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# This is true and corresponds to the horizontal

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Unformatted text preview: (or cyclic) rule applied to the state variables X, Y and Z is: (∂X/∂Y)Z (∂Y/∂Z)X (∂Z/∂X)Y = - 1 True statement. This is how we characterize the Euler’s rule! f) A, B and C are three different states of a system. If the change in energy ΔUAB from state A to state B is + 10 J and the change in energy ΔUBC from state B to state C is – 20 J, the change in energy ΔUCA from state C to state A is + 10 J. This is a true statement. ΔUAB + ΔUBC = + 10 J + (– 20 J) = - 10 J. Furthermore, ΔUAB + ΔUBC = U(B) – U(A) + U(C) – U(B) = U(C) – U(A) = ΔUAC and ΔUCA = - ΔUAC = 10 J 2 Question 5: (5 points) Which among the following statements is correct? a) At all intermolecular distances, repulsive and attractive interactions have the same strength but their associated energies have opposite signs. This statement is incorrect. If it was, intermolecular interactions would be always zero and all gases would be ideal and no liquid could ever form! b) Repulsions dominate at very short intermolecular distances and attractive interactions dominate at somewhat larger intermolecular distances. This is correct. c) Attractions dominate at very short intermolecular distances and repulsions dominate at somewhat larger intermolecular distances. This is incorrect! If this were true, matter would occupy no space at high pressure, as attractions would still be active as molecules get very close together! Conversely, some state of materials with densities intermediate between gas densities and liquid densities would be forbidden. d) The transformation of a gas into a liquid as a result of cooling is explained by the fact that repulsive forces between molecules vanish at low temperature while attractive forces do not. This is incorrect. As the temperature decreases, the kinetic energy of molecules decreases and attractions start to play a more important role. Molecules start to reside more frequently in the “dip” of the intermolecular energy curve. Repulsions are still present at very small intermolecular distances, preventing different molecules from occupying the same space (and insuring that in contrast with what can be observed with neutron stars, the density of liquids does not become anomalously large!) Question 6: (5 points) Which among the following statements is incorrect? a) During the reversible condensation of a gas both the pressure...
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## This note was uploaded on 01/26/2014 for the course CHEM 3615 taught by Professor Aresker during the Spring '07 term at Virginia Tech.

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