Enthalpy change of reaction is the energy transferred in a chemical reaction.

• A. The energy to break all bonds in the reactants
• B. The energy transferred in a chemical reaction
• C. The energy gained by the products
• D. The energy needed to catalyze a reaction

Answer: (B)

Enthalpy change of a reaction is about heat flowing between the reacting system and its surroundings when the reaction occurs at constant pressure. It tells you how much energy is transferred as heat: negative if heat is released (exothermic) and positive if heat is absorbed (endothermic). This is why the description “the energy transferred in a chemical reaction” is the best match. The energy to break bonds in the reactants is only part of the story and doesn’t by itself equal the overall heat exchanged, since energy is also released when new bonds form. The energy gained by the products isn’t a measure of heat transfer with the surroundings, and the energy needed to catalyze a reaction refers to activation energy, not the overall enthalpy change.