Heat capacity, entropy, and free energy of rubber hydrocarbon

Measurements of heat capacity were made on rubber hydrocarbon in its different forms from 14 to 320 K with an adiabatic vacuum-type calorimeter. At 14 K the heat capacity was found to be 0.064 j/g/"C for both the metastable amorphous and the crystalline forms. With increase in temperature, the heat capacity increases gradually up to a transition at about 199 K, the amorphous form having a little the greater value. At 199 K both forms undergo a transition of the second order, the heat capacity rising sharply. For the amorphous form above this transition the heat capacity rises gradually without discontinuity to the highest temperature of the measurements. The crystalline form undergoes fusion (a transition of the first order) at 284 K, the heat of fu sion being 16.7 jig. At 298.1 K the heat capacity of the rubber is 1.880 0.002 j/g/oC. Utilization of the data according to the third law of thermodynamics yields 1.881 0.010 j/g/oC for the entropy of rubber at 298.1 K. Combination of these with appropriate other data on entropies and heats of reaction yields 1.35 0.10 kj/g for the standard free energy of formation at 298.1 K of rubber from carbon (graphite) and gaseous hydrogen.