- Electrode potential is the potential difference established due to an electrode half reaction occurring at the surface of contact between metal and its ion solution.
- The potential associated with oxidation reaction is called oxidation potential, while the potential associated with reduction reaction is called reduction potential.
- The overall cell potential (emf) of a galvanic cell is the algebraic sum of the electrode potentials of anode and cathode. Ecell = Eoxi(anode) + Ered(cathode)
- When the galvanic cell operates, electrons are produced at the anode and move through the external circuit to the cathode due to the cell potential.
- Standard potential is the electrode or cell potential measured under standard conditions of 1 M concentration, 1 atm pressure and 25°C.
- According to IUPAC convention, standard potential of an electrode is taken as the standard reduction potential and the standard cell potential is given by: E∘cell = E∘(cathode) − E∘(anode)
Definitions [23]
Definition: Electrochemistry
Electrochemistry is the study of the production of electricity from energy which is released during spontaneous chemical reactions, as well as the use of electrical energy to bring about non-spontaneous chemical transformations.
Define the term cell constant.
In a conductivity cell, the distance l between the two electrodes and the area A of the electrodes are fixed. Therefore, the quantity `l/A` is a constant for a particular A conductivity cell. This quantity is termed as cell constant.
Definition: Conductance
The reciprocal of the electrical resistance is called the conductance.
\[G\propto\frac{1}{R}\]
The unit of conductance is ohm⁻¹ or mho and is denoted by Ω⁻¹. In SI, unit is S (Siemen).
Define anode
The electrode at which the oxidation occur is called anode.
Define the following term:
Fuel cell
Fuel cells are the galvanic cells in which the energy of combustion of fuels like hydrogen, methanol, etc., is directly converted into electrical energy.
Define cathode
The electrode at which the reduction occur is called cathode.
Definition: Standard cell potential (E°cell)
It is the cell potential when the concentrations of all the species are 1 M at 25°C, and the pressure of the gas involved is 1 atm at 25°C.
Define standard electrode potential.
Standard electrode potential is the difference of electrical potential between a metal electrode and the solution around it at equilibrium when all the substances involved are in their standard states.
The potential of an electrode assembly is referred to as the standard electrode potential when the following conditions are satisfied.
- The temperature of the electrode assembly is 298 K (25°C).
- The ion solution used in the assembly is of concentration 1 mol L−1.
- The pressure of the gas, if used in the assembly, is 1 atm.
Definition: Electrode Potential
The potential difference developed between the electrode and the electrolyte due to the loss or gain of electrons by the electrode is called the electrode potential.
Oxidation potential: M(s) ⇌ Mⁿ⁺(aq) + ne⁻ Reduction potential: Mⁿ⁺(aq) + ne⁻ ⇌ M(s) Reduction potential = − Oxidation potential
Definition: Cell Potential
The cell potential of the cell is the algebraic sum of the electrode potentials. Cell potential is called the electromotive force (e.m.f.) of the cell when no current is drawn through the cell.
Definition: Standard electrode potential (E°)
Standard electrode potential is the potential associated with the electrode reaction at an electrode when all solutes are 1 M, and all gases are at 1 atm and at 25°C (298 K).
Define Reference electrode
It is an electrode whose potential is arbitrarily taken as zero or is exactly known. Standard Hydrogen Electrode (SHE), calomel electrode, silver-silver chloride electrode and glass electrode are some examples of reference electrode.
Definition: Fuel Cell
A fuel cell is a galvanic cell in which the reactants are not placed within the cell, but are continuously supplied from outside, where one reactant acts as a fuel (such as hydrogen or methanol) and the other as an oxidant (such as oxygen).
Definition: Electrochemical Series
Based on the ease with which atoms of metals lose electrons to form positively charged ions, the metals are arranged in a series known as the electrochemical series.
or
The arrangement of electrodes in order of their decreasing standard reduction potentials is called electrochemical series.
Define electrochemical series.
The standard potentials of a number of electrodes have been determined using standard hydrogen electrodes. These electrodes with their half reactions are arranged according to their decreasing standard potentials; this arrangement is called an electrochemical series.
