烟酸的低温热容和标准摩尔生成焓

Low-Temperature Heat Capacities and Standard Molar Enthalpy of Formation of Nicotinic Acid

利用精密自动绝热热量计测量了分析纯烟酸在78～400K温区的低温热容.用最小二乘法将实验摩尔热容对温度进行拟合,得到了热容随温度变化的多项式方程.用此方程进行数值积分,得到在此温区每隔5K的舒平热容值和相对于298.15K时的热力学函数值.利用精密静止氧弹燃烧热量计测定了烟酸在298.15K时的恒体积燃烧能为·cU=-(24528.3±16.1)J·g-1.依据物质燃烧焓定义计算出烟酸的标准摩尔燃烧焓为:ΔcHm=-(3019.05±1.98)kJ·mol-1.最后,依据Hess定律计算出烟酸的标准摩尔生成焓为:ΔfHm=-(56.76±2.13)kJ·mol-1.

Low-temperature heat capacities of nicotinic acid （C6H5NO2） were measured by a precision automated adiabatic calorimeter over the temperature range from 78 to 400 K. A polynomial equation of the heat capacities as a function of the temperature was fitted by the least square method. Based on the fitted polynomial, the smoothed heat capacities and thermodynamic functions of the compound relative to the standard reference temperature 298.15 K were calculated at the interval of 5 K. The constant-volume energy of combustion of nicotinic acid at T=298.15 K was measured by a precision oxygen-bomb combustion calorimeter to be △cU=-（24528.3±16.1）J·g^-1. The standard molar enthalpy of combustion for the compound was determined to be ΔcHm^⊙=-（3019.05±1.98）kJ·mol^-1, according to the definition of the combustion heat. Finally, the standard molar enthalpy of formation for the compound was determined to be ΔfHm^⊙=-（56.76±2.13）kJ·mo^l-1 , in accordance with Hess law.