What is heat of reaction and how is it calculated?

Heat of reaction (delta_H_rxn) is the enthalpy change when reactants convert to products at specified conditions. Calculated from: delta_H_rxn = sum(n*H_products) - sum(n*H_reactants), using heats of formation or combustion. Standard conditions: 25C, 1 atm, species in standard states. Negative delta_H indicates exothermic (releases heat); positive indicates endothermic (absorbs heat). Temperature adjustment uses heat capacities: delta_H(T) = delta_H_298 + integral(delta_Cp*dT). Essential for reactor energy balances and safety analysis.