What changes of state are endothermic and exothermic?

1 Answer

Well, we got....



And alternatively....


Now whether a particular chemical or physical reaction is exothermic or endothermic depends on the balance between bonds formed, and bonds made. The formation of bonds RELEASES energy, i.e. this is an exothermic phenomenon, and the breaking of bonds REQUIRES energy, i.e. this is an endothermic phenomenon.

When a solid melts, strong bonds in the solid state ARE CERTAINLY BROKEN to give a more loosely ordered liquid state, where the contiguous molecules/particles are not held in a rigid, ordered array. And so this change of state should be endothermic.

Likewise, when a liquid evaporates, i.e. vaporizes, whatever bonds exist in the liquid phase are COMPLETELY broken, and the gaseous material exists more or less as discrete, individual molecules or particles. And certainly the same consideration applies to sublimation, the direct transition between solid and gas.

Now I have classified the entropy change associated with the phase changes in one direction only: solid to liquid to gas.... It is up to you to predict the reverse direction: gas to liquid to solid....