Chapter 18

Acid Base Equilibria

Properties of Acids and Bases

Acids Bases

Release H+ Release OH-

Neutralize OH- Neutralize H+

Proton Donors Proton Acceptors

Electron Pair Acceptors Electron Pair Donors

pH < 7 pH > 7

Taste Sour Taste Bitter, Feel Slippery

Litmus Paper (turns pink) Litmus paper (turns blue)

Reacts with most metals

to form H2(g)

Neutralization Reaction

Acid + Base Salt + H2O(l)

Strong acids and bases dissociate approximately

100%

Strong Acids: HCl, HBr, HI, HNO3, HClO4, HClO3,

H2SO4

Strong Bases: Soluble metal hydroxides

Weak acids and bases dissociate << 100%

Fig.

18.1

Weak Acid Equilibrium Expression

HA H+ + A-

Ka = [H+][A-] or

[HA]

Ka = [H3O+][A-]

[HA]

HA + H2O(l) H3O+ + A-

Fig.

18.2

Example

Write the equilibrium expression for the following

dissociation of benzoic acid:

Note: A 2 M solution only dissociates about 0.6%

O

O H

O

O

+ H

Fig.

18.3

Weak Base Characteristics

Weak bases, such as insoluble metal

hydroxides, dissociate << 100% in water---

releasing few OH- ions.

Weak bases, such as ammonia, amines and

amides, slightly react with water to release

OH- ions. This process is also called

dissociation.

Weak Base Equilibrium Expression

Kb = [HB+][OH-]

[B]

B + H2O(l) HB+ + OH-

Example

Write the equilibrium expression for the following

dissociation:

NH3 + H2O(l) NH4+ + OH-

Autoionization of Water

Water self-ionizes. Pure water dissociates, but very

little. It is a very weak electrolyte.

Write equilibrium expressions for the following

dissociations:

H2O(l) + H2O(l) H3O+ + OH-

H2O(l)H+ + OH-

Autoionization of Water Cont.

Using your equilibrium table/problem solving

technique learned in the last chapter,

determine the [H+] and [OH-] in water.

Kw = 1.0 X 10-14

Fig.

18.4:

Problem Solving

1. If HNO3 is added to H2O, the [H3O+] is

2.0 X 10-5M, what is the [OH-]?

2. If LiOH is added to water, the [OH-] is 2.0 X 10-5 M, what is the [H+]?

3. If H+ is added to water in which direction will the equilibrium shift?

4. If OH- is added to water in which direction will the equilibrium shift?

pH

The following relationships are often used to determine the pH and pOH of a solution.

pH = - log [H+] pOH = - log [OH-]

pKw = -log Kw 14 = pH + pOH

If the pH = 7, the solution is neutral; [H+] = [OH-]

If the pH < 7, the solution is acidic; [H+] > [OH-]

If the pH > 7, the solution is basic; [H+] < [OH-]

Fig. 18.5

Fig.

18.6

Fig. 18.7

Problem Solving

Determine the pH, pOH, [H+], [OH-] as appropriate for the following problems.

a. A sample of blood has a pH of 7.40.

b. A household ammonia solution has a [OH-]=7.94X10-3 M.

c. A sample of rain water has a pH of 5.40.

d. A 0.0400 M HBr solution.

e. A 0.0250 M NaOH solution.

More Problem Solving

1. What is the pH, pOH, [H+], [OH-] of a 0.20 M

Ba(OH)2 solution?

2. Consider a 3.00 L 0.00100 M HCl solution.

If 50.0 mL of 0.00900 M NaOH is added,

what is the pH of the solution?

Bronsted-Lowry Acids and Bases

Recall: Acids are proton donors and bases are

proton acceptors.

Fig.

18.8

An acid-base conjugate pair: Two species of an

acid-base reaction that differ by the loss or gain of a

proton (H+).

Application Time!

Look at the following acid-base reactions and

determine (a) the acid, (b) the base, and (c)

the acid-base conjugate pairs in each.

