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If endothermic reactions absorb heat from the surroundings, how can they be spontaneous, if heat only flows from high temp to low temp objects?

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    $\begingroup$ Heat does not necessarily flow from high temp to low temp objects. Open your fridge, and you'll see. How did it get cold? When you bought it, it was room temp, wasn't it? $\endgroup$ Mar 29, 2018 at 7:32
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    $\begingroup$ Another example would be the evaporation of water at room temperature. Evaporation is endothermic, yet water slowly evaporates into vapor in the atmosphere. $\endgroup$ Mar 29, 2018 at 9:20
  • $\begingroup$ A very similar example (which I find a bit easier to visualise) to Pritt Balagopal's is the melting of ice at room temperature. $\endgroup$ Mar 29, 2018 at 11:36

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A spontaneous endothermic reaction can occur when the changes in enthalpy and entropy yield a negative Gibbs free energy. An endothermic reaction can be spontaneous if entropy increases by more than the change in enthalpy.

$\Delta G = \Delta H - T\Delta S$

For endothermic r̥eactions, $\Delta H$> 0 and thus if the value of $-T\Delta S$ overshadows the $\Delta H$ value (usually at very high temperatures) then the overall value of $\Delta G$ becomes negative and thus the reaction becomes spontaneous.

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At the end of the day spontaneity boils down to the sign of "Gibbs free Energy" so if the $\Delta G= \Delta H- T\Delta S$ gives you a negative value then the process would be spontaneous.

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