Statistical Thermodynamics of Iodine Sublimation

Gas cylinders must be chained securely to the wall or laboratory bench (see pp. 644-646 and Appendix C). The handling of liquid nitrogen must be carried out properly. Do not allow any low-temperature liquid (A, or N2) to heat up in a confined ehamber, sinee large vapor pressures can develop, leading to an explosive rapture of the system (see Appendix C). 

This experiment is in some respects similar to two other experiments eoneeming enthalpy changes attending phase transformations, namely, Exps. 13 and 47. However, it differs from them in that the experimental data, whieh are vapor pressures of solid iodine at several 

This section will not be concerned with the Clatrsius-Clapeyron eqrration, which is discussed adequately in Exps. 13 and 47. The discussion here will focus on the application of statistical mechartics to the phase eqttilibrium 

It is required for eqtrilibritrm that the chemical potential of I2 be the same in the two 

The basic question is How can these chemical potentials be determined  

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Other experimental and theoretical methods have been developed for the determination of the heat of sublimation of solid iodine these too are suitable for undergraduate laboratory experiments or variations on this experiment. Henderson and Robarts have employed a photometer incorporating a He-Ne gas laser, the beam from which (attenuated by a CUSO4 solution) has a wavelength of 632.8 nm, in a hot band near the long-wavelength toe of the absorption band shown in Fig. 3. Stafford has proposed a thermodynamic treatment in which a free-energy function ifef), related to entropy, is used in calculations based on the third law of thermodynamics. In this method either heat capacity data or spectroscopic data are used, and as in the present statistical mechanical treatment, the heat of sublimation can be obtained from a measurement of the vapor pressure at only one temperature. 

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