How can you tell from an energy diagram whether a reaction is exothermic or endothermic?

• A. If the energy of products is lower than reactants, exothermic; if higher, endothermic.
• B. If the activation energy is zero, endothermic.
• C. The color change determines the type of reaction.
• D. Temperature changes always indicate endothermic reactions.

Answer: (A)

The key idea is to compare the energy levels of the reactants and the products on the diagram. If the products end up at a lower energy than the reactants, the system releases energy to the surroundings, so the reaction is exothermic (enthalpy change negative). If the products are at higher energy than the reactants, energy is absorbed from the surroundings, making the reaction endothermic (enthalpy change positive). The height of the barrier you see (activation energy) relates to how fast the reaction occurs, not whether it is exothermic or endothermic. Observing color changes or temperature shifts can be misleading, since they depend on factors beyond the thermodynamic difference shown in the diagram.