Activation Energy

[Clara_Nguyen_1D]

How can we tell based on activation energy if a reaction is endothermic or exothermic?

[Eunnie_Lee_3H]

I don't believe you can tell based on (solely) activation energy if a reaction is exothermic or endothermic. Looking at whether the reactants have more energy than the products should be a better way of figuring out whether or not a reaction is exothermic or endothermic.

If the reaction is endothermic, the activation energy has to at least match energy difference between the reactant and product plus a little more energy (since reactants have less energy than the products), so there might be a "higher chance" of a reaction being endothermic if the activation energy is large. However, there are definitely exothermic reactions with high activation energy, so it is much better to just look at reactant and product energy levels to determine if a reaction is exothermic or endothermic if you have that information.

[Rochelle Ellison 2H]

A reaction that produces products that are lower in energy than the reactants is exothermic and a reaction that produces products that are higher in energy than the reactants is endothermic. Both exothermic and endothermic reactions require activation energy and you would not be able to tell if the reaction were exothermic or endothermic if you were just given the activation energy for the forward reaction. However, if you were given the activation energy for both the forward and reverse reactions of a particular reaction then you could make a determination. If a reaction is endothermic in the forward direction then its activation energy is higher in its forward direction than its reverse direction. Then if the temperature is raised, the rate constant will increase more for the forward reaction than the reverse reaction. If the reaction is exothermic in the forward direction then the activation energy is higher in the reverse direction than the forward direction so if the temperature is raised the rate constant for the reverse reaction increases more than the rate constant for the forward reaction.

[Jeannie_Kim_1I]

The activation energy of an endothermic reaction is greater than that of an exothermic reaction, but you can't determine if a reaction is endothermic or exothermic by looking at the activation energy.