The heat of reaction, or reaction enthalpy, is an essential parameter to safely and successfully scale-up chemical processes. The heat of reaction is the energy that is released or absorbed when chemicals are transformed in a chemical reaction. It describes the change of the energy content when reactants are converted into products. While a reaction can be exothermic (heat releasing) or endothermic (heat absorbing), most of the reactions performed in the chemical and pharmaceutical industries are exothermic. Among others, the heat of reaction is one of the thermodynamic properties used in chemical development, scale-up, and safety to scale processes from the lab scale to manufacturing. The heat of reaction or reaction enthalpy is typically expressed as molar enthalpy in kJ/mol or as specific enthalpy in kJ/kg or kJ/L.