Definition: Conductivity
The electrical conductance of a conductor of unit length and unit area of cross section is called conductivity.
k = 1 / ρ
k = G × l / a
Definition: Electrolysis
The process of breaking down of an ionic compound in molten state or aqueous solution by passage of electricity is called electrolysis.
Definition: Molar conductivity
The electrical conductance of all the ions produced by 1 mole of an electrolyte dissolved in a given volume of solution is called molar conductivity.
\[{}_{\Lambda}=\frac{k}{c}\]
Definition: Electrolytic cell
An electrochemical cell in which non-spontaneous reaction is forced by passing direct current is called electrolytic cell.
Definition: Galvanic or Voltaic cell
An electrochemical cell in which spontaneous reaction produces electricity is called galvanic or voltaic cell.
Definition: Metallic conduction
The conduction of electricity through metals by movement of electrons is called metallic conduction.
Definition: Ionic conduction
The conduction of electricity through molten electrolytes or aqueous solutions by movement of ions is called electrolytic or ionic conduction.
Definition: Electrolyte
A substance which dissociates into ions in aqueous solution or molten state and conducts electricity is called electrolyte.
Theorems and Laws [2]
Law: Kohlrausch law
At infinite dilution, each ion migrates independently of the co-ion and contributes to the total molar conductivity of an electrolyte, irrespective of the nature of the other ion to which it is associated.
Degree of dissociation = `"Molar conductance at a given concentration"/ "Molar conductance at infinite dilution"` \[=\frac{\Lambda_{m}^{c}}{\Lambda_{m}^{\infty}}\]
Law: Kohlrausch’s Law of Independent Migration of Ions
- Kohlrausch’s law states that at infinite dilution each ion migrates independent of co-ion and contributes to total molar conductivity of an electrolyte irrespective of the nature of the other ion to which it is associated.
- At zero concentration (infinite dilution), both cation and anion contribute to the molar conductivity of the electrolyte.
- The molar conductivity at infinite dilution (Λ°) is the sum of molar conductivities of the cation and the anion.
- The mathematical expression of the law is:
Λ° = n₊ λ₊° + n₋ λ₋°
where λ₊° and λ₋° are molar conductivities of cation and anion, and n₊ and n₋ are the number of moles of cation and anion in the formula of electrolyte. - The law is useful in calculating molar conductivity at zero concentration and in determining Λ° values of weak electrolytes from those of strong electrolytes.
Key Points
Key Points: Terms Related to Conductance of Solution
| Quantity | Definition | Formula / Relation | Units |
|---|---|---|---|
| Resistance (R) | Opposition to the flow of current | R = V / I | Ohm (Ω) |
| Conductance (G) | The ability to conduct electricity | G = 1 / R | Siemens (S) |
| Resistivity (ρ) | Resistance of a conductor of unit length & area | ρ = RA / l | Ω m or Ω cm |
| Conductivity (κ) | Conductance of unit length & area | κ = 1 / ρ | S m⁻¹ or Ω⁻¹ cm⁻¹ |
| Cell constant | Ratio of distance to area | l / A | m⁻¹ or cm⁻¹ |
| Molar conductivity (Λₘ) | Conductance of 1 mole of electrolyte | Λₘ = κ / C | S m² mol⁻¹ |
Key Points: Electrochemical Cells
| Type | Electrolytic Cell | Galvanic (Voltaic) Cell |
|---|---|---|
| Energy conversion | Electrical → Chemical | Chemical → Electrical |
| Nature of reaction | Non-spontaneous | Spontaneous |
