What does a negative change in enthalpy (−∆H) indicate about a reaction?

A negative change in enthalpy (−∆H) indicates that the reaction is exothermic. In an exothermic reaction, heat is released from the system to the surroundings, meaning the system loses heat while the surroundings gain heat. This is consistent with the concept of enthalpy, which is a measure of the total heat content of a system.

In this context, when you see −∆H, you can infer that energy is exiting the system, contributing to a temperature increase in the surroundings. It's important to understand that while a negative enthalpy change does indicate that heat is being transferred during the reaction, it does not necessarily imply that the reaction is spontaneous, increases in internal energy, or classifies the reaction as endothermic. In fact, endothermic reactions are characterized by a positive change in enthalpy (∆H), where they absorb heat rather than release it.