A student writes that in every exothermic reaction the reactants must be at a higher energy level than the products. The energy profile shown supports this ...
Question 1 Report
A student writes that in every exothermic reaction the reactants must be at a higher energy level than the products. The energy profile shown supports this idea. 
Is the student correct?
Answer Details
The student is correct. In an exothermic reaction the chemicals give out energy to the surroundings, so they must end up with less energy than they started with. On the diagram that shows as the product level lying below the reactant level, which is exactly what this profile shows.
Expressed as a calculation, \(\Delta H = E_{\text{products}} - E_{\text{reactants}}\), and a lower product energy makes \(\Delta H\) negative, which is the sign convention for exothermic.
The claim that the products are always higher describes an endothermic reaction instead, and equal levels would mean no net energy change at all, which is not a real reaction outcome. Temperature does not decide the direction either; it affects the rate. Fix the rule now: exothermic means products below reactants, endothermic means products above.
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