Short on time? This page is the recap. For explanations and worked examples, use the full study guide.
Recap
- The course sorts reactions by what is transferred: electrons (redox), protons (acid-base), or ions joining into a solid (precipitation).
- Classify from the net ionic equation; spectator ions take no part, so leaving them out shows the real change.
- Oxidation = loss of electrons = oxidation number goes up. Reduction = gain of electrons = oxidation number goes down.
- Electrons move from the species oxidised to the species reduced. Electrons lost = electrons gained.
- Combustion is redox with O₂. Complete combustion of a hydrocarbon gives CO₂ + H₂O only.
Key relationships
| Type | What moves | Evidence in the equation |
|---|---|---|
| Redox | electrons | at least one oxidation number changes |
| Acid-base | a proton, H⁺ | one species loses an H⁺, another gains it; no oxidation number changes |
| Precipitation | ions come together | (aq) ions → ionic (s); no oxidation number changes |
Oxidation-number rules
| Rule | Value |
|---|---|
| Atom in an element | 0 |
| Monatomic ion | its charge |
| Sum in a formula | overall charge |
| F in compounds | −1 |
| Group 1 / group 2 metals | +1 / +2 |
| H | +1 (−1 in metal hydrides) |
| O | −2 (−1 in peroxides) |
Assumptions
- The equation is balanced for both atoms and charge.
- Oxidation numbers are bookkeeping: they assume all bonds are fully ionic. They are not real charges.
- Solubility: you are expected to know only that Na⁺, K⁺, NH₄⁺ and NO₃⁻ salts are soluble. Other solubility data will be given.
Mistakes to avoid
- “H⁺ present, so acid-base.” Metal + H⁺ → H₂ is redox.
- “Solid formed, so precipitation.” A metal coming out of solution is a redox product.
- Oxidation number ≠ ion charge. S in SO₄²⁻ is +6; there is no S⁶⁺ ion.
- Electron direction backwards. From the oxidised species to the reduced one.
- Choosing a sodium, potassium, ammonium or nitrate salt as the precipitate.
- Forgetting to multiply by coefficients when counting electrons lost and gained.
Quick self-check
- Oxidation number of Cr in Cr₂O₇²⁻? (+6)
- Oxidation number of P in PO₄³⁻? (+5)
- NH₄NO₃(aq) is mixed with KCl(aq). Could a precipitate form? (No: every possible product is an ammonium, potassium or nitrate salt, so all stay dissolved.)
- In Mg(s) + 2H⁺(aq) → Mg²⁺(aq) + H₂(g), which species is reduced? (H⁺: hydrogen goes from +1 to 0.)
Next: practice questions.