Types of reactions
Most reactions in a first chemistry course follow one of five patterns. Recognizing the pattern helps predict the products.
| Type | Pattern | Example |
|---|---|---|
| Synthesis | A + B → AB | 2 Mg + O2 → 2 MgO |
| Decomposition | AB → A + B | 2 H2O2 → 2 H2O + O2 |
| Single replacement | A + BC → AC + B | Zn + 2 HCl → ZnCl2 + H2 |
| Double replacement | AB + CD → AD + CB | AgNO3 + NaCl → AgCl + NaNO3 |
| Combustion | fuel + O2 → CO2 + H2O | CH4 + 2 O2 → CO2 + 2 H2O |
- Synthesis: two or more substances combine into one.
- Decomposition: one substance breaks into simpler ones.
- Single replacement: an element takes the place of another in a compound.
- Double replacement: two compounds swap partners. Often a solid, called a precipitate, forms.
- Combustion: a fuel containing carbon and hydrogen burns in oxygen.
Worked example: classify 2 KClO3 → 2 KCl + 3 O2
There is one reactant and it breaks into two simpler products. This is decomposition.
Common mistake
Confusing single and double replacement. Count the compounds among the reactants. One element plus one compound is single replacement. Two compounds is double replacement.