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Decomposition reactions involve breaking of bonds which requires energy. Therefore, they are generally endothermic.

But how can decomposition reactions release energy? Examples are the respiration reaction or the decomposition of vegetable matter.

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  • $\begingroup$ Pure decomposition reactions are exceedingly rare. Typically, some bonds are broken, and some other bonds are formed. $\endgroup$ – Ivan Neretin Feb 19 '18 at 13:37
  • $\begingroup$ Yes it's the bonds that are formed that release that huge amount of energy, which is more than the energy absorbed when bonds are broken in the case of your compost pile. $\endgroup$ – Kevin Kostlan Jul 17 '18 at 1:21
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It just can be so. Look for something that is favored by cooling instead of heating.

In the case of decomposition of something to pure elements that is exceedingly rare, but decomposition involving other compounds offers more possibilities. During hot processing of steel (in the hot rolled coil, actually) we can get this reaction which is favored by cooling below 570°C:

Wustite --> Magnetite + Iron

$4 \ce{Fe_{1-x}O} \rightarrow \ce{Fe_3O_4}+(1-4x)\ce{Fe}$

$x$ typically 0.05-0.10.

See the phase diagram here noting what happens to the $\ce{Fe_{1-x}O}$ phase on cooling.

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  • $\begingroup$ Note that Fe-Fe metallic bonds are formed, which is crucial to driving the reaction forward. We don't have free gaseous iron atoms flying off! $\endgroup$ – Kevin Kostlan Jul 17 '18 at 1:23

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