Factors That Cause Deviation from Ideal Behavior

Factors That Cause Deviation from Ideal Behavior... 

A gas that obeys these five postulates is an ideal gas. However, just as there are no ideal students, there are no ideal gases only gases that approach ideal behavior. We know that real gas particles do occupy a certain finite volume, and we know that there are interactions between real gas particles. These factors cause real gases to deviate a little from the ideal behavior of the Kinetic Molecular Theory. But a non-polar gas at a low pressure and high temperature would come pretty close to ideal behavior. Later in this chapter, we ll show how to modify our equations to account for non-ideal behavior. 

A field experiment was conducted at the Canadian Air Forces Base Borden, Ontario, to study the behavior of organic pollutants in a sand aquifer under natural conditions (Mackay et al., 1986). Figure 25.9 shows the results of two experiments, the first one for tetrachloroethene, the second one for chloride. Both substances were added as short pulses to the aquifer. The curves marked as ideal were computed according to Eqs. 25-20 or 25-23. The measured data clearly deviate from the ideal curve. The nonideal curves were constructed by Brusseau (1994) with a mathematical model that includes various factors causing nonideal behavior. 

Comment Notice that the first term, 1.003 atm, is the pressure corrected for molecular volume. This value is higher than the ideal value, 1.000 atm, because the volume in which the molecules are free to move is smaller than the container volume, 22.41 L. Thus, the molecules must collide more frequently with the container walls. The second factor, 0.013 atm, corrects for intermolecular forces. The intermolecular attractions between molecules reduce the pressure to 0.990 atm. We can conclude, therefore, that the intermolecular attractions are the main cause of the slight deviation of Cl2( ) from ideal behavior under the stated experimental conditions. 

Evaluation of CL performance requires a number of parameters that define the ideal electrocatalyst performance, allowing deviations from ideal behavior to be rationalized and quantified. Ideal electrocatalyst performance is achieved when the total Pt surface area per unit volume, Stot, is utilized and when reaction conditions at the reaction plane (or Helmholtz layer) near the catalyst surface are uniform throughout the layer. These conditions would render each portion of the catalyst surface equally active. Deviations from ideal behavior arise due to statistical underutilization of catalyst atoms, as well as nonuniform distributions of reactants and reaction rates at the reaction plane that are caused by transport effects. This section introduces the effectiveness factor ofPt utilization and addresses the hierarchy of structural effects from atomistic to macroscopic scales that determine its value. 

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