Exothermic reactions have negative enthalpy change

Steps:

• Define exothermic reaction: releases heat, so products have lower energy than reactants.
• Sketch energy profile: horizontal axis for reaction progress, vertical for energy; reactants start high, products end low.
• Calculate ΔH: subtract products' energy from reactants'; negative value confirms exothermic.
• Match to options: choose diagram with downward energy trend and labeled negative ΔH.

Why D is correct:

• Diagram D shows products below reactants level, matching ΔH = H_products - H_reactants < 0 for exothermic reactions.

Why the others are wrong:

• A: Energy rises from reactants to products, indicating endothermic (positive ΔH).
• B: Flat or no clear decrease, failing to show energy release.
• C: Incorrect ΔH sign (positive) despite possible profile shape.

Final answer: D