# Do all decomposition reactions involve heat? [closed]

When water decomposes into Hydrogen and Oxygen as in 2H2O—-> 2H2+O2 does this involve heat? Is it always implied that heat is involved in a decomposition reaction or does this just spontaneously occur? Chemistry 1st semester so excuse the fundamental nature of the question. Thanks

• Water: yes. Always: no. – Ivan Neretin Feb 10 '18 at 6:22

$$\ce{A -> B + C}$$
As a general rule, if the product side is relatively more stable, it will decompose without you having the to provide extra heat. In the case of the reaction you gave, $$\ce{H2O}$$ is more stable than $$\ce{H2}$$ and $$\ce{O2}$$, and thus you will have to provide heat for the breaking of bonds.
Check out the iron oxygen phase diagram. Wustite ($\ce{FeO}$) decomposes upon ... cooling, below 570°C, to magnetite ($\ce{Fe_3O_4}$) and metallic iron. Steel strip manufacturers deal with this reaction because it impacts the acid pickling of hot mill scale.