Oxidation and Reduction (OILRIG)

Redox reactions are reactions where oxidation and reduction happen at the same time. They can be described using oxygen, electrons, or oxidation numbers.

Key ideas

• Oxidation: gain of oxygen, loss of electrons, or increase in oxidation number.
• Reduction: loss of oxygen, gain of electrons, or decrease in oxidation number.
• OILRIG: Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons).

Seeing redox in different ways

• Oxygen transfer: burning magnesium is oxidation because magnesium gains oxygen.
  $$2\,\text{Mg} + \text{O}_2 \rightarrow 2\,\text{MgO}$$
• Electron transfer: in displacement reactions, a more reactive metal gives away electrons to a less reactive metal’s ions.
  $$\text{Zn} + \text{CuSO}_4 \rightarrow \text{ZnSO}_4 + \text{Cu}$$
• Oxidation numbers help track changes. If a substance’s oxidation number goes up, it is oxidised; if it goes down, it is reduced.

Oxidation numbers (simple rules)

• Elements on their own have 0 (e.g., $\text{Zn}$, $\text{O}_2$).
• A simple ion’s oxidation number equals its charge (e.g., $\text{Fe}^{3+}$ is +3).
• The sum in a compound is 0; in a polyatomic ion, it equals the ion’s charge.
• Usually: oxygen is −2, hydrogen is +1 (with a few exceptions).
• Roman numerals show oxidation number in names: iron(III) oxide is $\text{Fe}_2\text{O}_3$ (iron is +3); potassium manganate(VII) has Mn at +7.

Oxidising and reducing agents

• Oxidising agent: causes another substance to be oxidised and is itself reduced (gains electrons). Examples: acidified potassium manganate(VII), chlorine.
• Reducing agent: causes another substance to be reduced and is itself oxidised (loses electrons). Examples: carbon, hydrogen, reactive metals.

Colour change tests

• Acidified potassium manganate(VII): purple to colourless when it is reduced by a reducing agent.
• Aqueous potassium iodide: an oxidising agent turns iodide to iodine (colourless to brown). With starch, it becomes blue‑black.

Real‑world link

Rusting is oxidation of iron to iron(III) compounds when iron loses electrons in the presence of oxygen and water. Sacrificial protection works because a more reactive metal gives up electrons first (it is oxidised), protecting iron.