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Oxidation is loss of electrons and reduction is gain. An oxidising agent accepts electrons and is reduced; a reducing agent donates electrons and is oxidised. An element in its elemental form has oxidation state zero. The sum of oxidation states equals the overall charge. Oxygen is usually −2, with exceptions such as peroxides, and hydrogen is usually +1 except in metal hydrides. Roman numerals in compound names identify an oxidation state, not the number of atoms present.
Video by Eliot Rintoul. The video’s own board label is retained; follow the specification reference and scope notes on this lesson for assessed content.
Viewing focus: Assign oxidation states
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: What happens to an oxidising agent?
It is reduced. It accepts electrons from another species.
After watching 2: What is sulfur’s oxidation state in SO4²⁻?
+6. S − 8 = −2.
What you'll be able to do
Oxidation is loss of electrons and reduction is gain. An oxidising agent accepts electrons and is reduced; a reducing agent donates electrons and is oxidised. An element in its elemental form has oxidation state zero. The sum of oxidation states equals the overall charge. Oxygen is usually −2, with exceptions such as peroxides, and hydrogen is usually +1 except in metal hydrides. Roman numerals in compound names identify an oxidation state, not the number of atoms present.
Write an oxidation or reduction half-equation with the species that changes oxidation state. In acidic solution, balance oxygen using water and hydrogen using H+. Then balance the charge with electrons. The electrons belong on the reactant side for reduction and the product side for oxidation. Multiply the complete half-equations until electron counts match, add them, and cancel species appearing on both sides. Check charge as carefully as atom counts.
Disproportionation occurs when the same element is both oxidised and reduced. In chlorine reacting with cold dilute hydroxide, chlorine changes from zero to −1 in chloride and +1 in chlorate(I). A redox titration uses the balanced electron transfer to connect measured volumes and concentrations. Acidified permanganate is a useful oxidising agent: the endpoint is a persistent faint pink from a slight excess, not the first transient flash of colour. The correct acid and reaction conditions are part of the method.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: Combine Fe2+ → Fe3+ + e⁻ with MnO4⁻ + 8H+ + 5e⁻ → Mn2+ + 4H2O.
MnO4⁻ + 8H+ + 5Fe2+ → Mn2+ + 4H2O + 5Fe3+.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.