Decomposition reactions are chemical reactions in which a more complex molecule breaks down to make simpler ones. There are three types of decomposition reactions:
Thermal decomposition reactions;
Elctrolytic decomposition reactions;
Photo decomposition reactions.
Thermal decomposition - such reactions are usually endothermic, since energy in the form of heat is required to break the bonds of the more complex molecule. Examples include
#CaCO_(3(s)) + "heat" -> CaO_((s)) + CO_(2(g))# - calcium carbonate decomposes into calcium oxide and carbon dioxide when heated;
#2KClO_(3(s)) + "heat" -> 2KCl_((s)) + 3O_(2(g))# - potassium chlorate decomposes into potassium chloride and oxygen gas when ehated;
#2Fe(OH)_3 + "heat" -> Fe_2O_3 + 3H_2O# -ferric dioxide decomposes into ferric oxide and water when heated;
Electrolytic decomposition - such reactions occur when an electric current is passed through an aqueous solution of a compound. Two classic examples are the electrolysis of water
#2H_2O_((l)) -> 2H_(2(g)) + O_(2(g))# - water decomposes into hydrogen and oxygen in the presence of an electric current;
and the decomposition of sodium chloride
#2NaCl_((l)) -> 2Na_((l)) + Cl_(2(g))# - molten sodium chloride will decompose into molten sodium and chlorine gas;
Photo decomposition - these reactions occur in the presence of light (photons). Examples include
#2AgCl_((s)) + "sunlight" -> 2Ag_((s)) + Cl_(2(g))# - silver chloride decomposes into silver and chlorine in the presence of sunlight;
#2AgBr_((s)) + "sunlight" -> 2Ag_((s)) + 2Cl_(2(g))# - silver bromide decomposes into silver and chlorine in the presence of sunlight;
As a rule of thumb, most decomposition reactions are endothermic, since energy, either in the form of heat, electric current, or sunlight must be provided in order to break the bonds of the more complex molecule.