Final answer:
A potential energy diagram for an exothermic reaction should show a lower energy level for the products than the reactants, with the 'X' value indicating activation energy and 'Y' value representing a negative enthalpy change (ΔH).
Step-by-step explanation:
In a potential energy diagram, the y-axis represents the energy and the x-axis the reaction progress. The diagram shows how the potential energy changes as a system of reactants is transformed into products. For a reaction with a negative enthalpy change (ΔH), which indicates an exothermic reaction, the energy level of the products should be lower than that of the reactants.
The point labeled 'X' on such a diagram typically represents the activation energy (Ea), which is the energy required to initiate the reaction. This is the peak of the energy barrier that reactants must overcome to form the transition state. The value labeled 'Y' represents the change in enthalpy (ΔH) of the reaction, which, for an exothermic reaction, is the difference in energy between the reactants and the products and is negative. If the student's diagram displays these characteristics accurately, then it is correct for an exothermic reaction.