Quick questions on Redox and electrochemistry: WACE Year 12 Chemistry

Oxidation numbers track electron distribution and reveal what is oxidised and reduced. The key rules are: elements in their standard state are 0; oxygen is usually $-2$ (peroxides $-1$); hydrogen is $+1$ with non-metals and $-1$ in metal hydrides; a monatomic ion equals its charge; and the sum of oxidation numbers equals the overall charge of the species. An increase in oxidation number is oxidation; a decrease is reduction.

A redox reaction splits into an oxidation half-equation and a reduction half-equation. To balance one (in acidic solution): balance the atoms being oxidised or reduced, balance oxygen with $\text{H}_2\text{O}$, balance hydrogen with $\text{H}^+$, then balance charge with electrons. For example the reduction of dichromate:

The standard electrode potential ($E^{\circ}$) of a half-reaction measures its tendency to be reduced, relative to the standard hydrogen electrode ($E^{\circ} = 0\ \text{V}$), measured at $25^{\circ}\text{C}$, $100\ \text{kPa}$ and $1\ \text{mol L}^{-1}$ concentrations. The SCSA data booklet lists these as reduction half-equations. A more positive $E^{\circ}$ means a stronger oxidising agent (more readily reduced); a more negative $E^{\circ}$ means a stronger reducing agent (more readily oxidised). The species with the higher (more positive) $E^{\circ}$ is reduced; the other is oxidised.

A galvanic cell converts the energy of a spontaneous redox reaction into electrical energy. Two half-cells are connected by an external wire (electron path) and a salt bridge (ion path that keeps each half-cell neutral). Oxidation occurs at the anode (negative electrode in a galvanic cell) and reduction at the cathode (positive electrode); electrons flow through the wire from anode to cathode. The cell potential is

An electrolytic cell uses an external power supply to drive a non-spontaneous redox reaction (for example electrolysis of molten salts or aqueous solutions, electroplating, and electrorefining). Here the anode is positive and the cathode is negative, the reverse polarity of a galvanic cell, but oxidation still occurs at the anode and reduction at the cathode.