| Anode | Positive | Negative |
| Cathode | Negative | Positive |
| Electron flow | Cathode → Anode | Anode → Cathode |
| Salt bridge | Not required | Required |
Electrolysis of NaCl
1. Molten NaCl:
-
Oxidation: Cl⁻ → Cl₂ (gas)
-
Reduction: Na⁺ → Na (metal)
-
Products: Na (cathode), Cl₂ (anode)
2. Aqueous NaCl:
-
Oxidation: Cl⁻ → Cl₂
-
Reduction: H₂O → H₂ + OH⁻
-
Products: H₂ (cathode), Cl₂ (anode), NaOH formed
Key Points: Galvanic or Voltaic Cell
Components of a Galvanic Cell
| Component | Key Points |
|---|---|
| Electrodes | Surfaces where oxidation and reduction occur may be inert or active |
| Anode | Electrode where oxidation occurs; in a galvanic cell → negative electrode |
| Cathode | Electrode where reduction occurs; in a galvanic cell → positive electrode |
| Electrolyte | Substance that ionises in solution or molten state; provides ions for conduction; placed in separate containers (half-cells) |
| Salt Bridge (Structure) | U-shaped tube with electrolyte |
| Salt Bridge (Functions) | Completes electrical circuit; maintains electrical neutrality; prevents mixing of solutions |
6. Cell Notation
-
Anode written on the left, cathode on the right
-
Example:
Cu(s) | Cu²⁺(aq) || Ag⁺(aq) | Ag(s)
-
Single line (|) → phase boundary
-
Double line (||) → salt bridge
Key Points: Prediction of Reaction
| ΔG° | E°cell | Nature |
|---|---|---|
| ΔG° < 0 | E°cell > 0 | Spontaneous |
| ΔG° = 0 | E°cell = 0 | Equilibrium |
| ΔG° > 0 | E°cell < 0 | Non-spontaneous |
Key Points: Thermodynamics of Galvanic Cells
1. Gibbs Energy Relation
ΔG = −nFEcell
2. Relation with Equilibrium Constant
E°cell \[=\frac{0.0592}{n}\log_{10}K at 25^{\circ}C\]
Key Points: Reference Electrodes
Reference Electrodes
| Type | Examples |
|---|---|
| Primary reference electrode | Standard hydrogen electrode (SHE) |
| Secondary reference electrode | Calomel electrode, silver–silver chloride electrode, glass electrode |
Standard Hydrogen Electrode (SHE)
| Feature | Description |
|---|---|
| Standard potential | 0.00 V |
| Electrode | Platinum (Pt) |
| Gas | Hydrogen gas at 1 atm |
| Solution | 1 M acid solution |
| Half reaction | 2H⁺(aq) + 2e⁻ ⇌ H₂(g) |
Key Points: Galvanic Cells Useful in Day-to-day Life
Types of Voltaic Cells
| Type | Key Points | Examples |
|---|---|---|
| Primary cells | Cannot be recharged; reaction is irreversible | Dry cell |
| Secondary cells | Can be recharged; reaction reversible | Lead storage battery, Ni–Cd cell, Mercury cell |
Important Cells
| Cell | Anode | Cathode | Electrolyte |
|---|---|---|---|
| Dry cell | Zn | Carbon (graphite) | NH₄Cl + ZnCl₂ paste |
| Lead storage battery | Pb | PbO₂ | H₂SO₄ |
| Nickel–cadmium cell | Cd | NiO₂ | KOH solution |
| Mercury battery | Zn–Hg amalgam | HgO | KOH + ZnO paste |
Key Points: Fuel Cells
Reactions
- Anode:
2H₂ + 4OH⁻ → 4H₂O + 4e⁻ - Cathode:
O₂ + 4H₂O + 4e⁻ → 4OH⁻ - Overall reaction:
2H₂ + O₂ → 2H₂O
Applications
- Spacecraft (electric power)
- Power generators (homes, hospitals)
- Automobiles (experimental)
- Clean energy for industries
Drawbacks
- Hydrogen gas is hazardous
- High cost of hydrogen preparation
Key Points: Classification of Voltaic (or galvanic) cells
| Basis | Primary Cell | Secondary Cell |
|---|---|---|
| Meaning | A cell in which chemicals are consumed during current generation and cannot be regenerated. | A cell in which chemicals consumed during current generation can be regenerated by applying external potential. |
