How can endothermic reaction be spontaneous?

An endothermic reaction can only be spontaneous under standard conditions if the increase in molar is sufficient to overcome the endothermicity.

Under normal conditions if constant temperature and pressure, the spontaneity of reaction is determined by 2 factors: changes in , ##Delta##H and entropy, ##Delta##S. The overall change in is ##Delta##G = ##Delta##H - T##Delta##S

If ##Delta##G is negative the reaction is spontaneous. Usually this equation is dominated by enthalpy changes but in rare cases entropy can determine spontaneity.