Ideal enthalpy of vaporization

For a gas, — Hm is the change in enthalpy as the gas at pressure, p, is expanded into a vacuum, For a liquid (or solid), - Hm is the enthalpy change as the liquid (or solid) is vaporized (or sublimed) into a vacuum. It has been called the ideal enthalpy of vaporization (or sublimation) since it represents the enthalpy change as the liquid (or solid) becomes an ideal gas. 

E6.2 The fugacity of liquid water at 298.15 K is approximately 3,17 kPa. Take the ideal enthalpy of vaporization of water as 43.720 TmoD1, and calculate the fugacity of liquid water at 300 K. 

Conversion to the gross enthalpy of combustion requires subtraction of the ideal enthalpy of vaporization for water, (J/2) tAHv(H20,T)). This value is slightly larger in magnitude than the enthalpy of vaporization ... 

This chapter presents quantitative methods for calculation of enthalpies of vapor-phase and liquid-phase mixtures. These methods rely primarily on pure-component data, in particular ideal-vapor heat capacities and vapor-pressure data, both as functions of temperature. Vapor-phase corrections for nonideality are usually relatively small. Liquid-phase excess enthalpies are also usually not important. As indicated in Chapter 4, for mixtures containing noncondensable components, we restrict attention to liquid solutions which are dilute with respect to all noncondensable components. 

The calculated boiling point corresponds to 77°C the experimental value is 78°C. The small error probably comes from assuming that the enthalpy of vaporization is a constant over the temperature range of the question and assuming that the vapor behaves like an ideal gas. 

In this chapter, intermolecular forces are viewed as complications and nuisances it is the molecule per se that is of interest. Therefore, unless explicitly noted to the contrary, any species of interest in this chapter is to be assumed in the (ideal) gas phase. Most organic compounds are naturally liquids or solids under the thermochemically desired conditions, much less as found by the synthetically or mechanistically inclined chemist. Corrections are naturally made by using enthalpies of vaporization (v) and of sublimation ), defined by equations la and lb ... 

In making the derivation, you will need to assume that (i) the molar volume of the liquid is negligible when compared to that of the gas (ii) the gas behaves ideally and (iii) the molar enthalpy of vaporization is constant with temperature. 

Again following our earlier chapters as precedent, we will continue to view intermolec-ular forces as complications and nuisances to be avoided whenever possible. As such, unless explicitly noted to the contrary, any species to be discussed in this chapter will be assumed in the (ideal) gas phase. To interrelate these data with those for the liquid or solid state that characterizes most organic compounds as synthesized, reacted, purified and thermochemically investigated, it will be necessary to make corrections to the gaseous state by using enthalpies of vaporization and of sublimation. These are defined by equations 1 and 2... 

Another useful equation is the Clausius-Clapeyron equation. It states that, provided the ideal gas law holds and the enthalpy of vaporization, Aft, is independent of T (which is a reasonable assumption for a small temperature range), the slope of the vapor pressure curve is given by... 

H°j Molar ideal gas enthalpy of vapor phase, stage j. 

The workimg medium in refrigeration systems is called the refrigerant. Fluids used for this purpose should ideally be nontoxic and chemically inert, with low boiling points93,94. They should have high enthalpies of vaporization per unit mass, high vapor densities and low miscibilities with water, since the latter can lead to freezeup in the expansion devices or corrosion. 

Before solving the equations, we need system property data, which, in this case, are thermodynamic properties. Equations 3.2.9 and 3.2.11 states that we may obtain vapor pressures for water from steam tables, such as those compiled by Chaar et al. [13]. Equation 3.2.10 also states that we can find the enthalpy of vaporization in the steam tables. We assume that the air-water mixture is ideal to calculate the enthalpy of air, so we can use the mole-fraction average of the pure-component enthalpies. Equations 3.2.12 and 3.2.13 in Table 3.2.1 give the mole fraction average of the inlet and outlet enthalpy. Table 3.2.1 also lists pure component enthalpies for water vapor (Equations 3.2.14 and 3.2.16) and for air (Equa-... 

The molar enthalpy of vaporization of water at 373 K is 41.16 kj/mol. What fraction of this energy is used to change the internal energy of the water, and what fraction is used to do work against the atmosphere (Assume that water vapor is an ideal gas.)... 

The normal boiling point and enthalpy of vaporization are those calculated for the ideal gas from these tables. The boiling point found for the real gas (387.3 K) by Hieber and Woerner (5) is in good agreement. The selected enthalpy of vaporization, aH (298.15 K) = 10.69 kcal mol, Is based on the vapor pressure data of Scott et al., ( ), series III. Vapor pressure measurements of Hieber and Woerner (5), are in satisfactory agreement as shown below. 

The enthalpy of vaporization is calculated from the enthalpy of formation of the Vaporization studies are discussed in the ideal gas table (12). 

Ethanol s enthalpy of vaporization is 38.7 kJ mol at its normal boiling point, 78°C. Calculate q, w, AU, AS js, and AG when 1.00 mol ethanol is vaporized reversibly at 78°C and 1 atm. Assume that the vapor is an ideal gas and neglect the volume of liquid ethanol relative to that of its vapor. 

The solubilities of nonpolymeric solutes in supercritical fluids such as CO2 can be predicted if the properties that describe the solute-solute interactions are chosen properly (35). For polyaromatic hydrocarbons, examples of appropriate solute properties include the enthalpy of vaporization and the solute molecular volume. To further understand the role of solute-solvent interactions on solubilities and selectivities, it is instructive to define an enhancement factor E as the actual solubility, y>2, divided by the solubility in an ideal gas, with the result E = y2 This factor is a normalized solubility... 

Clausius-Clapeyron equation - An approximation to the Clapeyron equation applicable to liquid-gas and solid-gas equilibrium, in which one assumes an ideal gas with volume much greater than the condensed phase volume. For the liquid-gas case, it takes the form d(lnp)/dT = A HIRV- where R is the molar gas constant and A H is the molar enthalpy of vaporization. For the solid-gas case, A H is replaced by the molar enthalpy of sublimation, A H. 

Hexanes. There are five hexanes n-hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane and 2,3-dimethylbutane. For n-hexane there are three combustion measurements, all on the liquid, Jessup [18], Prosen and Rossini [27] and Good and Smith, [15]. These are in reasonable agreement. The results of Good and Smith [15] are selected, on the basis of improved technique and sample purity. The liquid phase value is corrected to the gas phase using enthalpy of vaporization data [Majer, 20] and an adjustment to the ideal gas state. 

---