Reaction Classification, Conservation Laws, and Thermochemical Calculations
A balanced equation describes conserved particles, quantitative proportions, and—when ΔH is included—energy transfer for a defined reaction.
Learning goals
- Classify reactions by particle rearrangement and energy transfer.
- Balance equations using conservation of atoms and charge.
- Interpret coefficients as mole ratios.
- Scale and reverse thermochemical equations correctly.
- Calculate transferred heat from reaction amount or temperature change.
1. Multiple ways to classify
| Pattern | General form | Key idea |
|---|---|---|
| Synthesis | A + B → AB | several reactants form fewer products |
| Decomposition | AB → A + B | one reactant forms several products |
| Single displacement | A + BC → AC + B | one species replaces another |
| Exchange | AB + CD → AD + CB | ions or groups exchange partners |
| Combustion | substance + O2 → oxides | rapid oxidation, often releasing energy |
One reaction may belong to several categories: combustion can also be redox and exothermic. Reversible/irreversible, homogeneous/heterogeneous, and catalytic/noncatalytic are other independent classification axes.
2. Conservation and balancing
Ordinary chemical reactions rearrange atoms; they do not create or destroy nuclei. Therefore the number of atoms of each element and total electric charge are equal on both sides. In a closed system, total mass is conserved. If gas escapes from an open vessel, measured vessel mass may change while total mass of vessel plus escaped matter remains conserved.
- Write correct formulas first.
- Count atoms on both sides.