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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

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  • $\begingroup$ Water: yes. Always: no. $\endgroup$ – Ivan Neretin Feb 10 '18 at 6:22
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Plainly put: no, decomposition reaction does not always have to entail providing heat. A decomposition reaction (in simple language) is when one compound breaks into multiple other compounds, like so:

$$\ce{A -> B + C}$$

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.

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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.

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