Electrochemistry

Meaning

At the very core of electrochemistry lies the transfer of electrons. Whenever we talk about electrochemical processes, we are talking about oxidation and reduction happening simultaneously. These are called Redox reactions. Oxidation is the loss of electrons (increase in oxidation state), and reduction is the gain of electrons (decrease in oxidation state).

Definitions

1. According to Gilbert N. Lewis and modern electronic concepts, oxidation is defined as the loss of one or more electrons by an atom, ion, or molecule, while reduction is the gain of electrons by an atom, ion, or molecule.
2. In the classical approach by Antoine Lavoisier and early chemists, oxidation was defined as the addition of oxygen or electronegative elements or removal of hydrogen, whereas reduction was the addition of hydrogen or electropositive elements or removal of oxygen.

Explanation

• The species that loses electrons is oxidized (and acts as a reducing agent).
• The species that gains electrons is reduced (and acts as an oxidizing agent).
• Mnemonic: OIL RIG (Oxidation Is Loss, Reduction Is Gain) and LEO says GER (Lose Electrons Oxidation, Gain Electrons Reduction).
• Oxidation and reduction always occur together.

Examples

• When zinc metal is dipped in copper sulfate, zinc atoms lose electrons (oxidation) and copper ions accept them (reduction).
• Burning of magnesium ribbon in air.

Real-Life Uses

• Cellular Respiration in Living Organisms.
• Water Treatment Plants (chlorination/oxidation).

Meaning

Conductance is the ease with which electric current flows through a substance via the movement of free ions in electrolytic solutions.

Definitions

1. Conductance ($G$) is the reciprocal of electrical resistance ($R$): $G = \frac{1}{R}$. Unit: Siemens (S) or $\text{ohm}^{-1}$.
2. Specific conductance or conductivity ($\kappa$) is the conductance of a solution enclosed in a cell with unit distance and unit cross-sectional area.

Explanation

• Resistance ($R$) is directly proportional to length ($l$) and inversely proportional to area ($A$): $R = \rho \frac{l}{A}$.
• Conductivity ($\kappa$) is the reciprocal of resistivity ($\rho$): $\kappa = \frac{1}{\rho}$.
• Temperature increase decreases solvent viscosity and increases ionic mobility.

Examples

• Aqueous $\text{NaCl}$ solution conducts electricity well.
• Pure distilled water is a poor conductor.

Meaning

Molar conductivity ($\Lambda_m$) tells us how effectively the ions produced by one mole of an electrolyte conduct electricity.

Definitions

1. Conducting power of all ions produced by dissolving one mole of an electrolyte in a given volume.
2. Mathematically: $\Lambda_m = \kappa \times V = \frac{\kappa \times 1000}{C}$.

Explanation

• Strong electrolytes show slow increase in $\Lambda_m$ upon dilution.
• Weak electrolytes show a sharp increase in $\Lambda_m$ upon dilution due to higher dissociation.

Examples

• Dissolving 1 mole of $\text{KCl}$ or acetic acid ($\text{CH}_3\text{COOH}$) in water.

Meaning

Limiting molar conductivity of an electrolyte can be represented as the sum of individual contributions of anions and cations.

Definitions

1. $\Lambda_m^\circ (\text{electrolyte}) = \nu_+ \lambda_+^\circ + \nu_- \lambda_-^\circ$.

Explanation

• At infinite dilution, ions are far apart and move completely independently without mutual interference.
• Used to calculate $\Lambda_m^\circ$ for weak electrolytes.

Meaning

Driving non-spontaneous reactions using electrical current.

Definitions

1. Faraday's First Law: $W = Z \times Q = Z \times I \times t$.
2. Faraday's Second Law: $\frac{W_1}{W_2} = \frac{E_1}{E_2}$.

Explanation

• One Faraday ($F$) $\approx 96500 \text{ C mol}^{-1}$.
• Discharge depends on standard reduction potentials.

Examples

• Electrolysis of molten $\text{NaCl}$, electroplating silver.

Meaning

Conversion between chemical and electrical energy using galvanic or electrolytic cells.

