Ideality, deviation from functions

The raie gas atoms reveal through their deviation from ideal gas behavior that electrostatics alone cannot account for all non-bonded interactions, because all multipole moments are zero. Therefore, no dipole-dipole or dipole-induced dipole interactions are possible. Van der Waals first described the forces that give rise to such deviations from the expected behavior. This type of interaction between two atoms can be formulated by a Lennaid-Jones [12-6] function Eq. (27)). 

By combining Equations (8.4) and (8.6) we can see that the partition function for a re system has a contribution due to ideal gas behaviour (the momenta) and a contributii due to the interactions between the particles. Any deviations from ideal gas behaviour a due to interactions within the system as a consequence of these interactions. This enabl us to write the partition function as ... 

Plotting the left side of Eq. (3-22) as a function of gives a curve with as the slope and E° as the intercept. Ionic interference causes this function to deviate from lineality at m 0, but the limiting (ideal) slope and intercept are approached as OT 0. Table 3-1 gives values of the left side of Eq. (3-22) as a function of The eoneentration axis is given as in the corresponding Fig. 3-1 beeause there are two ions present for each mole of a 1 -1 electrolyte and the concentration variable for one ion is simply the square root of the concentration of both ions taken together. 

Inhibitors are often iacluded ia formulations to iacrease the pot life and cute temperature so that coatings or mol dings can be convenientiy prepared. An ideal sUicone addition cure may combine iastant cure at elevated temperature with infinite pot life at ambient conditions. Unfortunately, real systems always deviate from this ideal situation. A proposed mechanism for inhibitor (I) function is an equUibtium involving the inhibitor, catalyst ligands (L), the sUicone—hydride groups, and the sUicone vinyl groups (177). 

From this equation, the temperature dependence of is known, and vice versa (21). The ideal-gas state at a pressure of 101.3 kPa (1 atm) is often regarded as a standard state, for which the heat capacities are denoted by CP and Real gases rarely depart significantly from ideaHty at near-ambient pressures (3) therefore, and usually represent good estimates of the heat capacities of real gases at low to moderate, eg, up to several hundred kPa, pressures. Otherwise thermodynamic excess functions are used to correct for deviations from ideal behavior when such situations occur (3). 

In an ideal fluid, the stresses are isotropic. There is no strength, so there are no shear stresses the normal stress and lateral stresses are equal and are identical to the pressure. On the other hand, a solid with strength can support shear stresses. However, when the applied stress greatly exceeds the yield stress of a solid, its behavior can be approximated by that of a fluid because the fractional deviations from stress isotropy are small. Under these conditions, the solid is considered to be hydrodynamic. In the absence of rate-dependent behavior such as viscous relaxation or heat conduction, the equation of state of an isotropic fluid or hydrodynamic solid can be expressed in terms of specific internal energy as a function of pressure and specific volume E(P, V). A familiar equation of state is that for an ideal gas... 

Since it is nearly impossible to find an ideal tracer, all measurements made with tracers deviate from measurements made with real contaminants. I he efficiency methods, when using real contaminants, are as near to direct measurement of function as is possible. Use of tracers can also give direct measurements, but the conditions of the tracer may differ so much from the real contaminant that these measurements must be seen as itidirect. 

The extent of deviation from ideal solution behavior and hence, the magnitude and arithmetic sign of the excess function, depend upon the nature of the interactions in the mixture. We will now give some representative examples. 

Equations 4.55 and 4.57 are the most convenient for the calculation of gas flowrate as a function of Pi and Pi under isothermal conditions. Some additional refinement can be added if a compressibility factor is introduced as defined by the relation Pv -= ZRT/M, for conditions where there are significant deviations from the ideal gas law (equation 2.15). 

This is an indication of the collective nature of the effect. Although collisions between hard spheres are instantaneous the model itself is not binary. Very careful analysis of the free-path distribution has been undertaken in an excellent old work [74], It showed quite definite although small deviations from Poissonian statistics not only in solids, but also in a liquid hard-sphere system. The mean free-path X is used as a scaling length to make a dimensionless free-path distribution, Xp, as a function of a free-path length r/X. In the zero-density limit this is an ideal exponential function (Ap)o- In a one-dimensional system this is an exact result, i.e., Xp/(Xp)0 = 1 at any density. In two dimensions the dense-fluid scaled free-path distributions agree quite well with each other, but not so well with the zero-density scaled distribution, which is represented by a horizontal line (Fig. 1.21(a)). The maximum deviation is about... 

