When studying for your IB exams, you’ll need to do a thorough review of the parts of the syllabus you find challenging. If you’re an IB Chemistry student who is a bit confused about Oxidation and Reduction, then this quick article is for you. It’s an excerpt from Cameron Lumsden’s Chemistry SL study guide (3rd edition for exams up to 2024), but is still relevant and useful for both HL and SL Chemistry students. Read on for a clear explanation of oxidation and reduction as well as helpful tips, including an easy to remember mnemonic device.
Oxidation and Reduction
Oxidation and reduction – “Redox” to chemists – reactions are those that involve a transfer of electrons. Don’t all reactions involve a transfer of elements? No! Acid/base reactions and precipitation reactions do not involve a transfer of electrons.
| Oxidation | Reduction | |
|---|---|---|
| Oxygen | Gain of oxygen | Loss of oxygen |
| Electrons | Loss of electrons | Gain of electrons |
| Hydrogen | Loss of hydrogen | Gain of hydrogen |
| Electrode | Anode | Cathode |
| Oxidation number | Becomes more positive | Becomes less positive |
In IB Chemistry, we are mostly concerned with the electronic definition of oxidation and reduction. The oxygen and hydrogen are included as additional information.
My favourite way to remember this is “AN OIL RIG CAT”. ANode is the electrode where Oxidation Is Loss of electrons occurs, and Reduction Is Gain of electrons at the CAThode.
Oxidation Numbers
Oxidation numbers are chemists’ way to keep an account of electrons.
Remember to be organised in your working for determining oxidation numbers.
It’s the bottom numbers that have to add to zero (or the overall ionic charge).
Redox Reactions
A redox reaction is one that has changes in oxidation number. There are lots of reactions which are not redox reactions – precipitations, acid/base reactions, etc.
Reactions that are always going to be redox reactions:
- Combustion reactions – elemental oxygen becoming a compound.
- Synthesis reactions – any element reacting with another to produce a compound.
- Ions changing charge – Fe2+ becoming Fe3+.
- Ions of the oxy-acids changing the number of oxygen atoms – SO32– becoming SO42–.
A fast way to pick off 99% of redox reactions is to look for an element as a reactant or product. This works because the oxidation state of any free element is zero, and in a compound, it is unlikely zero.
Oxidizing agent: a species that removes electrons from another.
Reducing agent: a species that donates electrons to another.
Common Oxidizing and Reducing Agents
Continue your IB Chemistry revision
You’re on your way to filling your knowledge gaps in IB Chemistry. Don’t stop now. What other areas of the syllabus do you find challenging? How are you planning to address your knowledge gaps before your IB exams? If you’re looking for a resource that can provide you with comprehensive, easy-to-follow explanations of syllabus topics as well as lots of practice questions to test your understanding then watch out for the new editions of our Chemistry SL and Chemistry HL study guides, coming in the summer/autumn of 2025. You can also find more quick-read resources on our IB Chemistry subject page.