3.3 Decomposition Reactions
Decomposition is the reverse of synthesis: one compound breaks into two or more simpler products, $$\text{AB} \rightarrow \text{A} + \text{B}$$ usually driven by heat ($\Delta$) or electricity.
Common Decomposition Patterns
- Metal carbonate → metal oxide + $\text{CO}_2$: $$\text{CaCO}_3(s) \xrightarrow{\Delta} \text{CaO}(s) + \text{CO}_2(g)$$
- Metal hydroxide → metal oxide + $\text{H}_2\text{O}$: $$\text{Mg}(\text{OH})_2(s) \xrightarrow{\Delta} \text{MgO}(s) + \text{H}_2\text{O}(g)$$
- Metal chlorate → metal chloride + $\text{O}_2$: $$2\,\text{KClO}_3(s) \xrightarrow{\Delta} 2\,\text{KCl}(s) + 3\,\text{O}_2(g)$$
- Hydrogen peroxide: $$2\,\text{H}_2\text{O}_2(aq) \rightarrow 2\,\text{H}_2\text{O}(l) + \text{O}_2(g)$$
- Electrolysis of water: $$2\,\text{H}_2\text{O}(l) \xrightarrow{\text{electricity}} 2\,\text{H}_2(g) + \text{O}_2(g)$$
Example: Heating baking soda (sodium hydrogen carbonate) gives sodium carbonate, water, and carbon dioxide:
$$2\,\text{NaHCO}_3(s) \xrightarrow{\Delta} \text{Na}_2\text{CO}_3(s) + \text{H}_2\text{O}(g) + \text{CO}_2(g)$$
Practice Problems
- Predict the products of heating solid MgCO3.
- Complete and balance: KClO3 → ? (with heat)
- What are the products of electrolyzing water?
- Is CaCO3 → CaO + CO2 a synthesis or decomposition reaction?
Answers: 1. MgO + CO2 2. 2 KClO3 → 2 KCl + 3 O2 3. H2 and O2 4. Decomposition