Decomposition of N205

In this special case entropy overwhelms enthalpy, and the reaction is spontaneous even though it absorbs heat:

DG⁰= +26.2 �33.5 = -7.3 kcal

The vector diagram for this reaction is shown at the upper left. The enthalpy arrow points up, but the entropy arrow points down and is so long that the sum of these two is a downward-pointing free energy arrow. Disordering is so potent a driving force in this reaction that it carries the reaction along with it to a state of higher enthalpy, thereby drawing heat energy from its surroundings and cooling them in the process. The cooling effect of melting ice cubes has the same origin.