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# ch7 solutions - Thermo Chapter 7 Questions on Concepts Q7.1...

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117 Thermo Chapter 7 Questions on Concepts Q7.1) Explain why the oscillations in the two-phase coexistence region using the Redlich-Kwong and van der Waals equations of state (see Figure 7.4) do not correspond to reality. The oscillations predict that as V increases, P will increase. No real gas exhibits this behavior. Q7.2) Explain the significance of the Boyle temperature. The Boyle temperature provides a way to classify the way in which z varies with P at low values of P for different gases. If T > T B , z increases with increasing P ; if T < T B , z decreases with increasing P . Q7.3) The value of the Boyle temperature increases with the strength of the attractive interactions between molecules. Arrange the Boyle temperatures of the gases Ar, CH 4 , and C 6 H 6 in increasing order. Ar < CH 4 < C 6 H 6 Q7.4) Will the fugacity coefficient of a gas above the Boyle temperature be less than one at low pressures? No. The integral 0 1 P z dP P is always greater than zero for this case. Therefore, γ > 1 for all P . Q7.5) Using the concept of the intermolecular potential, explain why two gases in corresponding states can be expected to have the same value for z . Two different gases will have different values for the depth of the intermolecular potential and for the distance at which the potential becomes positive. By normalizing P , T , and V to their critical values, the differences in the intermolecular potential are to a significant extent also normalized. Q7.6) By looking at the a and b values for the van der Waals equation of state, decide whether 1 mole of O 2 or H 2 O has the higher pressure at the same value of T and V . A is significantly larger for H 2 O. Therefore, the attractive forces between H 2 O molecules are greater than between O 2 molecules. Consequently, O 2 will have a higher pressure.

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