CHE 401 : ANALYTICAL CHEMISTRY
MODULE
3
CHEMICAL EQUILIBRIA OF AQUEOUS SOLUTIONS
INTRODUCTION
This module provides a fundamental approach to chemical equilibrium, including calculations of
chemical composition and of equilibrium concentrations for monoprotic acid/base systems. Buffer
solutions and its properties, which are extremely important in many areas of science, are also
discussed. The students will be given an introduction on the effect of electrolytes on aqueous
solutions.
INTENDED LEARNING OUTCOMES
At the end of the module, the following learning outcomes are expected to be acquired by the
students:
1. Understand the concepts and be familiar with the steps and techniques employed in
gravimetric and volumetric method of analysis.
2. Execute calculation techniques used in stoichiometric analysis.
3. Acquire skills in laboratory technique required to perform and plan chemical analysis and
systematically collect and interpret data obtained in quantitative analytical process
CONTENT OVERVIEW
Indicative contents included the following topics:
1. Aqueous solutions and chemical equilibria
2. Buffer solutions
3. Effect of electrolytes on chemical equilibria
4. Solving equilibrium problems for complex systems
END OF THE MODULE TEST
The following are the assessment provided to evaluate students level of understanding after the
discussion of the modules:
Coursework (10 points )
Solving problems involving chemical equilibria, buffer solution, effect of electrolytes and
complex systems
1
,CHE 401 : ANALYTICAL CHEMISTRY
Table of Contents
MODULE 3. CHEMICAL EQUILIBRIA OF AQUEOUS SOLUTION ............................... 3
3.1 AQUEOUS SOLUTIONS AND CHEMICAL EQUILIBRIA ........................................ 3
3.1.1 Classification of electrolytes ..................................................................................... 3
3.1.2 Acids and Bases ........................................................................................................ 4
3.1.3 Chemical Equilibrium ............................................................................................... 7
3.1.4 Equilibrium constant equation .................................................................................. 9
3.1.5 Solubility Product Constant .................................................................................... 12
3.1.6 Acid-Base Dissociation Constant ........................................................................... 15
3.2 BUFFER SOLUTION .................................................................................................... 19
3.2.1 The Henderson-Hasselbalch Equation .................................................................... 19
3.2.2 Properties of Buffer Solutions ................................................................................ 20
3.3.3 Weak Acid – Base ionization.................................................................................. 22
3.3 EFFECT OF ELECTROLYTES IN CHEMICAL EQUILIBRIA ................................. 23
3.3.1 The Effect of Ionic Strength ................................................................................... 23
3.3.2 Activity Coefficients ............................................................................................... 25
3.3.2 The Debye-Huckel Equation................................................................................... 27
3.4 END OF MODULE TEST ............................................................................................. 29
2
,CHE 401 : ANALYTICAL CHEMISTRY
MODULE 3. CHEMICAL EQUILIBRIA OF AQUEOUS SOLUTION
3.1 AQUEOUS SOLUTIONS AND CHEMICAL EQUILIBRIA
An aqueous solution is a solution in which the solvent is water. Water is widespread use as medium
in carrying out chemical analysis. An aqueous solution is shown in Fig 1.3 in which the solute is
NaOH and solvent is water.
Fig. 3.1 Aqueous solution of NaOH
In this module, the solute which will be discuss further are electrolytes. Solution of electrolytes are those
which forms ions when dissolved in solvent hence produce solutions that conduct electricity.
3.1.1 Classification of electrolytes
Electrolytes are classified as strong and weak. Strong electrolytes completely ionize in a solvent while the
weak electrolytes partially ionize. Weak electrolytes will not conduct electricity and solutions containing
equal concentration of strong electrolytes. Figure 3.2 shows the effect of ions on the electrical conductivity
of water.
(a) (b) (c)
Fig. 3.2 Aqueous solution of NaOH
3
, CHE 401 : ANALYTICAL CHEMISTRY
Pure water Fig 3.2 (a) or an aqueous solution of a nonelectrolyte allows almost no current to flow,
and the bulb does not light. A weak electrolyte Fig 3.2 (b) produces a few ions, allowing some
current to flow and the bulb to glow dimly. A strong electrolyte Fig 3.2 (c) produces many ions,
allowing more current to flow and the bulb to shine brightly.
Solutes that forms strong and weak electrolytes are presented in Table 3.1.
Table 3.1 Classification of electrolytes
Strong Weak
1. Inorganic acids such as HNO3, HClO4, 1. Many inorganic acids. Including H2CO3,
H2SO4, HCl, HI. HBr, HCIO3, HBrO3 H3BO3, H3PO4,H2S, H2SO3
2. Alkali and alkaline-earth hydroxides 2. Most organic acids
3. Most salts 3. Ammonia and most organic bases
4. Halides, cyanides and thiocyanates of Hg,
Zn. and Cd
Note : H2SO4 is completely dissociated into HSO4 and H3O+ ions and for this reason is classified as a strong
electrolyte. It should be noted however that the HSO4- ion is a weak electrolyte. Being only partially dissociated
into SO4 and H3O+.
H2SO4 → HSO4- + H3O+ strong electrolyte
HSO4- ↔ SO42- + H+ weak electrolyte
3.1.2 Acids and Bases
J. N. Bronsted in Denmark and J. M. Lowry in England proposed independently a theory of
acid/base behavior that is particularly useful in analytical chemistry in 1923. According to this
theory :
An acid is a proton donor while the base is a proton acceptor.
H+ + OH- ↔ H2O (3.1)
Acid Base
The hydronium ion (H+) donate 1 proton, hence behave as an acid. On the other way, the hydroxide
ion (OH-) accepts a proton, thus so called a base.
