Classifying reactions helps you predict them
There are far too many chemical reactions to memorize one by one. Sorting them into types, as in 1.6.4, does more than give them names: once you know the type, you can predict the products. “Metal + acid” suggests hydrogen gas; two salt solutions mixed together suggest swapping partners. This lesson builds on the four types from 1.6.4 and adds the fifth type used in US textbooks: combustion.
Five types of reactions
| Type | General pattern | Example |
|---|---|---|
| Synthesis化合反应 (also called combination) | A + B → AB | 2Na + Cl₂ → 2NaCl |
| Decomposition分解反应 | AB → A + B | CaCO₃ → CaO + CO₂ |
| Single replacement置换反应 | A + BC → AC + B | Zn + 2HCl → ZnCl₂ + H₂ |
| Double replacement复分解反应 | AB + CD → AD + CB | AgNO₃ + NaCl → AgCl + NaNO₃ |
| Combustion燃烧反应 | fuel + O₂ → oxides | CH₄ + 2O₂ → CO₂ + 2H₂O |
In a combustion reaction, a substance reacts with oxygen and releases energy as heat and light. When a compound of C and H (or C, H, and O) burns completely, it forms CO₂ and H₂O. With too little oxygen, incomplete combustion also forms CO and soot (1.8.2).
How Chinese textbooks classify reactions
Chinese textbooks use only the first four, called the four basic reaction types: 化合 (synthesis), 分解 (decomposition), 置换 (single replacement), and 复分解 (double replacement). The differences are all about combustion:
- Burning methane gives two products, so it is neither synthesis nor decomposition. Chinese textbooks say it belongs to none of the four basic types.
- Burning an element, such as carbon, magnesium, or hydrogen, gives one product. Chinese textbooks call it synthesis; US textbooks usually accept both synthesis and combustion.
- The Chinese word 燃烧 is broader than “combustion”: hydrogen also burns in chlorine, so in Chinese, burning does not require oxygen. In US textbooks, combustion means a reaction with oxygen.
Some reactions fit none of the five types. In a blast furnace, Fe₂O₃ + 3CO → 2Fe + 3CO₂, but CO is a compound, not an element, so this is not single replacement. Chinese high-school textbooks and AP Chemistry both classify reactions like this by electron transfer, as oxidation–reduction (redox) reactions.
Predicting products by type
| Type | Common patterns |
|---|---|
| Synthesis | metal + nonmetal → ionic compound (write the formula from the ion charges, 2.5.1); Na₂O, K₂O, CaO, or BaO + water → base (most other metal oxides, such as CuO and Fe₂O₃, don’t react with water); a nonmetal oxide such as CO₂ or SO₃ + water → acid |
| Decomposition | metal carbonate → metal oxide + CO₂; insoluble metal hydroxide → metal oxide + H₂O; H₂O₂ → H₂O + O₂ |
| Single replacement | a more active element replaces a less active one (2.7.2) |
| Double replacement | partners swap; a reaction happens only if a precipitate, a gas, or water forms (1.8.6, 2.7.3) |
| Combustion | a compound of C, H, and O + O₂ → CO₂ + H₂O |
The equations in the tables leave out states and conditions. The practice and examples below write them in full.
Start by sorting. The reactions appear in random order; pick the type of each one:
Combustion reactions are balanced in a fixed order: C first, then H, then O. Pick a fuel and step through it:
| C | H | O |
|---|
Example 1: identify the type and predict the products
Identify the type of each reaction, then write and balance the equation.
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The method. Count how many substances become how many: many → one is synthesis, one → many is decomposition, an element swapping with an element is single replacement, and two compounds swapping partners is double replacement. Then write the product formulas from the ion charges, and balance last.
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Aluminum burns in oxygen. Two elements form one compound: synthesis (and also combustion). Al³⁺ and O²⁻ have a least common multiple of 6, so the product is Al₂O₃. Balance O first (3O₂ and 2Al₂O₃ each have 6 O), then Al:
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Magnesium carbonate is heated. One compound breaks into two: decomposition. A metal carbonate gives a metal oxide and carbon dioxide, and the atoms already balance:
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Limewater is mixed with sodium carbonate solution. Two compounds swap partners: Ca²⁺ pairs with CO₃²⁻, and Na⁺ stays in solution with OH⁻. CaCO₃ is insoluble, so a precipitate forms and the reaction happens: double replacement. Na and OH each need a 2:
Industry uses this reaction to make sodium hydroxide.
