Heat of vaporization determination

Under certain conditions, it can be demonstrated that the vapor pressure of a substance, Pvap is related to the absolute temperatiu-e at which it is maintained, T, by the Clausius-Clapeyron equation. 

In this equation, R is the ideal gas constant, 6o is a constant that varies from one substance to another, and Ai/vap is the heat of vaporization of the substance of interest, assumed to be constant over the temperature range studied. In an experiment to determine the heat of vaporization of carbon tetrachloride, nine runs were performed. Their results are reproduced in Table 5A.5 (Simoni, 1998). If the Clausius-Clapeyron equation holds under these conditions. In p p can be described by a linear function of 1/T, from whose slope we can determine the heat of vaporization of carbon tetrachloride. 

This corresponds to a heat of vaporization of 32.44 0.22 kJ mol . In spite of these good ANOVA values, however, the residual plot (Fig. 5A.7) 

Variation of the vapor pressure of CCI4 with temperature 

In physical terms, the superiority of the quadratic model indicates that the heat of vaporization cannot be considered constant in this example. Using therefore the quadratic model and assuming that the derivative 

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The Clausius-Clapeyron equation implies that if we plot the natural log of the pressure of the gas phase versus inverse temperature, the slope of the resulting line is the heat of vaporization divided by the gas constant (R). A plot of In P (vapor pressure of water) versus inverse temperature is given in Figure 3. The calculated heat of vaporization (determined by multiplying the slope by R) is 10,400 cal/mol. The important aspect of Eq. (10) with regard to moisture sorption is the fact that increasing the temperature also increases the vapor pressure. 

The enthalpy of formation is that adopted by Calder and Giauque (i) from an analysis of the equilibrium data of McMorris and Yost (2) and Beeson and Yost (3). This yielded a enthalpy of formation of ICl(g), from which the heat of formation of ICl(cr) was calculated by using the heat of vaporization determined by Calder and Giauque (1.). 

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