4
MODULE
3
CHEMICAL EQUILIBRIA OF AQUEOUS SOLUTIONS
INTRODUCTION
This module provides a fundamental approach to chemical equilibrium, including calculations of
chemical composition and of equilibrium concentrations for monoprotic acid/base systems. Buffer
solutions and its properties, which are extremely important in many areas of science, are also
discussed. The students will be given an introduction on the effect of electrolytes on aqueous
solutions.
INTENDED LEARNING OUTCOMES
At the end of the module, the following learning outcomes are expected to be acquired by the
students:
1. Understand the concepts and be familiar with the steps and techniques employed in
gravimetric and volumetric method of analysis.
2. Execute calculation techniques used in stoichiometric analysis.
3. Acquire skills in laboratory technique required to perform and plan chemical analysis and
systematically collect and interpret data obtained in quantitative analytical process
CONTENT OVERVIEW
Indicative contents included the following topics:
1. Aqueous solutions and chemical equilibria
2. Buffer solutions
3. Effect of electrolytes on chemical equilibria
4. Solving equilibrium problems for complex systems
END OF THE MODULE TEST
The following are the assessment provided to evaluate students level of understanding after the
discussion of the modules:
Coursework (10 points )
Solving problems involving chemical equilibria, buffer solution, effect of electrolytes and
complex systems
1
,CHE 401 : ANALYTICAL CHEMISTRY
Table of Contents
MODULE 3. CHEMICAL EQUILIBRIA OF AQUEOUS SOLUTION ............................... 3
3.1 AQUEOUS SOLUTIONS AND CHEMICAL EQUILIBRIA ........................................ 3
3.1.1 Classification of electrolytes ..................................................................................... 3
3.1.2 Acids and Bases ........................................................................................................ 4
3.1.3 Chemical Equilibrium ............................................................................................... 7
3.1.4 Equilibrium constant equation .................................................................................. 9
3.1.5 Solubility Product Constant .................................................................................... 12
3.1.6 Acid-Base Dissociation Constant ........................................................................... 15
3.2 BUFFER SOLUTION .................................................................................................... 19
3.2.1 The Henderson-Hasselbalch Equation .................................................................... 19
3.2.2 Properties of Buffer Solutions ................................................................................ 20
3.3.3 Weak Acid – Base ionization.................................................................................. 22
3.3 EFFECT OF ELECTROLYTES IN CHEMICAL EQUILIBRIA ................................. 23
3.3.1 The Effect of Ionic Strength ................................................................................... 23
3.3.2 Activity Coefficients ............................................................................................... 25
3.3.2 The Debye-Huckel Equation................................................................................... 27
3.4 END OF MODULE TEST ............................................................................................. 29
2
,CHE 401 : ANALYTICAL CHEMISTRY
MODULE 3. CHEMICAL EQUILIBRIA OF AQUEOUS SOLUTION
3.1 AQUEOUS SOLUTIONS AND CHEMICAL EQUILIBRIA
An aqueous solution is a solution in which the solvent is water. Water is widespread use as medium
in carrying out chemical analysis. An aqueous solution is shown in Fig 1.3 in which the solute is
NaOH and solvent is water.
Fig. 3.1 Aqueous solution of NaOH
In this module, the solute which will be discuss further are electrolytes. Solution of electrolytes are those
which forms ions when dissolved in solvent hence produce solutions that conduct electricity.
3.1.1 Classification of electrolytes
Electrolytes are classified as strong and weak. Strong electrolytes completely ionize in a solvent while the
weak electrolytes partially ionize. Weak electrolytes will not conduct electricity and solutions containing
equal concentration of strong electrolytes. Figure 3.2 shows the effect of ions on the electrical conductivity
of water.
(a) (b) (c)
Fig. 3.2 Aqueous solution of NaOH
3
, CHE 401 : ANALYTICAL CHEMISTRY
Pure water Fig 3.2 (a) or an aqueous solution of a nonelectrolyte allows almost no current to flow,
and the bulb does not light. A weak electrolyte Fig 3.2 (b) produces a few ions, allowing some
current to flow and the bulb to glow dimly. A strong electrolyte Fig 3.2 (c) produces many ions,
allowing more current to flow and the bulb to shine brightly.
Solutes that forms strong and weak electrolytes are presented in Table 3.1.
Table 3.1 Classification of electrolytes
Strong Weak
1. Inorganic acids such as HNO3, HClO4, 1. Many inorganic acids. Including H2CO3,
H2SO4, HCl, HI. HBr, HCIO3, HBrO3 H3BO3, H3PO4,H2S, H2SO3
2. Alkali and alkaline-earth hydroxides 2. Most organic acids
3. Most salts 3. Ammonia and most organic bases
4. Halides, cyanides and thiocyanates of Hg,
Zn. and Cd
Note : H2SO4 is completely dissociated into HSO4 and H3O+ ions and for this reason is classified as a strong
electrolyte. It should be noted however that the HSO4- ion is a weak electrolyte. Being only partially dissociated
into SO4 and H3O+.
H2SO4 → HSO4- + H3O+ strong electrolyte
HSO4- ↔ SO42- + H+ weak electrolyte
3.1.2 Acids and Bases
J. N. Bronsted in Denmark and J. M. Lowry in England proposed independently a theory of
acid/base behavior that is particularly useful in analytical chemistry in 1923. According to this
theory :
An acid is a proton donor while the base is a proton acceptor.
H+ + OH- ↔ H2O (3.1)
Acid Base
The hydronium ion (H+) donate 1 proton, hence behave as an acid. On the other way, the hydroxide
ion (OH-) accepts a proton, thus so called a base.
4
