text: are exothermic reactions slower than endothermic reactions? how do you know which reaction is slow by looking at heat of reaction or activation energy? exothermic and endothermic indicate if the reaction produces or requires energy to take place. it has to do with whether the products have lower or higher energy (respectively) than the reactants.  the rate of the reaction depends on several factors (like nonscientific mentions). in terms of energy, the rate or reaction and the activation energy are associated via the arrhenius equation k=a*e^(-ea/rt).  so in fact the rate of the reaction will depend on the height of the activation energy (that peak you see in the potential energy diagrams for reactions). it doesn't matter where the level of the products is, only the height of the peak.  a very nice example is the reaction of h2 and o2. as we all know it is extremely exothermic but if you just mix 2 volumes of h2 and 1 of o2 practically nothing will happen. the activation energy is so high that the reaction occurs at an extremely low level and too slow pace for you to see it happening. if you light a match you provide enough energy for several molecules to overcome the high activation energy. because the reaction releases large amounts of heat this helps more molecules overcome the energy barrier and thus you have a violent (in a way autocatalytic) reaction, leading to an explosion.