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Chemistry 231

Chemistry 231. Real Gases. Real Gases. The ideal gas equation of state is not sufficient to describe the P,V, and T behaviour of most real gases. Most real gases depart from ideal behaviour at deviation from low temperature high pressure. Deviations from Ideal Gas Behaviour .

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Chemistry 231

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  1. Chemistry 231 Real Gases

  2. Real Gases • The ideal gas equation of state is not sufficient to describe the P,V, and T behaviour of most real gases. • Most real gases depart from ideal behaviour at deviation from • low temperature • high pressure

  3. Deviations from Ideal Gas Behaviour • Low Temperatures

  4. Attractive Forces in Real Gases • The variation of the potential energy of two molecules on their separation. • High positive potential energy (little separation) • Repulsive interactions • Intermediate separations • attractive interactions dominate • Large separations (on the right) • the potential energy is zero and there is no interaction between the molecules.

  5. Deviations from Ideal Gas Behaviour (cont’d) • High Pressures

  6. Deviations from Ideal Behaviour • Real gas molecules do attract one another (Pid = Pobs + constant) • Real gas molecules are not point masses (Vid = Vobs - const.)

  7. The Van der Waal's Equation of State • Vid = Vobs - nb • b is a constant for different gases • Pid = Pobs + a (n / V)2 • a is also different for different gases • Ideal gas Law Pid Vid = nRT • (Pobs + a (n / V)2) x (Vobs - nb) = nRT

  8. Critical Constants • Critical temperature (Tc) - the temperature above whicha gas cannot be liquefied • Critical pressure (Pc) – the minimum pressure that needs to be applied at Tc to bring about liquefaction

  9. Compression Factor • The compression factor

  10. The Boyle Temperature • Boyle temperature - for a van der Waal's gas, the Boyle temperature (TB) is written

  11. The Boyle Temperature #2 • For a perfect gas, the slope is zero • Boyle temperature • the slope is zero and the gas behaves perfectly over a wider range of conditions than at other temperatures.

  12. Critical Constants for Van der Waals’s Gases • At the critical point

  13. Reduced Variables • The reduced state variables are defined

  14. The Law of Corresponding States • All substances obey the same equation of state in terms of the reduced variables. • Degree of generality.

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