Carbon dioxide molar heat capacities

Table 6.1 Comparison of the mean molar heat capacities for carbon dioxide and hydrogen, and carbon monoxide and water...

Example 8.1 Using the values from Table 8.2, calculate the molar heat capacity of carbon dioxide (CO2) at 600 K (327°C). 

Calculate the molar heat capacity at constant pressure of carbon dioxide at (i) 300 K, (ii) 500 K, (iii) 1000 K, assuming ideal behavior. 

Calculate the molar heat capacity at constant pressure for carbon dioxide at 298.15 K. 

For miscible blend phases, these parameters need to be described as a function of the blend composition. In a first approach to describe the behavior of the present PPE/PS and SAN/PMMA phases, these phases will be regarded as ideal, homogeneously mixed blends. It appears reasonable to assume that the heat capacity, the molar mass of the repeat unit, as well as the weight content of carbon dioxide scale linearly with the weight content of the respective blend phase. Moreover, a constant value of the lattice coordination number for PPE/PS and for SAN/PMMA can be anticipated. Thus, the glass transition temperature of the gas-saturated PPE/SAN/SBM blend can be predicted as a function of the blend composition (Fig. 17). Obviously, both the compatibilization by SBM triblock terpolymers and the plasticizing effect of the absorbed carbon dioxide help to reduce the difference in glass transition temperature between PPE and SAN. 

For the PPE/PS phase, the previously described Chow equation can be combined with the Couchman equation to estimate the Tg as a function of the blend composition. The results are highlighted in Fig. 25. For the prediction, the heat capacity and the molar mass of the repeat unit of the PPE/PS blends is regarded to scale linearly with the mass content of the blend partners, and a constant lattice coordination number of z = 2 is used [75]. While the addition of PS to PPE allows one to reduce continuously the Tg in presence of carbon dioxide, the plasticization effect is less pronounced, mainly driven by the decreasing solubility via addition of PS. 

Heat capacity ratio, k = Cp/c, = 1.04 for gases with molar mass > 100. The value of k increases to 1.67 as the molar mass decreases. For air =1.4 and for such gases as ethylene, carbon dioxide, steam, sulfur dioxide, methane, ammonia = 1.2-1.3. Temperature rise between feed 1 and exit 2 ... 

The tabulated values for the enthalpy, entropy, and heat capacity are on a molar basis. In order to convert them to the specific property (per unit mass), divide by the molar mass of carbon dioxide (44.010 g/mol). 

Another consideration is the structural complexity of the molecules themselves. Carbon dioxide, CO2, and propane, C3Hg, have molar masses of 44 g/mol, yet the specific heat capacity of C3Hg(g) is substantially larger than that of C02(g). The reason is that CgHg molecules are structurally more complex than CO2 molecules, and CgHg molecules have more ways to absorb added energy. 

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