How oxidation numbers track electron transfer in reactions
Redox reactions involve both oxidation and reduction. In electron terms, oxidation is the loss of electrons and reduction is the gain of electrons. Oxidation numbers are a bookkeeping method: they are assigned by rules, such as zero for elements, −2 for oxygen in most compounds and +1 for hydrogen with non-metals, with the sum equal to the charge on the species. An increase in oxidation number means oxidation and a decrease means reduction.
The substance that is reduced is the oxidising agent, and the one that is oxidised is the reducing agent. In Zn + Cu²⁺ → Zn²⁺ + Cu, zinc is oxidised and acts as the reducing agent. Redox reactions include combination, decomposition, displacement and disproportionation, in which the same element is both oxidised and reduced, as when H₂O₂ decomposes into water and oxygen. Redox equations are balanced by the oxidation-number method or the ion-electron (half-reaction) method, adding H⁺ and H₂O in acidic medium or OH⁻ in basic medium.
Why it matters
Redox reactions power batteries, release energy from food through respiration, extract metals from ores and bleach fabrics. Potassium permanganate and dichromate titrations are used to measure concentrations in laboratories. Rusting and photosynthesis are also redox processes, and this topic leads directly into electrochemistry.
Key terms
- Oxidation number
- A number assigned to an atom showing its apparent charge in a compound.
- Oxidising agent
- A substance that accepts electrons and is itself reduced.
- Reducing agent
- A substance that donates electrons and is itself oxidised.
- Disproportionation
- A redox reaction in which one element is both oxidised and reduced.