Reduction & Oxidation

The Standard Electrode Potential ($E^\theta$) measures the tendency of a half-cell to be reduced relative to the Standard Hydrogen Electrode (SHE), which is assigned a value of $0.00\text{ V}$.

Standard conditions are mandatory for valid comparisons: a temperature of $298\text{ K}$, a pressure of $100\text{ kPa}$ for gases, and an ion concentration of $1.00\text{ mol dm}^{-3}$.

A more positive $E^\theta$ value indicates a stronger tendency for the species on the left of the half-equation to be reduced (acting as a stronger oxidising agent).

A more negative $E^\theta$ value indicates a stronger tendency for the species on the right of the half-equation to be oxidised (acting as a stronger reducing agent).

Cells are represented using a shorthand notation: Oxidised Species | Reduced Species || Reduced Species | Oxidised Species. The double vertical line ($||$) represents the salt bridge.

A single vertical line ($|$) represents a phase boundary, such as the interface between a solid metal electrode and an aqueous solution of its ions.

If a half-cell consists only of aqueous ions (e.g., $Fe^{2+}/Fe^{3+}$), an inert platinum (Pt) electrode must be used to provide a surface for electron transfer, and the ions are separated by a comma.

By convention, the half-cell undergoing oxidation (the more negative $E^\theta$) is written on the left, and the one undergoing reduction (the more positive $E^\theta$) is written on the right.

The Standard Cell Potential ($E^\theta_{cell}$) is calculated as $E^\theta_{reduction} - E^\theta_{oxidation}$ (or $E^\theta_{right} - E^\theta_{left}$).

A reaction is thermodynamically feasible if $E^\theta_{cell}$ is positive. This relates to the Gibbs free energy change via the equation $\Delta G^\theta = -nFE^\theta_{cell}$.

$E^\theta_{cell}$ is directly proportional to the total entropy change ($\Delta S_{total}$) and the natural log of the equilibrium constant ($\ln K$).

Feasibility does not guarantee a reaction will occur; high activation energy may make a feasible reaction kinetically stable (too slow to observe).

The 'Right-Hand' Rule: Always identify the more positive $E^\theta$ value first; this half-cell will always be the reduction half-cell (right side) in a feasible spontaneous reaction.

State Symbols: In cell notation and half-equations, always include state symbols ($s, aq, g, l$) as they define the phase boundaries required in the diagram.

Balancing Electrons: When combining half-equations, ensure the number of electrons lost equals the number of electrons gained. Note that multiplying a half-equation to balance electrons does NOT change its $E^\theta$ value.

Platinum Electrodes: Always check if a half-cell contains a solid metal. If it only contains gases or ions in solution, you must include 'Pt' in your cell representation.