Oxidation, reduction & redox
Oxidation and reduction by oxygen and electrons, oxidation states, and identifying oxidising and reducing agents.
At a basic level, oxidation is gain of oxygen and reduction is loss of oxygen. More generally, in terms of electrons, OIL RIG: Oxidation Is Loss of electrons, Reduction Is Gain. A redox reaction has both happening together.
Rules for assigning oxidation states:
- Uncombined element: 0 (e.g. each O in O₂ = 0)
- Monatomic ion: = charge on the ion (e.g. Cl⁻ = −1, Mg²⁺ = +2)
- Sum in a neutral compound = 0
- Sum in a polyatomic ion = overall charge (e.g. in , sum = −2)
- Group 1 in a compound: ; Group 2:
- Oxygen: usually (exceptions: peroxides e.g. H₂O₂, O = ; bonded to F e.g. OF₂, O = )
- Hydrogen: usually (exception: metal hydrides e.g. MgH₂, H = )
Worked example
Find the oxidation state of S in .
O = ; four oxygens = . Sum must = (charge on ion), so S
Worked example
Show is redox.
Cu: (decreases) → reduced. Zn: (increases) → oxidised. Both occur → redox. ✓
Identify whether a reaction is oxidation only, reduction only, redox, or neither. An oxidising agent accepts electrons (it is itself reduced); a reducing agent donates them. In disproportionation, the *same* element is both oxidised and reduced.
- Mixing up OIL RIG (oxidation is loss of electrons).
- Calling the species that *is* oxidised the oxidising agent (it is the reducing agent).
- Sign slips when balancing oxidation states to the overall charge.