In an endothermic reaction, the standard enthalpy change ΔH° is positive.

In an endothermic reaction, the standard enthalpy change ΔH° is positive.

• A. The standard enthalpy change is negative.
• B. The standard enthalpy change is zero.
• C. The standard enthalpy change ΔH° is positive.
• D. The enthalpy change is undefined.

Answer: (C)

Enthalpy change reflects heat flow into or out of a system at constant pressure. In an endothermic reaction, heat is absorbed from the surroundings, so the system’s enthalpy increases. Under standard conditions, this means the standard enthalpy change ΔH° is positive. If the reaction were exothermic, heat would be released and ΔH° would be negative; zero would imply no net heat transfer, which isn’t the case for a reaction with a defined enthalpy change. Therefore, a positive ΔH° correctly describes an endothermic process.

Enthalpy change reflects heat flow into or out of a system at constant pressure. In an endothermic reaction, heat is absorbed from the surroundings, so the system’s enthalpy increases. Under standard conditions, this means the standard enthalpy change ΔH° is positive. If the reaction were exothermic, heat would be released and ΔH° would be negative; zero would imply no net heat transfer, which isn’t the case for a reaction with a defined enthalpy change. Therefore, a positive ΔH° correctly describes an endothermic process.