Copyright © Cengage Learning. All rights reserved 1 Real Gases (Deviations from Ideal Gases) Under ordinary conditions, the assumption that real gases act as ideal gases is a good on. However, an ideal gas is a hypothetical concept. At the molecular level, deviations arise because ideal gas assumptions neglect two factors: 1. attractive forces between gas particles 2.finite volume of gas particles

Copyright © Cengage Learning. All rights reserved 2 We must correct for non-ideal gas behavior for: Pressure Pressure Temperature Temperature Under these conditions: Concentration of gas particles is high. Concentration of gas particles is high. Attractive forces become important. Attractive forces become important.

Plots of PV/nRT Versus P for Several Gases (200 K) Copyright © Cengage Learning. All rights reserved 3 Effect of Pressure

Copyright © Cengage Learning. All rights reserved 4 Pressure Pressure Low Pressures High Pressures Volume taken up by particle size is insignificant compared with container. Volume taken up by particle size is insignificant compared with container. Volume is decreased Container volume available to the particles is less Particle size becomes important (e.g., imagine particles actually touching each other)

Copyright © Cengage Learning. All rights reserved 5 Temperature Temperature Low Temperatures High Temperatures Slower translational movement Slower translational movement Increased intermolecular attractions Increased intermolecular attractions Faster translational movement Less chance for intermolecular attractions

We can plot the compressibility factor (PV/nRT) for gases. If the gas is ideal, it should always give a value of 1. Obviously, none of these gases are ‘ideal.’ Real gases Compressibility factor Pressure, atm H2H2 N2N2 CH 4 C2H4C2H4 NH 3

Real gases As pressure approaches zero, all gases approach ideal behavior. As pressure approaches zero, all gases approach ideal behavior. At high pressure, gases deviate significantly from ideal behavior. At high pressure, gases deviate significantly from ideal behavior. Why? Why? Attractive forces actually do exist between molecules. Attractive forces actually do exist between molecules. Molecules are not points -- they have volume. Molecules are not points -- they have volume.

Van der Waals equation This equation is a modification of the ideal gas relationship. It accounts for attractive forces and molecular volume. This equation is a modification of the ideal gas relationship. It accounts for attractive forces and molecular volume. P + an 2 V 2 (V - nb) = nRT () Correction for Molecular volume Correction for attractive forces between molecules

Van der Waals equation This equation is a modification of the ideal gas relationship. It accounts for attractive forces and molecular volume. This equation is a modification of the ideal gas relationship. It accounts for attractive forces and molecular volume. P + an 2 V 2 (V - nb) = nRT () Correction for Molecular volume Correction for attractive forces between molecules