Two Ways of Sorting the Same Reaction
Chemists sort reactions in two independent ways; one reaction normally takes a label from each.
The first sorts by pattern, the arrangement of reactants and products: synthesis, decomposition, single replacement, double replacement and combustion.
The second sorts by what changes hands. Acid-base reactions transfer H⁺ ions, precipitation reactions lock ions into an insoluble solid, and redox reactions transfer electrons; acid-base is developed in Topic 4.8 and redox in Topic 4.9.
The two systems overlap freely: one reaction can be a double replacement and a precipitation at the same time.
Synthesis and Decomposition: Opposite Patterns
A synthesis reaction builds one product from two or more reactants: A + B → AB. Two elements combining, as in 2Mg + O₂ → 2MgO, is the clearest case, but compounds can join too: CaO + H₂O → Ca(OH)₂.
A decomposition reaction runs the pattern in reverse: one reactant splits into two or more products, AB → A + B. Heating calcium carbonate gives CaCO₃ → CaO + CO₂, and heating potassium chlorate gives 2KClO₃ → 2KCl + 3O₂.
The products need not be elements: CaCO₃ gives two compounds, and only some decompositions, such as 2HgO → 2Hg + O₂, end at elements.
Single Replacement and the Activity Series
In a single replacement reaction, a free element takes the place of a similar element in a compound: A + BC → AC + B.
Zinc pushes copper out of copper(II) sulfate solution, Zn + CuSO₄ → ZnSO₄ + Cu, and chlorine does the same to bromide, Cl₂ + 2NaBr → 2NaCl + Br₂.
The activity series, the ranking of elements by reactivity, decides whether the swap happens: a free element displaces only elements below it.
Copper sits above silver, so Cu + 2AgNO₃ → Cu(NO₃)₂ + 2Ag runs. Copper sits below zinc, so copper metal in zinc sulfate solution does nothing: it is not reactive enough to displace the zinc.
Hydrogen also holds a place in the series. Metals above it displace it from dilute acids, as in Zn + 2HCl → ZnCl₂ + H₂; copper, below it, does not react.
Double Replacement: Swapping Partners
In a double replacement reaction, two compounds exchange ions: AB + CD → AD + CB. The way to predict the products is to swap the cations, giving each cation the other compound’s anion: AgNO₃ + NaCl → AgCl + NaNO₃.
Swapping writes candidate products, not a guaranteed reaction: it runs only if a precipitate, a gas or water leaves the solution, as insoluble AgCl does here.
When the product that leaves is a solid, as BaSO₄ in BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl, the reaction is also a precipitation.
Swapped formulas must balance charges: Al³⁺ with SO₄²⁻ gives Al₂(SO₄)₃, never AlSO₄.
Acid-Base Reactions: Salt, Water, and a Gas From Carbonates
An acid reacting with a metal hydroxide produces a salt and water: HCl + NaOH → NaCl + H₂O. The acid’s H⁺ and the base’s OH⁻ form the water; the salt pairs the base’s cation with the acid’s anion.
By pattern it is double replacement; by family it is acid-base, the neutralisation developed in Topic 4.8.
Carbonates behave as bases but add a third product. Acid plus a metal carbonate gives a salt, water and carbon dioxide: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂.
The fizzing is the CO₂ escaping: spot a carbonate among the reactants, and CO₂ must appear among the products.
Combustion: Burning in Oxygen
Combustion is a rapid reaction with oxygen, releasing energy as heat and light; oxygen is a required reactant.
A hydrocarbon burned in excess oxygen burns completely: every carbon ends as CO₂ and every hydrogen as H₂O. CH₄ + 2O₂ → CO₂ + 2H₂O and C₃H₈ + 5O₂ → 3CO₂ + 4H₂O follow that rule, so the products can be written before balancing.
In limited oxygen the burn is incomplete: carbon monoxide or soot (carbon) replaces some of the CO₂, so the CO₂ and H₂O rule holds only with oxygen in excess.
Metals burn to metal oxides: 2Mg + O₂ → 2MgO is a synthesis and a combustion at once. Every combustion is redox, the fuel losing electrons to oxygen.
