Properties of the enthalpy

For the change of a system between states 1 and 2, we have from (21) above (2-4) 

In the special case where the system is under a constant pressure p the last equation may be written 

The last term is the work of displacing the environment of the system at pressure p. If this is the only form of work, the last two terms cancel and therefore v tj  

The enthalpy change of a closed system is thus equal to the heat absorbed under ttuo restrictive conditions, namely, that there is constancy of pressure and that the only form of work is due to the volume change of the system. In a galvanic cell, where electrical work is done, the heat absorbed is not equal to the change in enthalpy. 

Unlike the internal energy, enthalpy is not a quantity which is necessarily conserved under conditions of isolation. Consider, for 

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The state function property of the enthalpy should be kept in mind for the next move of our discussion. In figure 2.1 we have decomposed reaction 2.1 in a series of steps whose net effect must correspond to the overall reaction. This means that the correct value for Asin//(2) is the solution enthalpy of 1 mol of oxygen in the (ethanol + water) mixture described—and not the solution enthalpy of the gas in pure water. Unfortunately, solution enthalpy data in organic liquid mixtures are not abundant in the chemical literature. So, either we are lucky to find them, we have the equipment to measure them in the laboratory, or we assume that the values will be identical to the ones in the pure solvent. The validity of this assumption depends on the system under discussion and on the accuracy needed for the final result, but in the present case it seems fair. Leaving further discussion to section 2.5, we shall take Asin//(2) = -12 4 kJ mol-1 [17],... 

Some properties of the enthalpy-concentration plot are as follows. The region in between the saturated vapor line and the saturated liquid line is the two-phase liquid-vapor region. From Table 11.1-1 for = 0.411, the vapor in equilibrium is y = 0.632. These two points are plotted in Fig. 11.6-1 and this tie line represents the... 

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