CO32- + H2O(l) HCO3

- + OH-

C2H3O2- + HNO2 HC2H3O2 + NO2

-

HCO3- + H2O(l) H3O+ + CO3

2-

HCO3- + H2O(l) OH- + H2CO3

Note: A substance that can act as an acid or a base is said to be amphiprotic.

Fig. 18.9: Strength relationship between acid-base conjugate pairs.

Lewis Acids and Bases

Recall: An acid can also be an electron pair

acceptor and a base can be an electron pair

donor.

Example: See the next slide

Label the Lewis acid and base. The product is

called an adduct.

Acid-Base Pair Strength Relationships

The higher the Ka, the stronger the acid, the

weaker the conjugate base.

Note: pKa = -log Ka

The higher the Ka, the lower the pKa.

The stronger the acid, the weaker the

conjugate base.

Weak Monoprotic Acid Problem

Solving

1. Determine the equilibrium constant for a 0.10 M phenol (HC6H5O) solution that has a pH of 5.43 at 250C. What is the degree of ionization of this weak acid?

2. What are the concentrations of nicotinic acid, hydogen ion, and nicotinate ion in a solution of 0.15 M nicotinic acid (HC6H4O2) at 25oC? What is the pH? What is the percent ionization?

Ka = 1.4 x 10-5

3. Determine the percent ionization for each of the

following solutions: 0.10 M HC2H3O2 and 0.0010

M HC2H3O2. Ka = 1.7 x 10-5

4. When 5.00 grains of aspirin are dissolved in 0.500

liters of solution, 0.325 grams of acetylsalicylic

acid (HC9H7O4) are present. What is the pH of the

solution?

Ka = 3.3 x 10-4

Polyprotic Acids

Polyprotic Acids are compounds that contain

more than one acidic hydrogen.

General Equilibrium Expressions for a

Triprotic Acid

H3A H+ + H2A- Ka1 =[H+][H2A-]

[H3A]

H2A- H+ + HA2- Ka2 =[H+][HA2-]

[H2A-]

HA2- H+ + A3- Ka3 =[HA2-]

[H+][A3-]

Note: Ka3 < Ka2 < Ka1 Why?

Problem Solving for Polyprotic Acids

1. Tartaric Acid, H2C4H4O6 is a diprotic acid.

What is the pH of a 0.10 M solution? What

is the concentration of C4H4O62-?

Ka1 = 9.2 x 10-4 and Ka2 = 4.3 x 10-5

2. What is the pH of a 0.10 M ascorbic acid

(H2Asc) solution?

Ka1 = 7.9 x 10-5 and Ka2 = 1.6 x 10-12

Weak Base Equilibria

Relationships worth knowing:

Kw = KaKb for weak acid-base conjugate pairs.

Write equilibrium expressions for each of the following

reactions and prove the above statement.

HB H+ + B-

B- + H2O HB + OH-

Weak bases include amines and amides such as

the following molecule.

Conjugate Base of a Weak Acid

The conjugate base (anion) of a weak acid can

react with water. Write a general reaction and

the equilibrium expression for this process.

Would you use Ka or Kb?

If given Ka how would you determine Kb?

Problem Solving for Weak Base

1. Determine Kb of a 6.0 M weak base having a

pH of 12.02.

2. Dissolved aniline, C6H5NH2, is used in

perfumes. What is the pH of a 0.035 M

solution?

Kb = 4.2 x 10-10

A- + H2O HA + OH-

Acid-Base Properties of Salt Solutions

Hydrolysis of an ion with water can produce an acidic or basic solution.

Examples: Determine if the aqueous solutions below are acidic, basic, or neutral.

Ammonium chloride

Ammonium cyanide

Iron(III) nitrate

Sodium chloride

Ammonium hypochlorite

Aluminum chloride

Problem Solving

1. What is the pH of a 0.10 M sodium acetate

solution? pKa = 4.74

2. If 3.00 g of KClO are added to 100 mL of

water, what is the pH of the solution.

Ka = 2.9 x 10-8