Answer: it is a curve line that resembles a hill, where the reactants start at an intermediate height (on the left side), the activated complex is at the peak, and the products are in the through (the lowest level) at the right side.
Explanation:
1) The potential energy diagram shows the potential energy of the spieces in the vertical axis, as they progress from the reactants to the products (algong the horizontal axis. The horizontal axis represents the progress of the reaction. The reactants are in the left extreme of the graph, the activated complex is in the middle, and the products are in the right end.
2) In any chemical reaction (either exothermic or endothermic), the products are at a certain energy level that is below the energy level of the activated complex. The acitvated complex is at the ighest energy level (the peak of the curve) , which is the activation energy, this is the energy required to the reaction proceed from the reactants to the products.
3) Since an exothermic reaction releases heat (energy) the energy (enthalpy) of the products is lower than the energy of the reactants. That is why the right end of the curve is lower than the left end.
Since, one image is worther than one thousand words, I attache a picture of the energy graph for an exothermic reaction. See the figure attached.
