Classifying reactions and tracking energy
A chemical equation can be classified in several independent ways: by how substances rearrange, by energy exchange, by oxidation-state change, by reversibility and by catalyst participation. One reaction can belong to several categories at once.
1. Classification by particle rearrangement
| Type | Pattern | Example |
|---|---|---|
| Combination | A + B → AB | 2Mg + O2 → 2MgO |
| Decomposition | AB → A + B | CaCO3 → CaO + CO2 |
| Displacement | A + BC → AC + B | Zn + 2HCl → ZnCl2 + H2 |
| Exchange | AB + CD → AD + CB | HCl + NaOH → NaCl + H2O |
These patterns describe composition, not the detailed mechanism. Always balance the equation before comparing particle counts. Coefficients change amounts; subscripts are part of a substance's identity.
2. Thermal effect
An exothermic reaction transfers energy from the reacting system to the surroundings; the surroundings often warm. Products lie at lower chemical energy than reactants, so $ΔH<0$. An endothermic reaction absorbs energy from the surroundings; products lie higher, so $ΔH>0$. Temperature change is evidence only when heat exchange with the environment is considered.