Electrochemical Cells and Electrode Potentials

This lesson covers electrochemical cells, standard electrode potentials (E°), the electrochemical series, calculating cell EMF, and predicting feasibility of reactions.

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Electrochemical Cells

An electrochemical cell converts chemical energy into electrical energy (or vice versa). It consists of two half-cells, each containing an electrode in contact with an electrolyte.

Half-Cells

A metal/metal ion half-cell consists of a metal electrode dipping into a solution of its ions. Example: Zn(s) in ZnSO₄(aq) — the half-equation is Zn²⁺(aq) + 2e⁻ ⇌ Zn(s)

A non-metal half-cell requires a platinum electrode as an inert conductor. Example: Pt electrode in a solution containing Fe²⁺ and Fe³⁺ — the half-equation is Fe³⁺(aq) + e⁻ ⇌ Fe²⁺(aq)

A gas half-cell requires a gas bubbled over a platinum electrode. Example: The standard hydrogen electrode (SHE): 2H⁺(aq) + 2e⁻ ⇌ H₂(g)

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Standard Electrode Potential (E°)

Key Definition: The standard electrode potential E° is the EMF of a half-cell measured relative to the standard hydrogen electrode (SHE) under standard conditions.

Standard conditions:

• Temperature: 298 K (25°C)
• Concentrations: 1.00 mol dm⁻³
• Gas pressures: 100 kPa (or 1 atm)

The standard hydrogen electrode is assigned E° = 0.00 V by convention.

The Electrochemical Series

Half-equations are listed as reductions: Oxidised form + ne⁻ ⇌ Reduced form

Selected standard electrode potentials:

Half-equation	E° / V
Li⁺ + e⁻ ⇌ Li	−3.04
Zn²⁺ + 2e⁻ ⇌ Zn	−0.76
Fe²⁺ + 2e⁻ ⇌ Fe	−0.44
2H⁺ + 2e⁻ ⇌ H₂	0.00
Cu²⁺ + 2e⁻ ⇌ Cu	+0.34
I₂ + 2e⁻ ⇌ 2I⁻	+0.54
Ag⁺ + e⁻ ⇌ Ag	+0.80
Br₂ + 2e⁻ ⇌ 2Br⁻	+1.07
Cl₂ + 2e⁻ ⇌ 2Cl⁻	+1.36
MnO₄⁻ + 8H⁺ + 5e⁻ ⇌ Mn²⁺ + 4H₂O	+1.51
F₂ + 2e⁻ ⇌ 2F⁻	+2.87

• More negative E° → stronger reducing agent (more easily oxidised)
• More positive E° → stronger oxidising agent (more easily reduced)

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Calculating Cell EMF

The EMF of a cell is calculated using:

E°cell = E°(cathode) − E°(anode)

Or equivalently:

E°cell = E°(more positive half-cell) − E°(less positive half-cell)

The half-cell with the more positive E° is the cathode (reduction occurs). The half-cell with the less positive E° is the anode (oxidation occurs). Electrons flow from the anode to the cathode in the external circuit.