What does a negative heat of reaction indicate about a reaction?

A negative heat of reaction indicates that energy is released during the reaction process, which characterizes an exothermic reaction. In an exothermic reaction, the system loses heat to the surroundings, resulting in an overall decrease in internal energy. This typically means that the products have lower energy than the reactants, reinforcing the concept that energy is being released.

For instance, many combustion reactions exhibit this characteristic: when a fuel burns, it releases heat and light, indicating that the reaction is exothermic and has a negative heat of reaction.

In contrast, an endothermic reaction would have a positive heat of reaction, signifying that energy is absorbed from the surroundings. A reaction at equilibrium does not favor either the forward or reverse direction with respect to the energy changes occurring, while an incomplete reaction suggests that reactants are still present without fully converting to products. Thus, these scenarios can be distinguished from an exothermic reaction indicated by a negative heat of reaction.