Figure 3.8. The transformation of a rectangular into a normal distribution. The rectangle at the lower left shows the probability density (idealized observed frequency of events) for a random generator versus x in the range 0 < jc < 1. The curve at the upper left is the cumulative probability CP versus deviation z function introduced in Section 1.2.1. At right, a normal distribution probability density PD is shown. The dotted line marked with an open square indicates the transformation for a random number smaller or equal to 0.5, the dot-dashed line starting from the filled square is for a random number larger than 0.5.

Using Equation 5 and 6, it is possible to calculate the crosslinking density distribution as a function of the birth conversion (0). Figure 1 shows one of the calculation results. Though it is quite often assumed that the crosslinking density is the same for all polymer molecules, this assumption is not valid for free radical polymerization. Generally, this distribution becomes significant when the conditions deviate from the idealized Flory s conditions, namely, 1) the reactivities of all types of... 

Figure 4.6 Reaction velocity as a function of enzyme concentration for a non-ideal enzymatic activity assay. Note the deviations from the expected linear relationship at low and at high enzyme concentration.

To account for differences in the Hill coefficient, enzyme inhibition data are best ht to Equation (5.4) or (5.5). In measuring the concentration-response function for small molecule inhibitors of most target enzymes, one will hnd that the majority of compounds display Hill coefficient close to unity. However, it is not uncommon to hnd examples of individual compounds for which the Hill coefficient is signihcandy greater than or less than unity. When this occurs, the cause of the deviation from expected behavior is often reflective of non-ideal behavior of the compound, rather than a true reflection of some fundamental mechanism of enzyme-inhibitor interactions. Some common causes for such behavior are presented below. 

Both enthalpy and entropy can be calculated from an equation of state to predict the deviation from ideal gas behavior. Having calculated the ideal gas enthalpy or entropy from experimentally correlated data, the enthalpy or entropy departure function from the reference state can then be calculated from an equation of state. 

Most real solutions cannot be described in the ideal solution approximation and it is convenient to describe the behaviour of real systems in terms of deviations from the ideal behaviour. Molar excess functions are defined as... 

The imaging process is influenced by interactions between the wave functions of the substrate and tip that result in significant deviation from the idealized square barrier profile. This is particularly true at small tip-substrate separations, where the barrier collapses below the vacuum level, as indicated in Fig. 8 for the Al-Al junction [56,64]. These calculations indicate that for typical STM imaging conditions, the top of the barrier... 

Virial Function. A useful form of expression for deviations from the ideal gas law is the virial equation,... 

DEVIATIONS FROM IDEALITY IN TERMS OF EXCESS THERMODYNAMIC FUNCTIONS 373... 

Various functions have been used to express the deviation of observed behavior of solutions from that expected for ideal systems. Some functions, such as the activity coefficient, are most convenient for measuring deviations from ideality for a particular component of a solution. However, the most convenient measure for the solution as a whole, especially for mixtures of nonelectrolytes, is the series of excess functions (1) (3), which are defined in the foUowing way. 

Redlich and Kister [4] suggested a convenient way to represent gI as a function of composition that permits convenient classification of various kinds of deviation from ideality. From Equation (16.57)... 

Although the description of deviations from ideality in terms of the excess Gibbs function gives us one quantity instead of the two activity coefficients of the two components of a binary solution, we still need to calculate the activity coefficients first, as observed in Equation (16.57). 

Deviations from ideality in real solutions have been discussed in some detail to provide an experimental and theoretical basis for precise calculations of changes in the Gibbs function for transformations involving solutions. We shall continue our discussions of the principles of chemical thermodynamics with a consideration of some typical calculations of changes in Gibbs function in real solutions. 

So far, we have seen that deviation from ideal behavior may affect one or more thermodynamic magnitudes (e.g., enthalpy, entropy, volume). In some cases, we are able to associate macroscopic interactions with real (microscopic) interactions of the various ions in the mixture (for instance, coulombic and repulsive interactions in the quasi-chemical approximation). In practice, it may happen that none of the models discussed above is able to explain, with reasonable approximation, the macroscopic behavior of mixtures, as experimentally observed. In such cases (or whenever the numeric value of the energy term for a given substance is more important than actual comprehension of the mixing process), we adopt general (and more flexible) equations for the excess functions. 

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