A single-step energy diagram shows reactants at 75 kJ, a peak at 140 kJ, and products at 70 kJ. Which statement best describes the reaction based on the diagram?
- Exothermic, because the peak is above the reactants
- Endothermic, because the products are slightly lower in energy than the reactants
- Exothermic, because the products are slightly lower in energy than the reactants (correct answer)
- Endothermic, because the curve rises before it falls
Explanation: This question tests your ability to interpret reaction energy diagrams that show how energy changes as a reaction proceeds from reactants to products, including identifying activation energy and determining whether the reaction is exothermic or endothermic. A reaction energy diagram plots energy (y-axis) against reaction progress (x-axis, showing the journey from reactants on left to products on right): the curve starts at the reactant energy level, rises to a peak (the transition state—highest energy point as bonds are breaking and forming), then descends to the product energy level. Two key measurements come from this diagram: (1) ACTIVATION ENERGY is the height from the reactant level UP to the peak (the energy barrier that must be overcome to start the reaction—like pushing a boulder uphill), and (2) OVERALL ENERGY CHANGE (ΔH) is the height difference between reactants and products (positive if products higher = endothermic, negative if products lower = exothermic). The activation energy tells you how hard it is to START the reaction, while the overall change tells you whether energy is released or absorbed OVERALL! With reactants at 75 kJ and products at 70 kJ, the products are 5 kJ LOWER than reactants—a small but definite energy release, making this an EXOTHERMIC reaction (the peak at 140 kJ shows a high activation barrier, but doesn't affect the overall energy change). Choice C correctly identifies this as exothermic because the products (70 kJ) are slightly lower in energy than the reactants (75 kJ), indicating a net release of 5 kJ. Choice A incorrectly focuses on the peak being above reactants (which relates to activation energy, not overall change), while B gets the energy comparison backwards. Reading energy diagrams—the three-level method: (1) LOCATE REACTANTS (starting point, left side): note their energy level height. (2) LOCATE PEAK (highest point on curve): note its height. Activation energy = peak height MINUS reactant height (the climb from start to top). (3) LOCATE PRODUCTS (ending point, right side): note their energy level. Overall energy change = product height MINUS reactant height (positive if products higher, negative if products lower). Even a SMALL drop from reactants to products means exothermic!