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Chapter 23 Calculation of Enthalpy Changes

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Chapter 23 Calculation of Enthalpy Changes Basic principle II Second class Dr. Arkan Jasim Hadi CHAPTER 23 CALCULATION OF ENTHALPY CHANGES 1.Equations to estimate the enthalpy of a phase transition 2.Heat capacity and other equations 3.Tables 4.Enthalpy charts 5.Computer databases 23.1 Phase Transitions Phase transitions occur from the solid to the liquid phase and from the liquid to the gas phase, and the reverse? During these transitions very large changes in the value of the enthalpy (and internal energy) for a substance occur, changes called latent heat changes, because they occur without any noticeable change in temperature. Because of the relatively large enthalpy change associated with phase transitions, it is important to accurately ascertain latent heats when applying energy balances that involve them. For a single phase, the enthalpy varies as a function of the temperature, as illustrated in Figure 23.1. The enthalpy changes that take place within a single phase are often called "sensible heat" changes. Basic principle II Second class Dr. Arkan Jasim Hadi The enthalpy changes for the phase transitions are termed Heat of fusion (ΔH fusion): The enthalpy change for the phase transition of melting. Heat of solidification: The negative of the heat of fusion. Heat of sublimation: The enthalpy change of a solid directly to vapor. Heat of vaporization (ΔHv): The enthalpy changes for the phase transition of a liquid to a vapor. Heat of condensation: The negative of the heat of vaporization. Latent heat: An enthalpy change that involves a phase transition. Phase transitions: A change from the solid to the liquid phase, from the liquid to the gas phase, from the solid to the gas phase, or the respective reverse changes. Sensible heat: An enthalpy change that does not involve a phase transition. Changes in ΔU can be calculated by an equation analogous to Equation (23.1) with Cv substituted for Cp and values of the internal energy phases transitions substituted for the values of the enthalpies of the phase transitions. Equations to Estimate Heats of Vaporization 1. The Clapeyron Equation The Clapeyron equation is an exact thermodynamic relationship between the slope of the vapor-pressure curve and the molar heat of vaporization and the other variables listed below: Basic principle II Second class Dr. Arkan Jasim Hadi where p* = vapor pressure T = absolute temperature Δ ̂ = molar heat of vaporization at T = molar volume of gas or liquid as indicated by the subscript g or l Figure 23.2 shows how the value of the slope dp / d(ln T) can be used to estimate the specific heat of vaporization. Any consistent set of units may be used. vapor pressure: The pressure exerted by a vapor in equilibrium with the solid or liquid phase of the same substance. Also, the partial pressure of the substance in the atmosphere above the solid or the liquid. If the vapor and liquid of a pure component are in equilibrium, then the equilibrium pressure is called the vapor pressure, which we-will denote by p*. At a given temperature there is only one pressure at which the liquid and vapor phases of a pure substance may exist in equilibrium. Either phase alone may exist, of course, over a wide range of conditions. Equation (23.2) can be used to calculate Δ ̂ , and to check the internal consistency of data if a function for the vapor pressure of a substance is known so that you can evaluate dp* /dT. If you assume that (a) is negligible in comparison with . (b) The ideal gas law is applicable for the vapor: Then Basic principle II Second class Dr. Arkan Jasim Hadi If you further assume that ̂ is constant over the temperature range of interest, integration of Equation (23.3) yields the Clausius-Clapeyron equation 2. Chen's Equation: An equation that yields values of ΔHv (in kJ/gmol) to within 2% is Chen's equation*: where Tb is the normal boiling point of the liquid in K, Tc, is the critical temperature in K, and pc is the critical pressure in atmospheres. 4. Watson's Equation: Watsont found empirically that below the critical temperature the ratio of two heats of vaporization could be related by where ΔHv1 = heat of vaporization of a pure liquid at T1 ΔHv2= heat of vaporization of the same liquid at T2 EXAMPLE 23.2 Comparisons of Various Sources to Estimate the Heat of Vaporization Use (a) the Clausius-Clapeyron equation, (b) Chen's equation, and (c) Riedel's equation to estimate the heat of vaporization of acetone at its normal boiling point, and compare with the experimental value of 30.2 kJ/gmol listed in Appendix C. Basic principle II Second class Dr. Arkan Jasim Hadi .
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