In a reaction energy diagram, which label represents the overall enthalpy change (ΔH), and how can you determine if the reaction is exothermic or endothermic?

• A. ΔH is the difference between the energy of products and reactants; if products are lower in energy, the reaction is exothermic.
• B. ΔH is the energy at the peak of the diagram; if the peak is higher than reactants, the reaction is exothermic.
• C. ΔH is the difference between the energy of reactants and the transition state; if reactants are higher, the reaction is endothermic.
• D. ΔH is the difference between the activation energy and the energy of reactants; if activation energy is higher, the reaction is endothermic.

Answer: (A)

Verified step by step guidance

1. Understand that the overall enthalpy change (\$\Delta H\$) in a reaction energy diagram is defined as the difference in energy between the products and the reactants.
2. Identify the energy level of the reactants and the products on the diagram. \$\Delta H = E_{products} - E_{reactants}\$.
3. Determine the sign of \$\Delta H\$: if the products have lower energy than the reactants (\$E_{products} < E_{reactants}\$), then \$\Delta H\$ is negative, indicating an exothermic reaction where energy is released.
4. If the products have higher energy than the reactants (\$E_{products} > E_{reactants}\$), then \$\Delta H\$ is positive, indicating an endothermic reaction where energy is absorbed.
5. Note that the peak of the diagram represents the transition state and relates to activation energy, not the overall enthalpy change \$\Delta H\$.