A chemist examines a single-step reaction whose energy diagram is described as follows:
• The vertical axis is labeled in kilojoules of potential energy (kJ).
• The horizontal axis is an arbitrary reaction coordinate that begins at 0 and ends at 100 (dimensionless units).
• The curve starts at 40 kJ, rises to a maximum of 75 kJ at a reaction coordinate of 40, and finishes at 65 kJ at a reaction coordinate of 100.
Use this verbal description to analyze the reaction thermochemistry.
(a) State what physical quantities are represented on the vertical and horizontal axes of an energy diagram for a chemical reaction. Explain why these particular axes are chosen.
(b) Using the numerical values provided, calculate ΔH for the forward reaction. Include the appropriate sign and units.
(c) Classify the forward reaction as endothermic or exothermic. Justify your answer with reference to the energy changes you calculated and the relative positions of reactants and products on the diagram.
(d) For the reverse reaction, determine the sign of ΔH and classify it as endothermic or exothermic. Describe how the appearance of the energy diagram would change if the reverse reaction were drawn left-to-right instead of right-to-left.