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Short Notes: Electrochemistry
Electrochemical Cells
| Feature | Galvanic Cell | Electrolytic Cell |
|---|---|---|
| Energy Conversion | Chemical → Electrical | Electrical → Chemical |
| Spontaneity | Spontaneous (ΔG < 0) | Non-spontaneous (ΔG > 0) |
| Anode | Negative (-), Oxidation | Positive (+), Oxidation |
| Cathode | Positive (+), Reduction | Negative (-), Reduction |
| Example | Daniell cell | Electrolysis of water |
Cell Notation (Daniell Cell)
- Representation: Zn(s) | Zn2+(aq) || Cu2+(aq) | Cu(s)
- Single line (|) = phase boundary
- Double line (||) = salt bridge
- Anode on left, Cathode on right
Electrode Potential
- Standard Electrode Potential (E0): Measured at 298K, 1M concentration, 1 bar pressure
- Standard Hydrogen Electrode (SHE): Reference electrode, E0 = 0.00 V
- E0cell = E0cathode - E0anode
- Higher reduction potential → Better oxidizing agent
- Lower reduction potential → Better reducing agent
Nernst Equation
- For cell: Ecell = E0cell - (0.059/n) log Q (at 298K)
- General form: Ecell = E0cell - (RT/nF) ln Q
- For electrode: E = E0 - (0.059/n) log [Reduced form]/[Oxidized form]
- At equilibrium: Ecell = 0, Q = Keq
Gibbs Energy and EMF
- ΔG = -nFEcell
- ΔG0 = -nFE0cell
- ΔG0 = -RT ln K
- Combining: E0cell = (0.059/n) log K (at 298K)
- n = number of electrons, F = 96500 C/mol
Conductance
| Term | Symbol | Formula | Unit |
|---|---|---|---|
| Resistance | R | R = ρ(l/A) | Ohm (Ω) |
| Conductance | G | G = 1/R | Siemens (S) or mho |
| Resistivity | ρ | ρ = RA/l | Ω·m |
| Conductivity | κ (kappa) | κ = 1/ρ = l/RA | S/m or S·cm-1 |
| Molar Conductivity | Λm | Λm = κ × 1000/M | S·cm2·mol-1 |
Kohlrausch's Law
- Statement: Λ0m = λ0cation + λ0anion
- Λ0m = Molar conductivity at infinite dilution
- Applications:
- Calculate Λ0m for weak electrolytes
- Determine degree of dissociation: α = Λm / Λ0m
- Calculate dissociation constant: Ka = (Cα2) / (1 - α)
Variation of Conductivity
- Conductivity (κ): Increases with dilution initially, then decreases
- Molar Conductivity (Λm): Always increases with dilution
- Strong electrolytes: Λm increases slowly with dilution
- Weak electrolytes: Λm increases sharply with dilution
Faraday's Laws of Electrolysis
- First Law: m ∝ Q (mass deposited ∝ charge passed)
- Second Law: m ∝ E (mass ∝ equivalent weight)
- Combined: m = (E × I × t) / 96500 = ZIt
- Z = Electrochemical equivalent = E/96500
- Q = It (charge = current × time)
Important Batteries
| Battery | Anode | Cathode | Voltage |
|---|---|---|---|
| Dry Cell (Leclanche) | Zn | Carbon rod + MnO2 | 1.5 V |
| Mercury Cell | Zn-Hg | HgO + Carbon | 1.35 V |
| Lead Storage Battery | Pb | PbO2 | 2 V (per cell) |
Fuel Cells
- H2-O2 Fuel Cell:
- Anode: 2H2 + 4OH- → 4H2O + 4e-
- Cathode: O2 + 2H2O + 4e- → 4OH-
- Overall: 2H2 + O2 → 2H2O
- Efficiency ~70%
Corrosion
- Rusting of Iron: Electrochemical process
- Anode: 2Fe → 2Fe2+ + 4e-
- Cathode: O2 + 4H+ + 4e- → 2H2O
- Overall: 2Fe2+ + 2H2O + O2 → Fe2O3·H2O (rust)
- Prevention: Galvanization, painting, alloying, cathodic protection
NEET Focus:
- Nernst equation calculations
- E0cell calculation and spontaneity
- Kohlrausch's law applications
- Faraday's laws numerical problems
- Battery reactions
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FAQs on Short Notes: Electrochemistry
| 1. What is electrochemistry? |  |
Ans. Electrochemistry is the branch of chemistry that deals with the relationship between electricity and chemical reactions. It involves the study of how electrical energy is produced from chemical reactions and how chemical changes can be induced by passing an electric current through a substance.
| 2. What are the types of electrochemical cells? | |
Ans. There are two main types of electrochemical cells: galvanic cells and electrolytic cells. Galvanic cells convert chemical energy into electrical energy through spontaneous reactions, while electrolytic cells use electrical energy to drive non-spontaneous chemical reactions.
| 3. What is the significance of the Nernst equation in electrochemistry? | |
Ans. The Nernst equation is significant as it relates the cell potential to the concentrations of the reactants and products in an electrochemical reaction. It helps to calculate the equilibrium potential of a cell under non-standard conditions, providing insights into the spontaneity and direction of the electrochemical reaction.
| 4. How does the standard electrode potential influence electrochemical reactions? | |
Ans. The standard electrode potential is a measure of the tendency of a species to be reduced. It influences electrochemical reactions by determining the voltage of a half-cell compared to a standard hydrogen electrode. A higher standard electrode potential indicates a greater likelihood of reduction, thereby affecting the overall cell potential and feasibility of the reaction.
| 5. What role do electrolytes play in electrochemistry? | |
Ans. Electrolytes are substances that dissociate into ions in solution and conduct electricity. In electrochemistry, they are crucial as they facilitate the movement of ions between the anode and cathode in electrochemical cells, enabling the conduction of electric current and the progression of chemical reactions.
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