In heat of reaction

The heat of reaction, also known as enthalpy of reaction ($\Delta H_{rxn}$), is the change in enthalpy that occurs during a chemical reaction. It represents the amount of heat absorbed or released when reactants are converted into products under constant pressure. A negative $\Delta H_{rxn}$ indicates an exothermic reaction (heat released), while a positive $\Delta H_{rxn}$ indicates an endothermic reaction (heat absorbed).