Enthalpies of Reaction:

If the chemical composition of the FRM is known, the potential exists to calculate the enthalpies of reaction from heats of formation and heat capacity data.^{3, 4} The standard procedure is to "cool" the reactants down from the reaction temperature to a reference state (temperature) of 25 ºC, compute the heat of reaction at 25 ºC, and then heat the products back up to the reaction temperature.⁴ Here, we will illustrate this simple procedure for a gypsum-based FRM. Gypsum, which contains two molecules of water for each molecule of calcium sulfate, undergoes two dehydration reactions when exposed to elevated temperatures, first converting to calcium sulfate hemihydrate and then to the anhydrite form of calcium sulfate. The heat capacities and heats of formation (Hf) of the relevant compounds are provided in Table 1. ^{3, 4} Care must be taken to consider water in its gas phase form as the reaction temperatures being considered are always above 100 ºC. Using these properties and the known dehydration reaction stoichiometries (e.g., CaSO₄−2H₂O → CaSO₄−0.5H₂O + 1.5 H₂O and CaSO₄−0.5H₂O → CaSO₄ + 0.5 H₂O), heats of reaction of 3.01 kJ/g water lost at 150 ºC and 2.35 kJ/g water lost at 250 ºC are calculated for the dehydrations to hemihydrate and anhydrite, respectively. These values are in reasonable agreement with those recently summarized for gypsum plasterboard by Thomas.⁵ These values could then be multiplied by the corresponding measured mass loss in these temperature ranges (from Figure 1 for example) to obtain the enthalpy changes due to reactions for a particular FRM during fire exposure. Similar calculations can be employed for portland-cement based and intumescent FRMs, as long as their specific decomposition reactions and corresponding thermophysical properties are known. ^{6, 7} It is worth noting that not all reactions in commercially available FRMs are endothermic in nature, as organic components may provide significant exotherms, further supplementing the energy being provided by a fire.