Phase change vaporization

A V = volume change accompanying the phase change / = vapor pressure... 

Enthalpy Defining Enthalpy AH of Phase Changes Vaporization and Electricity... 

The establishment of an equilibrium vapor pressure, as described in Section 12.2, involved two of the.se phase changes vaporization and condensation. Figure 12.31 summarizes the various types of phase changes. 

The reverse sequence of phase changes—vapor liquid solid—are all exothermic. In each case, heat must be given off. Gases must give offbeat in order to condense into the liquid phase, and liquids must give offbeat in order to solidify or crystallize. This brings up an important point to remember The reverse of any endothermic process is an exothermic process, and vice versa. 

If, in going from 0 K to T, a substance undergoes phase changes (fusion, vaporization, etc) at and Tg with molar enthalpies of transition AHy, one can write... 

The vaporization process describes the phase change liquid (1) gas (g), so from the definition of AH we can write... 

In most utibty boilers, steam pressure regulation is achieved by the throttling of turbine control values where steam generated by the boiler is admitted into the steam turbine. Some modem steam generators have been designed to operate at pressures above the critical point where the phase change between Hquid and vapor does not occur. 

Molecular Nature of Steam. The molecular stmcture of steam is not as weU known as that of ice or water. During the water—steam phase change, rotation of molecules and vibration of atoms within the water molecules do not change considerably, but translation movement increases, accounting for the volume increase when water is evaporated at subcritical pressures. There are indications that even in the steam phase some H2O molecules are associated in small clusters of two or more molecules (4). Values for the dimerization enthalpy and entropy of water have been deterrnined from measurements of the pressure dependence of the thermal conductivity of water vapor at 358—386 K (85—112°C) and 13.3—133.3 kPa (100—1000 torr). These measurements yield the estimated upper limits of equiUbrium constants, for cluster formation in steam, where n is the number of molecules in a cluster. 

If the fluid is at its boiling point, then volume is generated through the phase change that occurs upon vaporization ... 

Notice from the figure that the effect of temperature on entropy is due almost entirely to phase changes. The slope of the curve is small in regions where only one phase is present. In contrast, there is a large jump in entropy when the solid melts and an even larger one when the liquid vaporizes. This behavior is typical of all substances melting and vaporization are accompanied by relatively large increases in entropy. 

The heat accompanying the phase change (2) is 1.44 kcal/mole. This is much less than the molar heat of vaporization of water, 10 kcal/mole. Table 5-II contrasts the melting points and the heats of melting per mole (the molar heat of melting, or the molar heat of fusion) of the same pure substances listed in Table 5-1. 

Let us compare the behavior of these two systems during a phase change. Consider, first, how water acts when it is frozen or vaporized. Pure water freezes at a fixed temperature, 0°C. If we freeze half of a water sample to ice, remove the ice, melt it in another container, and compare the separate samples, we find that the two fractions of the original sample are indistinguishable. 

We have encountered equilibrium before—in our we considered the liquid-gas equilibrium that consideration of phase changes. In Section 5-1.2 fixes the vapor pressure of a liquid, and in Sec-142... 

A phase change in which the molecules become further separated, such as vaporization, requires energy to break intermolecular attractions and is therefore endothermic. Phase changes that increase molecular contact, such as freezing, are exothermic because energy is given off when attractions form between molecules. 

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