Redox and Electron Transfer

Redox reactions happen when electrons move from one substance to another. Oxidation and reduction always occur together. These reactions explain rusting of iron, how bleach works, and how batteries produce electricity.

What are oxidation and reduction?

• Oxidation = gain of oxygen, loss of electrons, or an increase in oxidation number.
• Reduction = loss of oxygen, gain of electrons, or a decrease in oxidation number.

Memory aid: OIL RIG — Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons).

Electron transfer and half‑equations

Electrons move from the reducing agent (electron donor) to the oxidising agent (electron acceptor). We show this with half‑equations:

Oxidation (loss of electrons):

$$\mathrm{Mg \rightarrow Mg^{2+} + 2e^-}$$

Reduction (gain of electrons):

$$\mathrm{O_2 + 4e^- \rightarrow 2O^{2-}}$$

Oxidation numbers (oxidation states)

Oxidation numbers help track electron movement. Roman numerals in names show oxidation number, e.g. iron(II) = Fe with +2; iron(III) = Fe with +3; manganate(VII) means Mn is +7.

• Uncombined elements have 0: $\mathrm{Na}$, $\mathrm{O_2}$, $\mathrm{Cl_2}$.
• Monatomic ion = its charge: $\mathrm{Na^+}$ is +1, $\mathrm{Cl^-}$ is −1.
• Sum in a compound = 0: in $\mathrm{H_2O}$, H is +1 each and O is −2.
• Sum in a polyatomic ion = ion charge: in $\mathrm{SO_4^{2-}}$ the total is −2, so S is +6.

Roman numerals example: iron(III) chloride is $\mathrm{FeCl_3}$ because Fe is +3 and each Cl is −1.

Identifying oxidising and reducing agents

• An oxidising agent oxidises another substance and is itself reduced (it gains electrons).
• A reducing agent reduces another substance and is itself oxidised (it loses electrons).

Colour tests you need

• Acidified aqueous potassium manganate(VII), $\mathrm{KMnO_4}$: a strong oxidising agent. Purple solution turns colourless when it is reduced by a reducing agent.
• Aqueous potassium iodide, $\mathrm{KI}$: an oxidising agent turns colourless $\mathrm{I^-}$ into brown $\mathrm{I_2}$; with starch, a blue‑black colour appears. A reducing agent can turn iodine back to colourless iodide.