Free energy decides spontaneity — not enthalpy, not speed
A reaction is spontaneous when delta G is negative, and delta G depends on both enthalpy and entropy through delta G equals delta H minus T delta S. Two consequences trip students up. An endothermic reaction can still be spontaneous if it increases entropy enough and the temperature is high enough, which is why ice melts. And spontaneity says nothing whatever about rate: the oxidation of glucose is enormously spontaneous and yet sugar sits on a shelf unchanged for years, because spontaneity is a statement about the difference between initial and final states while rate is a statement about the activation energy barrier between them.