Definitions

1. EMF is the maximum potential difference between two half-cells.
2. Standard electrode potential ($E^\circ$) is measured at $1 \text{ atm}$, $298 \text{ K}$, and $1 \text{ mol L}^{-1}$ concentration referenced against SHE ($0\text{V}$).

Explanation

• Oxidation at Anode (An Ox), Reduction at Cathode (Red Cat).
• $E_{\text{cell}}^\circ = E_{\text{cathode}}^\circ - E_{\text{anode}}^\circ$.

Meaning

Calculating cell potential under non-standard conditions and connecting it with free energy.

Definitions

1. Nernst Equation: $E_{\text{cell}} = E_{\text{cell}}^\circ - \frac{0.0591}{n} \log Q$ (at $298\text{K}$).
2. Gibbs Free Energy: $\Delta G = -nFE_{\text{cell}}$ and $\Delta G^\circ = -nFE_{\text{cell}}^\circ$.

Explanation

• Positive $E_{\text{cell}}$ means spontaneous reaction ($\Delta G$ is negative).
• Pure solids and liquids have activity equal to 1 and are excluded from $Q$.

Meaning

Portable chemical-to-electrical energy storage devices.

Definitions

1. Primary Battery: Non-rechargeable (e.g., dry cell).
2. Secondary Battery: Rechargeable (e.g., lead storage battery, Li-ion).
3. Fuel Cell: Continuously supplied with external fuel (e.g., $\text{H}_2-\text{O}_2$ fuel cell).

Meaning

Electrochemical deterioration of metal surfaces via atmospheric redox reactions.

Definitions

1. Rusting of iron: Formation of hydrated ferric oxide ($\text{Fe}_2\text{O}_3 \cdot x\text{H}_2\text{O}$).

Explanation

• Requires both moisture and oxygen simultaneously.
• Prevented via painting, galvanization, and sacrificial anodes.

Galvanic Cell vs. Electrolytic Cell

Primary vs. Secondary Batteries

• PEM Fuel Cells: Zero-emission hydrogen buses and cars.
• Lithium-Sulfur & Solid-State Batteries: Next-gen high-density energy storage.
• Chlor-Alkali Process: Industrial brine electrolysis for $\text{Cl}_2$ and $\text{NaOH}$.
• Wastewater Treatment: Electrochemical destruction of toxic pollutants.

Problem 1

Calculate $E_{\text{cell}}^\circ$ and $\Delta G^\circ$ for: $\text{Mg(s)} + \text{Cu}^{2+}(\text{aq}) \rightarrow \text{Mg}^{2+}(\text{aq}) + \text{Cu(s)}$ given $E^\circ_{\text{Mg}^{2+}/\text{Mg}} = -2.37 \text{ V}$, $E^\circ_{\text{Cu}^{2+}/\text{Cu}} = +0.34 \text{ V}$.

Step 1: Identify Anode (Mg, -2.37 V) and Cathode (Cu, +0.34 V).
Step 2: E_cell° = E_cathode° - E_anode° = (+0.34) - (-2.37) = +2.71 V.
Step 3: Electrons transferred (n) = 2.
Step 4: ΔG° = -nFE_cell° = -2 × 96500 × 2.71 = -523,030 J/mol = -523.03 kJ/mol.
        

Problem 2

Resistance of $0.1 \text{ M}$ $\text{KCl}$ is $100 \ \Omega$ (conductivity $1.29 \times 10^{-2} \text{ S cm}^{-1}$). Resistance of $0.02 \text{ M}$ $\text{KCl}$ is $520 \ \Omega$. Find conductivity and molar conductivity of $0.02 \text{ M}$ solution.

Step 1: Cell Constant (G*) = Conductivity × Resistance = 1.29 × 10^-2 × 100 = 1.29 cm^-1.
Step 2: Conductivity (κ) of 0.02 M = G* / R = 1.29 / 520 = 2.48 × 10^-3 S cm^-1.
Step 3: Molar Conductivity (Λm) = (κ × 1000) / C = (2.48 × 10^-3 × 1000) / 0.02 = 124 S cm^2 mol^-1.
        

