Final answer:
The heat value (qrxn) corresponds to the heat released or absorbed per amount of reactants, while the enthalpy change of the reaction (ΔHrxn) depends on the energies of bond breaking and formation and the state of reactants and products.
Step-by-step explanation:
The heat value (qrxn) is based on the amount of heat released or absorbed during a chemical reaction for a specific quantity of reactants. The enthalpy change of the reaction (ΔHrxn) is based on the energies involved in breaking and forming chemical bonds, factoring in the amount of materials and the physical states of the reactants and products. For example, when 1 mole of hydrogen gas and 0.5 mole of oxygen gas react to form liquid water (ℓ2O), the reaction releases 286 kJ of heat, but if gaseous water forms, only 242 kJ of heat are released, demonstrating the dependency on the physical state of the water formed.
Enthalpy, which is the heat content of a system at constant pressure, can be measured and represented through thermochemical equations, which clearly state the enthalpy changes along with the physical states of reactants and products. Also, Hess's law allows calculation of enthalpy change for reactions by summing the enthalpy changes of individual reactions that constitute the overall process. The standard enthalpy of formation, ΔHf, involves the formation of 1 mole of a substance from its elemental form at standard conditions, and it can be used to calculate the standard enthalpy of reaction, ΔHrxn.