Argon molar heat capacity

We can see how the values of heat capacities depend on molecular properties by using the relations in Section 6.7. We start with a simple system, a monatomic ideal gas such as argon. We saw in Section 6.7 that the molar internal energy of a monatomic ideal gas at a temperature T is RT and that the change in molar internal energy when the temperature is changed by AT is A(Jm = jRAT. It follows from Eq. 12a that the molar heat capacity at constant volume is... 

For monatomic gases such as helium, argon, or xenon, etc., the molar heat capacity at constant volume is approximately 3(cal)/(g mole)(K). In the same system of units R is about 2 so the heat capacity at constant pressure for an ideal, monatomic gas is around 5(cal)/(g mole)(K). 

Suppose 1.000 mol argon (assumed to be an ideal gas) is confined in a strong, rigid container of volume 22.41 L at 273.15 K. The system is heated until 3.000 kJ (3000 J) of heat has been added. The molar heat capacity of the gas does not change during the heating and equals 12.47 J moU. ... 

For simple solutes such as argon, the partial molar heat capacity of s in an ideal gas phase is due to the translational degrees of freedom only, and this does not change upon transferring of s from an ideal gas into the liquid. Therefore, whatever excess value of either Cp s or ACp is ascribed to the structural changes induced by the addition of the solute to water. 

The partial molar heat capacity of gases is usually larger in water than in other solvents. This quantity is obtained from the second derivative of experimental curves and therefore is generally not very accurate. Nevertheless, the difference between water and other solvents is considered to be quite clear-cut. As an example, the partial molar heat capacity of argon in water at room temperature is about 50 cal/mole deg, whereas in ethanol, methanol, or p-dioxane, it is almost zero. Table 7.2 includes some information on the partial molar heat capacity of methane in water and in nonaqueous solvents. 

As an example, the molar constant-volume heat capacity of argon is 12.8 J-K 1-mol 1, and so the corresponding constant-pressure value is 12.8 + 8.3... 

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