Principles of Electrochemistry and Ionic Solutions
Activity and Activity Coefficient
Activity (a): Activity is the effective concentration of species in solution.
Where:
- a = Activity
- γ = Activity coefficient
- C = Concentration
For an ideal solution: a = C
For a real solution: a = γC
For an electrolyte: a = a₊ · a₋
Activity and Mean Ionic Activity of Electrolytes
For an electrolyte: a = (a₊)ν₊(a₋)ν₋
Mean ionic activity (a±): a± = (a)1/ν
Mean ionic activity coefficient (γ±): γ± = (γ₊ν₊ · γ₋ν₋)1/ν
For a 1:1 electrolyte: a± = γ±C
Ionic Strength
Ionic strength measures the total ionic concentration in a solution.
Where:
- I = Ionic strength
- C = Concentration
- Z = Charge
Example: NaCl
Ionic Strength of Bi-trivalent Electrolytes
Example: Dissociation of Al₂(SO₄)₃
Debye-Huckel Limiting Law
This law describes the relation between the activity coefficient and ionic strength.
Where:
- A = Constant
- Z = Charge
- I = Ionic strength
For a 1:1 electrolyte: log γ± = -A |Z₊Z₋| √I
PAGE 2 (Next 4 Answers)
Classification of Galvanic Cells
Galvanic cells are classified into:
- Chemical cells
- Concentration cells
Chemical Cells vs. Concentration Cells
Chemical Cell: EMF is produced due to a chemical reaction. Example: Daniel Cell.
Concentration Cell: EMF is produced due to a concentration difference. Example: H₂ electrode at different pressures.
Types of Concentration Cells
There are two types:
- With transference
- Without transference
Standard EMF of Concentration Cells
Why is the standard EMF (E°) equal to 0 for a concentration cell? Since the electrodes are the same, the standard potentials cancel out. Therefore, the EMF is due to the concentration difference only.
Liquid Junction Potential
Liquid junction potential is the potential difference developed at the junction of two electrolyte solutions of different concentrations due to the unequal migration of ions.
How Liquid Junction Potential Arises
When two electrolyte solutions of different concentrations are in contact:
- H⁺ ions move faster than Cl⁻ ions.
- Faster ions diffuse quickly to the dilute side.
- Slower ions remain behind.
- Charge separation occurs.
- A potential difference develops.
Thus, liquid junction potential arises due to unequal ionic mobility. For example, hydrogen ions diffuse faster, causing a potential to develop.
Minimizing Liquid Junction Potential
Liquid junction potential is minimized or eliminated by using a Salt Bridge (the most common method). A salt bridge contains electrolytes like KCl, KNO₃, or NH₄NO₃. These electrolytes have equal ionic mobility, reduce charge separation, and minimize liquid junction potential.
The Salt Bridge
A salt bridge is a device used in an electrochemical cell to connect two half-cells and complete the electrical circuit by allowing the flow of ions. It usually consists of a U-tube filled with an inert electrolyte like KCl or KNO₃ in agar-agar or gelatin.
Functions of a Salt Bridge
- Completes the electrical circuit.
- Maintains electrical neutrality in both half-cells.
- Prevents mixing of the two solutions.
- Minimizes liquid junction potential.
- Allows migration of ions.
Why KCl and KNO₃ are Used
- Equal ionic mobility: This reduces liquid junction potential.
- Chemically inert: They do not react with electrolytes in the half-cells.
- Highly soluble: They provide good ionic conductivity.
- Stable electrolytes: They do not disturb electrode reactions.
Electrolysis and Overvoltage
Decomposition potential is the minimum external voltage required to start electrolysis and decompose an electrolyte.
Polarization is the decrease in electrode potential due to the accumulation of products on the electrode surface during electrolysis.
Overvoltage is the difference between the actual electrode potential and the theoretical electrode potential.
Tafel’s Theory
Tafel’s theory states that overvoltage increases logarithmically with an increase in current density.
η = a + b log i
Where:
- η = Overvoltage
- a and b = Tafel constants
- i = Current density
When current density increases:
- The rate of the electrochemical reaction increases.
- More gas forms on the electrode surface.
- Polarization increases.
- Overvoltage increases.