Deep out in the turbulent waters of the North Sea, massive offshore platforms face severe corrosive environments. Engineers protect structural steel legs using sacrificial anodes (zinc or aluminum blocks) and Impressed Current Cathodic Protection (ICCP) systems based on exact redox and electrode potential principles.

1. What is the unit of molar conductivity?
   Answer: B) $\text{S cm}^2 \text{ mol}^{-1}$
   Explanation: Derived from $\frac{\kappa \times 1000}{C}$.
2. During aqueous $\text{NaCl}$ electrolysis, what liberates at the cathode?
   Answer: C) Hydrogen gas
   Explanation: Water has a higher reduction potential than sodium ions.
3. Which of the following is a secondary battery?
   Answer: C) Lead storage battery
   Explanation: Rechargeable via external DC current.
4. What happens to molar conductivity on infinite dilution for weak electrolytes?
   Answer: B) It increases steeply and approaches a limiting value
   Explanation: Degree of dissociation increases sharply.
5. In a galvanic cell, oxidation takes place at:
   Answer: B) Anode
   Explanation: Oxidation always occurs at the anode (An Ox).
6. What is $E_{\text{cell}}$ when a galvanic cell reaches chemical equilibrium?
   Answer: C) Zero
   Explanation: Net potential drops to zero at equilibrium ($\Delta G = 0$).
7. Which metal is used as a sacrificial anode for underground pipelines?
   Answer: C) Magnesium
   Explanation: It has a more negative reduction potential than iron.
8. One Faraday of electricity is equal to:
   Answer: B) Charge on one mole of electrons ($96500 \text{ C mol}^{-1}$)
9. Which law relates limiting molar conductivity to constituent ions?
   Answer: C) Kohlrausch's Law
10. Main product at the anode during molten $\text{NaCl}$ electrolysis:
    Answer: B) Chlorine gas

1. State Faraday's First Law of Electrolysis and write its mathematical expression.
   Answer: $W = Z \times I \times t$, where $W$ is mass deposited and $Z$ is electrochemical equivalent.
2. Why does conductivity decrease with dilution while molar conductivity increases?
   Answer: Conductivity drops because ions per unit volume decrease, whereas molar conductivity increases because the total volume containing 1 mole expands significantly.
3. Write the Nernst equation for general cell reaction $a\text{A} + b\text{B} \rightarrow c\text{C} + d\text{D}$.
   Answer: $E_{\text{cell}} = E_{\text{cell}}^\circ - \frac{0.0591}{n} \log \left(\frac{[\text{C}]^c [\text{D}]^d}{[\text{A}]^a [\text{B}]^b}\right)$ at $298\text{K}$.
4. What is the role of the salt bridge in a galvanic cell?
   Answer: Completes internal electrical circuit and maintains electrical neutrality.
5. Why can't sodium metal be obtained by aqueous $\text{NaCl}$ electrolysis?
   Answer: Water reduces preferentially at the cathode instead of sodium ions due to higher reduction potential.
6. State Kohlrausch's Law of Independent Migration of Ions.
   Answer: Limiting molar conductivity of an electrolyte is the sum of individual ionic contributions.
7. What are fuel cells? Give an example.
   Answer: Galvanic cells converting fuel combustion energy directly to electricity (e.g., Hydrogen-Oxygen fuel cell).
8. Why doesn't iron rust in dry air or oxygen-free water?
   Answer: Rusting requires both moisture and oxygen simultaneously to establish electrochemical micro-cells.
9. Relationship between standard Gibbs free energy and standard cell potential?
   Answer: $\Delta G^\circ = -nFE_{\text{cell}}^\circ$.
10. Give two methods to prevent rusting of iron.
    Answer: Galvanization and sacrificial protection.

• AN OX / RED CAT: Anode = Oxidation, Reduction = Cathode.
• OIL RIG: Oxidation Is Loss, Reduction Is Gain of electrons.
• FAT CAT: Electrons flow From Anode To CAthode; Cathode gets fat.

Memory Trick: Always check standard reduction potentials to determine anode and cathode locations. Lower potential = Anode (Oxidation).