product constant of water at a specific temperature. At 25C, Kw has a value of 1.0 x 10-14.

Acid-Base EquilibriaCalculate the concentration of H+ in (a) a solution in which [OH-] is 0.010 M; (b) a solution in which [OH-] is 1.8 x 10-9 M. (Unless otherwise stated, the temperature is 25C.) (a) Kw = [H+][OH-] (b) Kw = [H+][OH-] Kw 1.0 x 10-14 Kw 1.0 x 10-14 H+ = = H+ = = -] [OH 0.010 [OH-] 1.8 x 10-9 H+ = 1.0 x 10-12 M Is this solution acidic or basic? H+ = 5.6 x 10-6 M Is this solution acidic or basic?

What is the pH and pOH of each of the solutions given above? Recall: pH = -log [H+], pOH = -log [OH-]

Acid-Base Equilibria Relative strengths of acids and bases A strong acid completely transfers its protons while its conjugate base has negligible tendency to be protonated. A weak acid only partially dissociates and its conjugate base is a weak base. A substance with negligible acidity (e.g. CH4) has a conjugate base that is very strong.

Acid-Base Equilibria Strong acids and strong basesHCl(aq) H+(aq) + Cl-(aq) Kc = [H+][Cl-] [HCl] Since HCl is a strong acid and dissociates almost completely in the solution, [HCl] is almost zero. Therefore, HCl(aq) H+(aq) + Cl-(aq) Kc = [H+][Cl-] [HCl] [H+] = [Cl-] = initial concentration of HCl before dissociation took place] The above trend goes for strong bases.

(a) Calculate the pH of 0.10 M HNO3 (b) Calculate the pH of 1.0 x 10-10 M HCl (c) Calculate the pH of 0.0011 M Ca(OH)2

Acid-Base Equilibria Weak acids Consider the ionization of a weak acid in water:

Ka is the acid dissociation constant at a specific temperature. Just like in molecular equilibrium, an ICE table is used in solving acid-base equilibrium problems.

Acid-Base Equilibria Weak acids

The following approximation can be used since Ka is very small:

Acid-Base Equilibria Weak acids

When can we do the approximation? The approximation is valid if the result is 5% of the initial concentration.

The pH of a 0.10 M solution of formic acid (HCOOH) is 2.39. What is the Ka of the acid? (1.8 x 10-4)

Acid-Base Equilibria Percent ionization Another measure of acid strength is percent ionization which has the formula:

The extent of dissociation of a weak acid depends on the initial concentration of the acid.

Acid-Base Equilibria Weak bases and Base dissociation constant

What is the pH of a 0.26 M methylamine solution (Kb = 4.4 x 10-4)? What is the value of Ka for the conjugate acid of methylamine? Acid-base properties of Salts A salt is one of the products of an acid-base reaction (aside from water). The cation of a salt is from a base and its anion is from an acid. Salt hydrolysis refers to the reaction of a cation or an anion of a salt, or both, with water. Salts can be neutral, acidic, or basic depending on the strength of the acid and base that reacted to form the salt.

The conjugate base has a Kb value at a specific temperature. What is the value of Kb for acetate anion?

Acid-Base Equilibria Acid-base properties of Salts If a strong acid and a strong base react, the resulting salt is neutral. If a strong acid and a weak base react, the resulting salt is acidic. If a weak acid and a strong base react, the resulting salt is basic. If a weak acid and a weak base react, the pH of the resulting solution depends on the Kb of the conjugate base and Ka of the conjugate acid.

Acid-Base Equilibria Acid-base properties of Salts Tell whether the following salts are neutral, acidic or basic: (a) LiClO4 (b) Na3PO4 (c) NH4CN Calculate the pH of a 0.24 M sodium formate solution (HCOONa). Ka for HCOOH = 1.7 x 10-4 Calculate the pH of a 0.10 M NH4Cl solution. The Kb value for NH3 = 1.8 x 10-5.

Buffers A buffer solution is a solution consisting of (1) a weak acid or base and (2) its salt; both must be present. A buffer solution has the ability to resist changes in pH upon addition of small amounts of either acid or base. An example of a buffer solution is made up acetic acid (CH3COOH) and acetate anion (CH3COO-). If an acid is added to the system, the acetate ion will serve as a base to neutralize the added acid: If a base is added to the system, the acetic acid will serve as the acid to neutralize the added acid:

Which of the following are buffer systems? (a) KF/HF; (b) KBr/HBr; (c) Na2CO3/NaHCO3