The heat of reaction refers to the amount of heat absorbed or released during a chemical reaction. It is the difference in enthalpy between the products and reactants of a reaction. A positive value for $\Delta H$ indicates an endothermic reaction, meaning that heat is absorbed from the surroundings to break bonds in the reactants and form new bonds in the products. Conversely, a negative value for $\Delta H$ indicates an exothermic reaction, meaning that heat is released to the surroundings as bonds are formed and broken during the reaction.

Enthalpy reaction $(\Delta H_{rxn})$:

Enthalpy reaction is a specific type of heat of reaction that refers to the change in enthalpy that occurs during a chemical reaction at constant pressure. It is defined as the difference between the total enthalpy of the products and the total enthalpy of the reactants:

$$\Delta H_{rxn} = \Sigma H_{products} - \Sigma H_{reactants}$$

The enthalpy of a substance refers to its total energy content, including internal energy and the energy associated with its position and motion. In the context of chemical reactions, the change in enthalpy represents the net energy absorbed or released during the conversion of reactants to products.

In summary, the heat of reaction is a general term that refers to the amount of heat involved in a chemical reaction, while enthalpy reaction specifically refers to the change in enthalpy that occurs at constant pressure during a chemical reaction.