part 16: pH

1. Arrhenius theory

2. Bronsted-Lowry

3. Lewis theory

Theories of Acids and Bases

Svante Arrhenius

Arrhenius theory was proposed by?

H+

In Arrhenius theory, acids are substances that dissociates when dissolved in water and produces _____________

OH-

In Arrhenius theory, bases are substances that dissociate and release ______________ ions

1. Arrhenius theory is limited to behavior of acids and bases in aqueous solutions and not in non-aqueous media

2. Many substances do not have H+ yet they behave as acids

3. Many substances do not have OH- yet behave as bases

Limitations of Arrhenius theory

1. Johannes Bronsted

2. Thomas Lowry

Bronsted-Lowry theory was proposed by?

donate

In Bronsted-Lowry theory, acids are Hydrogen containing substance that _______ a proton (H+ ) to another

accept

In Bronsted-Lowry theory, bases are substances that _______ proton

Gilbert Lewis

Lewis theory was proposed by?

accept

In Lewis theory, an acid is a molecule or ion that can ________ a pair of electrons to form a dative bond

donate

In Lewis theory, a base is a molecule or ion that can ________ an electron pair to form a dative bond

Equilibrium dissociation constant

• Quantitative measure of the amount of dissociation in aqueous solutions

• Quantitative measure of the strength of an acid or base in solution

Acidity Constant or Acid Ionization Constant

“ka” is also known as?

↑ acidity

↑ ka = ___ acidity

↑ acidity

pka = -logka

↓ pka = ___ acidity

Basicity Constant or Base Ionization Constant

“kb” is also known as?

↑ basicity

↑ kb = ___ basicity

↑ basicity

pkb = -logkb

↓ pKb= ___ basicity

Formula for strong acids and bases

Formula for weak acids and weak bases

2.15

Sample problem: pH

Compute for the pH of a solution containing 0.5 M boric acid with ka of 1x10-4 .

3.09

Sample problem: pH

What is the pH of a 0.01 M benzoic acid solution?

• Given: benzoic acid

• ka= 6.5 x 10 -5

11.54

Sample problem: pH

A sodium hydroxide solution has [OH- ] = 3.50 x 10-3 M. What is its pH?

10.87

Sample problem: pH

What is the pH of 0.032 M ammonia solution with Kb of 1.71 x10 -5

Buffer

• Solutions that resists pH change even if a small amount of strong acid or strong base is added

• It protects the formulation from a sudden change in pH

1. Carbonic acid

2. Bicarbonate

Primary buffer in blood

1. Acid buffer

2. Basic buffer

Types of Buffer

Weak acid + Conjugate base

Acid buffer

Weak base + Conjugate acid

Basic buffer

Henderson-Hasselbalch Equation

• Buffer equation is also known as?

• Used to calculate the pH of a buffer solution

p.s.: take note of its spelling! 🙂

Henderson-Hasselbalch Equation for Weak acid

Henderson-Hasselbalch Equation for Weak base

4.56

Sample problem: pH

Calculate the pH of the buffer if the molar concentration of acetic acid is 0.03 M and that of sodium acetate is 0.02 M. The pKa for acetic acid 4.74

9.33

Sample problem: pH

What is the pH of a buffer solution that contains 0.24 M NH3 and 0.20 M NH4Cl?

• Given: ka= 5.6 x10-10

Buffer capacity

• Effectiveness of buffer in minimizing pH changes

• The magnitude of resistance of a buffer to pH changes

• The amount in gram equivalents per liter (Normality) of strong acid or strong base required to be added to a solution to change its pH unit by 1

1. Buffer effect

2. Buffer efficiency

3. Buffer index

4. Buffer value

Buffer capacity is also known as?

Van Slykes Equation

Buffer capacity equation

Van Slykes Equation

pH = pKa

Maximum Buffer Capacity

Bmax = 0.756 x C

Bmax = ?

0.092

Sample problem: pH

What is the maximum buffer capacity of a buffer solution containing 0.1 M of weak base and 0.06 M of its conjugate acid?