Three definitions
- Oxygen and hydrogen: gaining oxygen or losing hydrogen is oxidation; the reverse is reduction.
- Electrons: losing electrons is oxidation; gaining them is reduction.
- Oxidation numbers: an increase is oxidation, a decrease reduction. This is the most general definition.
Coupling and agents
Electrons lost must be gained by something else, so oxidation and reduction occur together and the electrons transferred balance. The substance that oxidises another and is itself reduced is the oxidising agent; the one that reduces another and is itself oxidised is the reducing agent. In CuO + H₂ → Cu + H₂O, H₂ is the reducing agent and CuO the oxidising agent.
Oxidation numbers and balancing
Elements have oxidation number 0; in compounds H is usually +1 and O usually -2, and the numbers in a neutral compound sum to zero. Complex equations are balanced by splitting them into oxidation and reduction half-reactions, balancing atoms and charge, equalising electrons and recombining.
Examples
Rusting, combustion, metal extraction, battery charging and discharging and electrolysis are all redox processes. In living things photosynthesis oxidises water and reduces carbon dioxide, while cellular respiration oxidises glucose to release energy.
References
- Atkins, P., & Jones, L. (2010). Chemical Principles: The Quest for Insight (5th ed.). W. H. Freeman.
- Zumdahl, S. S., & Zumdahl, S. A. (2014). Chemistry (9th ed.). Cengage.
- IUPAC. (2019). Compendium of Chemical Terminology (the Gold Book), entries 'oxidation' and 'reduction'. International Union of Pure and Applied Chemistry.