Decomposition reactions are initiated by the addition of energy.
A decomposition reaction is a chemical reaction in which some chemical bonds in a compound are broken and simpler substances are formed. The breaking of chemical bonds requires the addition of energy, usually in the form of heat.
When a compound is heated, its atoms move about more vigourously. This movement can break chemical bonds. For example, if calcium carbonate is strongly heated, it decomposes into calcium oxide and carbon dioxide.
CaCO₃(s) → CaO(s) + CO₂(g)
In some compounds, the energy needed for decomposition is so small that it can be supplied by a minor shock, such as a physical impact. For example, nitrogen triiodide, NI₃, decomposes explosively into purple iodine vapour and nitrogen gas when touched by a feather.
2NI₃(s) → 3I₂(g) + N₂(g)
This video shows how heating a sample of sodium bicarbonate will cause its decoposition.