Thermodynamics of dibenzo-p-dioxin and 1,2,3,4-tetrachlorodibenzo-p-dioxin fromT = 5 K toT = 490 K

Abstract In adiabatic low-pressure and dynamic calorimeters the temperature dependence of the standard molar heat capacity C p , m o of dibenzo- p -dioxin and 1,2,3,4-tetrachlorodibenzo- p -dioxin have been determined at temperatures in the range T  =  5 K to T  =  490 K: from T  =  5 K to T  =  340 K with an accuracy of about 0.2 per cent and with an accuracy of 0.5 per cent to 1.5 per cent between T  =  340 K and T  =  490 K. The temperatures, enthalpies, and entropies of melting of the above compounds have been determined. The experimental data were used to calculate the thermodynamic functions C p , m o  /  R , Δ 0 T H m o  / ( R · K ), Δ 0 T S m o  /  R , and Φ m o  =  Δ 0 T S m o  −  Δ 0 T H m o  /  T (where R is the universal gas constant) in the range T  →  0 to T  =  490 K. The isochoric heat capacity C V , m of both dioxins has been estimated over the range T  →  0 to T fus . The effect of substitution of four hydrogen atoms by chlorine atoms on the lattice and atomic components of the isochoric heat capacity was considered.