161ASpring2010-Lecture6

161ASpring2010-Lecture6 - 1 Department of Chemistry Seminar...

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Unformatted text preview: 1 Department of Chemistry Seminar Series Professor Jin Zhong Zhang Department of Chemistry University of California @ Santa Cruz 2 Real Gases Under many conditions, real gases do not follow the ideal gas law ...-- Intermolecular forces of attraction cause the measured pressure of a real gas to be less than expected-- When molecules are close together, the volume of the molecules themselves becomes a significant fraction of the total volume of a gas EOS ( 29 2 2 n a P V nb nRT V + - = van der Waals equation 3 The van der Waals model of real gases ( 29 T Nk Nb V V aN P B =- + 2 2 For real gases both quantitative and qualitative deviations from the ideal gas model r U ( r ) van der Waals attraction repulsion attraction Nb V V eff- = 2 2 V aN P P eff + = Electric interactions between electro-neutral molecules : The van der Waals equation of state for real gases-3-2-1 1 2 3 4 1.5 2.0 2.5 3.0 3.5 4.0 distance U ( r )-3-2-1 1 2 3 4 1.5 2.0 2.5 3.0 3.5 4.0 distance U ( r )-3-2-1 1 2 3 4 1.5 2.0 2.5 3.0 3.5 4.0 distance 4 van der Waals Equation Corrections for real gas behavior are made using the parameters a and b a accounts for intermolecular attractions in real gases b accounts for the real volumes of gases EOS 5 Compression Factor The molar volume V m =V/n is the volume taken up per mole of gas The compression factor (Z) shows deviation from ideal gas behavior. Z=1 for an ideal gas. Z<1 means the gas is more easily compressed. Z>1 means its more difficult to compress. At very low densities (pressures) the gases approach ideal behavior At Intermediate densities(pressures) attractive contributions dominate and the gases are more compressible (generally) than...
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This note was uploaded on 09/08/2010 for the course CHEM 160 at San Jose State University .

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161ASpring2010-Lecture6 - 1 Department of Chemistry Seminar...

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