Real gasesReal gases do not obey the perfect gas law exactly because molecules interact with each other.

Two important inter-molecular forces exist: repulsive forces and attractive forces.

1.5 The van der Waals equationFor the perfect gas:1). The size of molecules is negligible;2). Molecules do not interact; pV=nRTFor real gases:1). The molecule itself occupies a volume; 2). There are interactions among molecules; (V -V )=nRT(p+p)

Repulsive forces between molecules assist expansion and attractive forces assist compression. Attractive forces are effective over several molecular diameters. They are important when the molecules are fairly close together. Attractive forces are ineffective when the molecules are far apart.Molecular interactions

The compression factorThe compression factor, Z, is defined asFor a perfect gas, Z = 1 under all conditions; the deviation of Z from 1 is a measure of departure from perfect behaviour.

Results by plotting the compression factor, Z, against pressure for several gases at 0 . At very low pressures: Z < 1.

At high pressures: Z> 1. Repulsive forces are now dominant. At intermediate pressures: Z

Penyimpangan Perilaku Ideal1 mol gas idealPV = nRT5.8Gaya Tolak-menolakGaya Tarik-menarik

In the van der Waals equation,

where n2a/V2 represents the effect on pressure to intermolecular attractions or repulsions. Table 5.4 gives values of van der Waals constants for various gases.

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Also, in the van der Waals equation, where nb represents the volume occupied by n moles of molecules.

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(p+p) (V -V ) =nRTThe van der Waals equation

The van der Waals Equation

General form of the van der Waals equation:

Real Gases: Deviations from Ideal Behavior

Corrects for molecular volume

Corrects for molecular attraction

5.8Persamaan Van der WaalsUntuk gas nonideal

If sulfur dioxide were an ideal gas, the pressure at 0C exerted by 1.000 mol occupying 22.41 L would be 1.000 atm. Use the van der Waals equation to estimate the real pressure.

values for SO2a = 6.865 L2.atm/mol2b = 0.05679 L/mol

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The real pressure exerted by 1.00 mol of SO2 at STP is slightly less than the ideal pressure.

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