Oxidation

Oxidation is the loss of electrons by a species. It can also be described as the gain of oxygen or the loss of hydrogen, and it always makes the oxidation number of that species increase. Malay: pengoksidaan · Chinese: 氧化.

In this chapter the electron definition is the one the marking scheme rewards, so learn it with the direction of electron movement fixed firmly: in oxidation, electrons leave the species. The memory aid OIL RIG, Oxidation Is Loss, captures exactly this. Because the species gives up negative electrons, it becomes more positive, so its oxidation number rises. The older definitions still appear in questions: a substance is oxidised when it gains oxygen, as carbon does when it forms carbon dioxide, or when it loses hydrogen.

Example. When magnesium burns in oxygen, each magnesium atom loses two electrons to become Mg²⁺, so its oxidation number rises from 0 to +2 and magnesium is oxidised. In a displacement reaction, zinc added to copper(II) sulfate loses electrons (Zn to Zn²⁺ plus 2e-), so the zinc is the species that is oxidised.

Confusion to avoid. Do not mix oxidation up with reduction, its paired opposite. Reduction is the gain of electrons and a fall in oxidation number. A reliable check is the direction of change: if the oxidation number goes up the species is oxidised, and if it goes down the species is reduced. Never conclude that a substance is oxidised only because oxygen is present without tracking where the electrons actually go.

Oxidation never happens by itself; it is always matched by the reduction of another species, which is why this chapter treats the two together as redox. When you write the oxidation half-equation, the electrons always appear on the right-hand side, and their number equals the rise in oxidation number multiplied by the atoms involved. Writing a correct half-equation for the oxidation step, and stating clearly that electrons are lost, is a routine task in SPM Chemistry.