Chapter 7 · Chemical Reactions
7.9Oxidation–Reduction Reactions
11 min · two checks
Predict
In 2 Mg + O₂ → 2 MgO, which element loses electrons?
The idea
- Define oxidation and reduction as electron loss and gain.
- Spot a redox reaction from an elemental reactant becoming a compound.
Oxidation is loss of electrons. Reduction is gain of electrons. They happen together, because the electrons have to go somewhere. The memory aid OIL RIG — oxidation is loss, reduction is gain — is enough for now. The substance that loses electrons is oxidized and is the reducing agent. The substance that gains electrons is reduced and is the oxidizing agent. The names feel backwards until you remember that the agent causes the other process.
A reaction is redox if electrons move, which you can often see without numbers: a pure element on one side appears in a compound on the other. Metal plus acid that produces hydrogen gas is redox. Combustion is redox. Precipitation of AgCl from Ag⁺ and Cl⁻ is not redox; the ions trade partners and keep their charges. Oxidation states, next in a later chapter, make the hidden cases obvious. Double replacement is usually not redox. Single replacement usually is.
Keep these
- Oxidation: electrons lost. Reduction: electrons gained.
- The reducing agent is oxidized. The oxidizing agent is reduced.
- Element → compound is a common redox clue. Simple precipitation is not redox.