Is an exothermic reaction characterized as having positive or negative delta H?

In the context of thermodynamics, an exothermic reaction is characterized by the release of energy, particularly in the form of heat, to its surroundings. This release of energy leads to a decrease in the internal energy of the system undergoing the reaction.

The change in enthalpy (delta H) for a process measures the heat content exchanged at constant pressure. For an exothermic reaction, as heat is released, the enthalpy of the products is lower than that of the reactants, resulting in a negative change in enthalpy. Therefore, in an exothermic reaction, delta H is negative, confirming that energy is being lost to the surroundings.

In contrast, endothermic reactions, which absorb heat, have a positive delta H, as the system gains energy from its surroundings. Thus, the correct characterization of an exothermic reaction is one with a negative delta H.