Energy changes – identifying exothermic reactions In an exothermic chemical reaction, how do the energies of reactants and products compare?

Introduction / Context:
Classifying reactions as exothermic or endothermic is fundamental to reactor design and safety. Energy balance, heat removal, and temperature control hinge on knowing the enthalpy change and relative energy levels of reactants and products.

Given Data / Assumptions:

• Exothermic reaction: releases heat to surroundings.
• Comparison at the same reference temperature and pressure.

Concept / Approach:
For an exothermic reaction, the standard enthalpy change ΔH is negative. This means products lie at a lower enthalpy (energy) level than reactants. The energy difference is liberated as heat. Temperature of the mixture may rise if heat is not removed, but “higher temperature” is an outcome, not a definition of energy level comparison.

Step-by-Step Solution:

Verification / Alternative check:
Potential energy diagrams show reactants at a higher enthalpy plateau transitioning to products at a lower plateau, with heat released to maintain energy conservation.

Why Other Options Are Wrong:

• Higher temperature (option a): Not a reliable comparison; temperature depends on heat removal and heat capacity.
• Less or same energy (options c, d): Contradicts ΔH < 0 definition.

Common Pitfalls:
Equating temperature directly with energy content; enthalpy depends on both temperature and composition. Always compare enthalpies at a common reference to classify exo/endothermicity.

Final Answer:
Reactants possess more energy than products