4.7 Reaction Types

Types of Chemical Reactions

Browse 13 examples across all 5 reaction types with 3D molecule diagrams. Each entry shows the balanced equation, identification tips, and common mistakes to avoid.

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Topic 4.7

Types of Chemical Reactions

Identify a reaction as acid–base, oxidation–reduction, or precipitation.

The CED reduces reaction classification to a single question: what is being transferred?

Acid–base reactions involve transfer of one or more protons (H⁺ ions) between chemical species.

Oxidation–reduction (redox) reactions involve transfer of one or more electrons, as indicated by changes in oxidation numbers. Electrons are transferred from the species that is oxidized to the species that is reduced. Combustion is an important subclass: a species reacts with oxygen gas, and for hydrocarbons complete combustion produces CO₂ and H₂O.

Assigning oxidation numbers to each atom in reactants and products is often the most effective way to identify what was oxidized and what was reduced. The rules, in priority order:

  1. A free element is 0.
  2. A monatomic ion equals its charge.
  3. Fluorine is always −1.
  4. Oxygen is usually −2 (−1 in peroxides).
  5. Hydrogen is +1 with nonmetals, −1 in metal hydrides.
  6. The sum equals 0 for a neutral compound, or the charge for a polyatomic ion.

Precipitation reactions frequently involve mixing ions in aqueous solution to produce an insoluble or sparingly soluble ionic compound. The one solubility fact the CED requires: all sodium, potassium, ammonium, and nitrate salts are soluble in water.

Memory device: OIL RIG — Oxidation Is Loss of electrons, Reduction Is Gain. Oxidation number goes up on oxidation.

Key points

  • Protons transferred → acid–base. Electrons transferred → redox. Insoluble solid forms → precipitation.
  • Any reaction with a free element on one side and that element in a compound on the other is redox.
  • The only required solubility rule: Na⁺, K⁺, NH₄⁺, and NO₃⁻ salts are always soluble.
  • Oxidation numbers are a bookkeeping device — they are not actual charges on atoms in covalent compounds.

Common mistakes

  • Oxygen is −1 in peroxides (H₂O₂, Na₂O₂), not −2.
  • Hydrogen is −1 in metal hydrides (NaH, CaH₂).
  • Oxidation number ≠ formal charge ≠ actual charge. Three different bookkeeping systems.
  • Do not memorize long solubility tables. The CED excludes rote memorization beyond the four soluble families.

Worked example

Classify each reaction and, for any redox reaction, identify the species oxidized and reduced. (a) 2 KI(aq) + Pb(NO₃)₂(aq) → PbI₂(s) + 2 KNO₃(aq) (b) HNO₃(aq) + KOH(aq) → KNO₃(aq) + H₂O(l) (c) 2 Al(s) + 3 Cl₂(g) → 2 AlCl₃(s)

(a) Precipitation. Lead(II) iodide is insoluble and separates as a solid. No oxidation numbers change (Pb stays +2, I stays −1, K stays +1, NO₃ stays −1). Net ionic: Pb²⁺(aq) + 2 I⁻(aq) → PbI₂(s).

(b) Acid–base. A proton is transferred from HNO₃ to OH⁻. Both are strong, so the net ionic equation is H⁺(aq) + OH⁻(aq) → H₂O(l).

(c) Redox. Aluminum goes from 0 (free element) to +3 — it is oxidized, losing 3 electrons per atom. Chlorine goes from 0 (free element) to −1 — it is reduced, gaining 1 electron per atom. Electrons transfer from Al to Cl. This is also a synthesis reaction.

Full notes for topic 4.7 →