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A common mistake. Writing aluminum oxide as AlO or Al₃O₂. The charges fix the formula; when you balance, change only the coefficients, never the subscripts.
Example 2: balance the combustion of hexane
Hexane, C₆H₁₄, is one of the compounds in gasoline. Write the balanced equation for its complete combustion.
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Write reactants and products. In complete combustion of a hydrocarbon, all the C becomes CO₂ and all the H becomes H₂O: C₆H₁₄ + O₂ → CO₂ + H₂O.
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Balance the elements that appear in only one product first. O appears in O₂, CO₂, and H₂O, so leave it for last; C and H each appear in only one product.
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Balance C. 6 C atoms make 6 CO₂.
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Balance H. 14 H atoms make 7 H₂O.
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Balance O. The right side has
O atoms, which needs 19/2 O₂, not a whole number.
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Multiply everything by 2. Equations use the lowest whole-number coefficients:
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Check. Left: 12 C, 28 H, 38 O. Right: 12 C, 28 H, 24 + 14 = 38 O. Balanced.
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A common mistake. Changing only the O₂ coefficient to 19 and forgetting to double the others.
- CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)
Swapping partners gives H₂CO₃, which breaks down into water and carbon dioxide: double replacement.
- 2HgO(s) → 2Hg(l) + O₂(g) (heated)
One compound breaks into two substances: decomposition.
- Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)
Magnesium replaces hydrogen in the acid: single replacement.
- C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(l)
A hydrocarbon reacts with oxygen to form CO₂ and H₂O: combustion. It is none of the four basic types in Chinese textbooks.
- CO₂(g) + H₂O(l) → H₂CO₃(aq)
A nonmetal oxide and water form an acid: synthesis.
- BaCl₂(aq) + H₂SO₄(aq) → BaSO₄(s) + 2HCl(aq)
Partners swap and barium sulfate precipitates: double replacement.
- 2NaHCO₃(s) → Na₂CO₃(s) + H₂O(g) + CO₂(g) (heated)
One compound breaks into three: decomposition. The products include CO₂ and H₂O, but no oxygen reacts, so it is not combustion.
- Cu(s) + 2AgNO₃(aq) → Cu(NO₃)₂(aq) + 2Ag(s)
Copper replaces silver in its salt: single replacement.
- NH₃(g) + HCl(g) → NH₄Cl(s)
Two substances form one: synthesis.
Common mistakes
Mistake 1: “Any reaction with an element in it is single replacement.” Single replacement needs element + compound → new element + new compound. 2H₂ + O₂ → 2H₂O is synthesis, CH₄ + 2O₂ → CO₂ + 2H₂O is combustion, and Fe₂O₃ + 3CO → 2Fe + 3CO₂ has no element among the reactants at all.
Mistake 2: “Two compounds forming two compounds is always double replacement.” The partners must actually swap. In CO₂ + Ca(OH)₂ → CaCO₃ + H₂O, no partners are exchanged, so it is not double replacement.
Mistake 3: “Every combustion reaction is a synthesis reaction.” Only burning an element, such as C, Mg, or H₂, gives a single product. Burning a compound such as methane or ethanol gives two products, CO₂ and H₂O, so it is not synthesis.
Quick check
Which reaction is a single replacement reaction?
Iron (an element) and copper(II) sulfate (a compound) form copper (an element) and iron(II) sulfate (a compound). CO in the first option is not an element, the third is synthesis, and the fourth is double replacement.
When ethane, C₂H₆, burns completely and the equation is balanced with the lowest whole numbers, what is the coefficient of O₂?
2 C give 2CO₂ and 6 H give 3H₂O, so the right side has 7 O atoms and needs 7/2 O₂. Doubling gives 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O.
Summary
- The five US types are synthesis, decomposition, single replacement, double replacement, and combustion. The first four match the four basic types in Chinese textbooks.
- Combustion is a reaction with oxygen; compounds of C, H, and O burn completely to CO₂ and H₂O. Chinese textbooks put methane combustion in none of their four types, and burning an element counts as both synthesis and combustion.
- The type lets you predict the products. Balance combustion in the order C → H → O, and double everything if O₂ ends up with a fraction.
On to AP (for later)
AP Chemistry Unit 4 drops the five types and sorts reactions into three classes: precipitation, acid–base, and oxidation–reduction. Combustion and single replacement are both redox reactions (Unit 4 introduces oxidation numbers), and most double replacement reactions are precipitation or acid–base reactions. Balancing combustion reactions and doing stoichiometry with them still comes up often on the exam.