Identify an energy diagram showing the energy of the reaction as a function of the progress of the reaction.

a potential energy diagram of a chemical reaction shows the net change in energy as reactants change into products.

the net change in energy corresponds to the change in gibb's free energy (delta g) for a reaction. this also relates to the change in  enthalpy  (delta h) for a reaction. the gain or loss of energy by the reaction from or to the surroundings indicates whether a reaction is endothermic or exothermic.

an endothermic reaction involves a net increase in potential energy as reactants change into products. energy is gained by the system from the surroundings. so, delta h is positive. this type of reaction will feel cold, or absorb heat.

an exothermic reaction involves a net decrease, so dealt h is negative. this type of reaction will feel warm or give of heat.

frequency or wavelenth of em radiation in a vacuum