An exothermic reaction is apparently one that releases energy to it's environment. According to Wikipedia, an explosion is a rapid increase in volume & energy.

Does this mean that an explosion is basically an exothermic reaction?

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    $\begingroup$ The distinguishing feature of an explosion is how fast it happens. There are plenty of slow exothermic reactions, like digestion of food. $\endgroup$
    – matt_black
    Oct 26 '12 at 23:11

The key parts to the definition of an explosion are "rapid" and "increase in volume".

Exothermic reactions do not necessarily meet these requirements. Exothermic reactions need only release energy to the surroundings by converting chemical potential energy into kinetic energy.

For example, the combustion of methane is very exothermic, with a standard enthalpy change of -890 kJ/mol.

$\ce{CH4(g) + 2O2(g)->CO2(g) +2H2O(g)} \space \space \space \space \Delta_c H^o=-890 \text{ kJ/mol}$

However, at room temperature this reaction is very slow. Mixtures of oxygen and methane do not spontaneously ignite without a spark source. Also, there is no change in pressure with this reaction, since there are 3 moles of gas on both sides of the equation.

On the other hand, consider nitroglycerin. Yes, it's combustion is significantly exothermic. More importantly, the combustion happens rapidly, needs only the heat of friction or the energy of impact to ignite, and produces significantly more gas than it consumes:

$\ce{4C3H5O9}(l) \ce{+1O2(g)->12CO2(g) +10H2O(g) +6N2(g)} \space \space \space \space \Delta_c H^o=-5660 \text{ kJ/mol}$

All thermodynamic data were mined from the NIST Chemistry Webbook.

  • $\begingroup$ May we then say an explosion is a special case of an exothermic reaction? $\endgroup$
    – Everyone
    Oct 27 '12 at 5:33
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    $\begingroup$ In some rare cases, substances may be explosive without evolving gas. An example is silver acetylide (here's a demo of its detonation). Furthermore, some substances are still explosive though their decomposition is only slightly exothermic, or conceivably even slightly endothermic, so long as the resulting reaction results in a large enough entropic gain. These are termed entropic explosions. $\endgroup$ Sep 14 '14 at 12:55

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