Arrow in Energy Diagrams Shows Enthalpy Change

Steps:

• Recognize the diagram as an energy profile for a chemical reaction, plotting energy vs. reaction progress.
• Identify the arrow as the vertical difference between reactant and product energy levels.
• Recall that this difference quantifies the net energy absorbed or released during the reaction.
• Match the arrow to the definition of enthalpy change (ΔH), the heat transferred at constant pressure.

Why D is correct:

• Enthalpy change (ΔH) is defined as the difference in enthalpy between products and reactants (ΔH = H_products - H_reactants), directly represented by the arrow's length and direction in energy diagrams.

Why the others are wrong:

• A: Endothermic change indicates ΔH > 0 (upward arrow), but the arrow itself represents the magnitude and sign of ΔH, not just endothermic processes.
• B: Activation energy is the hump from reactants to the transition state, not the overall arrow between levels.
• C: Reactant energy is the starting horizontal line, not the arrow showing change.

Final answer: D