Redox: The Electron-Transfer Test
A redox reaction is one in which electrons are transferred, and the reliable sign is that oxidation numbers change. The substance that loses electrons is oxidised; the one that gains them is reduced.
In Zn + CuSO₄ → ZnSO₄ + Cu, zinc moves from 0 to +2 in Zn²⁺, losing two electrons per atom: zinc is oxidised. Copper moves from +2 to 0: copper ions are reduced.
In Cl₂ + 2NaBr → 2NaCl + Br₂, chlorine falls from 0 to −1 and is reduced; bromide rises from −1 to 0 and is oxidised.
Single replacement reactions are always redox, because a neutral atom and an ion trade places. Double replacement and acid-base reactions are never redox: no ion changes its charge, so no oxidation number moves.
Worked Examples
Example 1: predicting and classifying. Iron filings in copper(II) sulfate solution. Iron is above copper in the activity series, so the replacement runs: Fe + CuSO₄ → FeSO₄ + Cu. Pattern: single replacement. Family: redox, iron moving 0 to +2 while copper moves +2 to 0. The blue colour fades as Cu²⁺ leaves solution, and brown copper coats the iron.
Example 2: combustion with numbers. 8.02 g of methane (0.500 mol) burns completely: CH₄ + 2O₂ → CO₂ + 2H₂O. Products: 0.500 mol of CO₂ (0.500 × 44.01 = 22.0 g) and 1.00 mol of H₂O (18.0 g); oxygen used: 1.00 mol (32.0 g). Mass check: 8.02 + 32.0 = 40.0 g in, 22.0 + 18.0 = 40.0 g out.
Example 3: a double replacement carried through. BaCl₂ and Na₂SO₄ solutions are mixed. Swap the cations: Ba²⁺ takes SO₄²⁻, Na⁺ takes Cl⁻. BaSO₄ is insoluble, so BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl runs: a double replacement and a precipitation. With 0.0100 mol of BaCl₂ and excess sulfate, 0.0100 mol of BaSO₄ forms: 0.0100 × 233.39 = 2.33 g.
Traps That Cost Marks in Topic 4.7
Giving a reaction only one label. BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl is a double replacement and a precipitation; Zn + CuSO₄ → ZnSO₄ + Cu is a single replacement and redox. A question asking what is “also true” is usually fishing for the second label.
Assuming every cation swap is a reaction. Swapping partners only proposes products: if both are soluble and no gas or water forms, every ion stays in solution and nothing has reacted. NaCl with KNO₃ is the standard case.
Ignoring the activity series. A single replacement runs only when the free element sits above the element it would displace: silver does nothing in copper(II) nitrate (below copper), while copper reacts in silver nitrate (above silver).
Losing the carbon dioxide from a carbonate. Acid plus carbonate makes three products: a salt, water and CO₂. Stopping at the salt and water drops a product and leaves the equation unbalanced.
Writing charge-unbalanced formulas for swapped products. An ionic product must be neutral overall: aluminium sulfate is Al₂(SO₄)₃, its +6 from two Al³⁺ ions balancing −6 from three SO₄²⁻ ions. AlSO₄ carries a net charge and cannot be a product.
Confusing combustion with synthesis, or with the absence of redox. 2Mg + O₂ → 2MgO is synthesis and combustion in one equation, and every combustion is redox. Test each label on its own terms.
Key Relationships
- Pattern forms: synthesis A + B → AB; decomposition AB → A + B; single replacement A + BC → AC + B; double replacement AB + CD → AD + CB; combustion: fuel + O₂ → oxides
- The three families: acid-base transfers H⁺, precipitation locks ions into an insoluble solid, redox transfers electrons; pattern and family labels overlap freely
- A hydrocarbon in excess oxygen gives CO₂ and H₂O only; in limited oxygen, expect carbon monoxide or carbon as well
- Acid + metal hydroxide → salt + water; acid + metal carbonate → salt + water + CO₂
- Activity series: a free element displaces only elements ranked below it; metals above hydrogen displace H₂ from dilute acids, metals below it do not
- Double replacement prediction: swap the cations, write charge-balanced formulas, then check for a precipitate, a gas or water; none of the three means no reaction
- Single replacement and combustion are always redox; double replacement and acid-base reactions never are, because no oxidation number changes
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