Types of Chemical Reactions
High-Yield Summary
- Six reaction types cover most MCAT reactions: combination, decomposition, combustion, single-displacement, double-displacement, neutralization.
- Combustion and single-displacement reactions are inherently redox (electron transfer, oxidation state change); neutralization is a specific case of double-displacement.
- Double-displacement reactions proceed when a product leaves as a precipitate/gas, or two species combine into a weak (undissociated) electrolyte.
- Recognizing the reaction type is often the fastest way to predict an equation's products.
The Six Reaction Types
| Type | Pattern / Example |
|---|---|
| Combination (synthesis) | A + B → C — e.g., 2 Mg + O₂ → 2 MgO |
| Decomposition | A → B + C (heat/radiation/electrolysis) — e.g., 2 HgO → 2 Hg + O₂ |
| Combustion | Fuel (hydrocarbon) + O₂ → CO₂ + H₂O; a redox reaction |
| Single-displacement | Atom/ion in a compound replaced by another element — e.g., Cu + 2 AgNO₃ → Cu(NO₃)₂ + 2 Ag; a redox reaction |
| Double-displacement (metathesis) | Elements from 2 compounds swap — e.g., CaCl₂ + 2 AgNO₃ → 2 AgCl↓ + Ca(NO₃)₂ |
| Neutralization | Acid + base → salt (+ usually water) — e.g., HCl + NaOH → NaCl + H₂O |
Must Know
- Combustion and single-displacement reactions are ALWAYS redox reactions — electrons transfer and oxidation states change.
- Neutralization is a special case of double-displacement, not a separate mechanism.
- Double-displacement reactions are driven forward by removing a product from solution (precipitate ↓, gas, or a weak/undissociated electrolyte).
Common MCAT Trap
- Don't confuse combination (A + B → C, fewer products) with decomposition (A → B + C, more products) — they're mirror images.
- A reaction can look like combustion (fuel + O₂) without producing exactly CO₂ and H₂O if the fuel isn't a simple hydrocarbon — check what the reactants and products actually are, don't assume by pattern-matching alone.
- Single-displacement swaps ONE element into a compound; double-displacement swaps parts of TWO compounds — count which side has a lone element vs. two full compounds to tell them apart.
Quick Recall
Zn + CuSO₄ → ZnSO₄ + Cu — what type of reaction is this, and why?
What two reaction types are inherently redox reactions?
What condition drives a double-displacement reaction to completion?