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Section7_Real_Gases

# Section7_Real_Gases - States of Matter I Real Gases By the...

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States of Matter: I Real Gases By the end of this lecture you will be able to: (1) Differentiate between real and ideal gases. (2) Understand how the ideal gas law can be modified to account for the differences. (3) State and manipulate the van der Waals equation. (4) Know what intermolecular forces and dipole moments are. (5) Correlate the dipole moment of a molecule to its intramolecular bonding. (6) Know what polar covalent bonds are and their relationship to electronegativity.

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(7) Understand attractive forces between non-polar molecules. (8) Understand how the size, polarity and symmetry of molecules correlate with the van der Waals equation.
(1) What is a real gas ? All gases are real gases. The ideal gas is simply a concept and does not actually exist. Real gases behave like the ideal gas at low pressure and high temperature . In real gases: (a) Molecules have significant volume. (b) There are significant forces between molecules.

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The ideal gas equation can be modified to account for these two factors. (a) Because molecules have volume, the actual volume available to the particles is (V nb ) where n is the number of moles of gas and b is the molar volume of the molecules themselves. The factor nb is a volume correction . ) V ( RT ' P nb n ! = nb must have units of volume (L) b has units L mol -1 {atm = mol × L atm mol -1 K -1 × K / L}
(b) Because attractive forces between molecules decrease the frequency of collisions with the walls of the container, the pressure exerted by real gases is lower than predicted by the ideal gas equation. A pressure correction must be applied to the ideal gas law to account for this. 2 obs V ' P P ! " # \$ % & = n a

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a ( n /V) 2 must have units of pressure (atm) a has units atm L 2 mol -2 {atm = atm L 2 mol -2 × (mol / L) 2 } Thus: 2 obs V ) V ( RT P ! " # \$ % & = n a nb n This gives the van der Waals equation : ( ) RT V V P 2 obs n nb n a = !
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