Nonideal solution definition

Whenever the solute and solvent exhibit significant degrees of mutual attraction, deviations from the simple relationships will be observed. The properties of these nonideal solutions must be determined by the balance of attractive and disruptive forces. When a definite attraction can exist between the solute and solvent, the vapor pressure of each component is normally decreased. The overall vapor pressure of the system will then exhibit significant deviations from linearity in its concentration dependence, as is illustrated in Fig. 10B. 

Begin with the equilibrium relationship given by Eq. (1-8) and the definition of the ideal solution K value given by Eq. (1-10) and obtain the following formulas for the Kb method for nonideal solutions... 

It is important to note that these definitions of supersaturation assume an ideal solution with an activity coefficient of 1. It is common practice to ignore activity coefficients in most cases and employ concentrations in expressions of supersaturation, however, in very nonideal solutions and in precise studies of crystal growth and nucleation, activity coefficients are often used. 

Pi, P2, etc., are called the partial pressures of the solution gases and equations (7.3) are now normally used as the definition of partial pressure even though in real, nonideal solutions they give a quantity that is not equivalent to the original meaning, i.e., the pressure a gas would exert if it alone occupied the total volume. ... 

The fiigacity has the dimension of pressure. Often we want a nondimensional representation of the fugacity, for example, in mass-action (chemical equilibrium) calculations. We will see in Chapter 12 that this requirement leads naturally to the definition of the activity. Furthermore, when we apply the ideal solution idea to nonideal solutions, we will need a measure of departure from ideality, just as the compressibility factor z is a measure of departure from ideal gas behavior. The logical choice for that measure is the activity coefficient, defined below. We will see that the activity and activity coefficient are dimensionless, and that for ideal solutions and many practical solutions the activity is equal to the mol fraction. 

In this definition, the activity coefficient takes account of nonideal liquid-phase behavior for an ideal liquid solution, the coefficient for each species equals 1. Similarly, the fugacity coefficient represents deviation of the vapor phase from ideal gas behavior and is equal to 1 for each species when the gas obeys the ideal gas law. Finally, the fugacity takes the place of vapor pressure when the pure vapor fails to show ideal gas behavior, either because of high pressure or as a result of vapor-phase association or dissociation. Methods for calculating all three of these follow. 

The product 7 + 7- is experimentally measurable. The quantity (7 referred to as the mean molal activity coefficient The mean ionic molality is defined as im+mJ) and is simply m for a univalent-univalent electrolyte. Summarizing these definitions for a nonideal, univalent-univalent solution, where the solute is component 2. 

The definition of pH in terms of [H ] neglects any correction for nonideality of the solutions. 

The lUPAC definition of the SHE is as follows [1] The SHE consists of a Pt electrode in contact with a solution of H+ at unit activity and saturated with H2 gas with a fugacity referred to the standard pressure of 10 Pa. Clearly, the solution and H2 gas are hypothetical in this definition, so nonideality of both should be taken into account if the SHE is to be used in a lab. Eor the H2 gas, it can easily be done using the H2 fugacity coefficients to be calculated from the van der Waals equation with constants available from [Chapter 10, Table 10.24], Eor the aqueous solution, a few experiments should be carried out using relatively dilute acidic solutions [e.g., HCl(aq)], and then an extrapolation should be carried out to the infinitely diluted solution as described in the following chapter (Section 5.11). 

If, as shown above, for ideal gas mixtures the fugacity of one species in the mixture is equal to its partial pressure, then we would like to extend that simple idea to nonideal gas mixtures, and to solutions of liquids and solids. We can, using the definition of an ideal solution. An ideal solution is like an ideal gas in the following respects ... 

Apart from a few exceptions, polyelectrolytes such as proteins and nucleic acids need a definite pH range and the presence of counterions for stability in aqueous solution. Usually this condition is reaUzed by use of dilute salt or buffer solutions. In this case only weak interactions between macromolecules and low-molecular solutes occur, thus reflecting only small contributions from thermodynamic nonideality. 

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