| Recharging | Cannot be recharged. | Can be recharged by applying external potential. |
| Reaction Nature | Cell reaction cannot be reversed. | Cell reaction is reversed during charging. |
| Working Type | Works only as galvanic cell. | Acts as galvanic cell during discharge and electrolytic cell during charging. |
| Examples | Dry cell | Lead storage battery, Mercury cell, Nickel-cadmium cell |
Key Points:
Key Points: Fuel cells
| Topic | Details |
|---|---|
| Meaning | A fuel cell converts chemical energy of a redox reaction directly into electrical energy. |
| Fuel & Oxidant | In hydrogen–oxygen fuel cell, hydrogen acts as fuel and oxygen acts as oxidizing agent. |
| Anode Reaction (Oxidation) |
Hydrogen is oxidized at anode (–). 2H₂(g) + 4OH⁻(aq) → 4H₂O(l) + 4e⁻ |
| Cathode Reaction (Reduction) |
Oxygen is reduced at cathode (+). O₂(g) + 2H₂O(l) + 4e⁻ → 4OH⁻(aq) |
| Overall Reaction |
Sum of anode and cathode reactions. 2H₂(g) + O₂(g) → 2H₂O(l) E°cell = 1.23 V |
| Advantages | Non-polluting (water is product) and high efficiency (~70%). |
| Drawback | Hydrogen gas is hazardous and costly to prepare. |
| Applications | Used in automobiles (experimental), space programmes and power generation. |
Important Questions [24]
- Calculate molar conductivity for 0.5 M BaCl2 if its conductivity at 298K is 0.01 Ω–1 cm–1.
- What is the SI unit of molar conductivity?
- The value of [H3O+] in mol lit–1 of 0.001 M acetic acid solution (Ka = 1.8 × 10–5) is ______.
- The Molar Conductivity of 0.05 M Bacl2 Solution at 25° C is 223ω-1 Cm2 Mol-1 . What is Its Conductivity?
- 0.05 M NaOH solution offered a resistance of 31.6 Ω in a conductivity cell at 298 K. If the cell constant of the cell is 0.367 cm^-1, calculate the molar conductivity of NaOH solution.
- Derive a Relation Between δH and δU for a Chemical Reaction. Draw Neat Labelled Diagram of Calomel Electrode.
- Construct a Labelled Diagram for the Following Cell
- Write Cathode and Anode Reaction in a Fuel Cell.
- How Many Faradays of Electricity Are Required to Produce 6 G of Mg from Mgcl2?
- Answer the following in one or two sentences. Write any two functions of salt bridge.
- What is a salt bridge?
- What are the functions of a salt bridge in a galvanic cell?
- The standard potential of the cell in the following reaction is ______. CdA(s)+CuA(1M)2+⟶CdA(1M)2++CuA(s) ECdECu(ECd∘=-0.403V,ECu∘=0.334V)
- Calculate standard Gibbs energy change at 25°C for the cell reaction. CdA(s)+SnA(aq)2+⟶CdA(aq)2++SnA(s) EAcd0 = –0.403 V, EASn0 = –0.136 V
- Write any two difficulties of setting SHE (Standard Hydrogen Electrode).
- Draw neat, labelled diagram of standard hydrogen electrode (SHE).
- Define Reference Electrode
- Write the net cell reaction during discharging of lead accumulator.
- Draw a neat labelled diagram of a lead accumulator.
- What Are Fuel Cells?
- Draw labelled diagram of H2 – O2 fuel cell.
- Write two applications of fuel cells.
- Define electrochemical series.
- Write applications of electrochemical series.
Concepts [13]
- Introduction to Electrochemistry
- Electric Conduction
- Electrical Conductance of Solution
- Electrochemical Cells
- Electrolytic Cells
- Galvanic or Voltaic Cell
- Electrode Potential and Cell Potential
- Thermodynamics of Galvanic Cells
- Reference Electrodes
- Galvanic Cells Useful in Day-to-day Life
- Fuel Cells
- Electrochemical Series (Electromotive Series)
- Overview of Electrochemistry
