Enthalpy of Formation, Heat Capacity and Entropy of Melamine

Abstract

The thermodynamic properties of melamine were studied by multiple thermochemical methods. A bomb combustion calorimeter was used to determine the combustion heat of melamine at 298.15 K. According to the results, we calculated the standard molar enthalpy of combustion and standard molar enthalpy of formation of melamine: △(subscript c)H(superscript θ subscript m)=(-2455.17±4.65) kJ•mol^(-1); △(subscript f)H(superscript θ subscript m)=(-763.38±5.16) kJ•mol^(-1). The bond enthalpy of C≈N (between single bond and double bond) in melamine was then estimated to be 458.30 kJ•mol^(-1) according to the ralationship between bond enthalpy and combustion enthalpy. This value is larger than that of C-N but smaller than that of C=N. Heat capacity measurements were carried out in a small sample adiabatic calorimeter from 80 to 385 K. We obtained △(subscript f)H(superscript θ subscript m) at different temperatures between 80 and 385 K using the heat capacity data. Through calculation with the values of heat capacity, the relationship between the standard molar enthalpy of formation and temperature is also presented as a functional equation. We also measured the thermol stability of melamine by the thermogravimetry-differential scanning calorimetry (TG-DSC) technique, which showed a thermal decomposition peak at 603.37 K for the DSC curve.