三聚氰胺的生成焓、热容和熵

通过多种热化学方法研究了三聚氰胺的热力学性质. 首先用氧弹式燃烧热量计测定了三聚氰胺在298.15 K 时的燃烧热, 根据燃烧热结果, 计算出三聚氰胺的标准摩尔燃烧焓和标准摩尔生成焓, 分别为: △cHΘm=(-2455.17±4.65) kJ·mol-1; △fHΘm =(-763.38±5.16) kJ·mol-1. 然后根据键焓与燃烧焓之间的关系, 估算出三聚氰胺中的C≈N(此键介于单键与双键之间)键能为458.30 kJ·mol-1, 此值介于碳氮单键键能和双键键能之间. 通过绝热热量计测定了三聚氰胺从80到385 K的低温热容. 根据热容值, 计算了此温度区间的标准摩尔生成焓, 其与温度呈线性关系. 另外, 三聚氰胺的热稳定性也用热重-差示扫描量热(TG-DSC)法进行了分析, 确定其分解的DSC曲线的峰顶温度为603.37 K.

Enthalpy of Formation, Heat Capacity and Entropy of Melamine

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:△_cH~(Θ)_m=(-2455.17±4.65)kJ·mol~(-1),;△_fH~(Θ)_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 △_fH~(Θ)_m at different temperatures between 80 and 385K 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.