3.012 PS 4 3.0123.012 PS 4 3.012 Issued: 10.06.04 Fall 2004 due: 10.13.04 5pm 1. A particular cubic crystal has a lattice constant of 7 Å. It is irradiated with X-rays produced by an X-ray tube with a molybdenum target. The strongest intensity produced from such a target (one of the two metals that are most commonly used to produce radiation for diffraction; the other is copper) has a wavelength of 0.7107 Å. Upon irradiating the crystal it is found that the (310) plane produces a diffracted beam at an angle, θ, of 53.3840. a. Compute the value of the interplanar spacing, d, between (310) planes. b. How many wavelengths of path difference are present in the beam scattered from adjacent (310) planes in the stack for the diffraction peak that was observed at θ = 53.3840 ? 2. Two patterns are attached. Both have translational periodicity as well as additional symmetry. a. Sketch in each pattern a set of lattice points that correspond to the translations that map each pattern into congruence with itself. (Use bold, plump points, if you please, so that the grader can readily distinguish the points that you have drawn in.) b. Draw on each pattern the conventional unit cell (that is, one using the shortest pair of translations that describe a cell with the shape that conforms to the symmetry that is present) c. Draw in, using standard symbols (bold lines for mirrors, dashed lines for glides, and small rectangular polyhedra for rotation axes), the location of all symmetry elements that you are able to find within your cell. d. Using the descriptions of the seventeen plane groups taken from the “International Tables for X-Ray Crystallography” (Included in the notes from Lecture #9) identify the plane group that corresponds to each of your patterns and the symbol used to represent it. (And, while you’re at it, confirm that you have been successful in finding all the symmetry elements that are present.) 3.012 PS 4 1 of 63.012 PS 4 2 of 63.012 PS 4 3 of 63. Logic of the Second Law. Following are a collection of statements concerning the second law. Determine if each statement is true or false, and provide a brief (1-2 sentence) explanation. a. When two materials A and B with initial temperatures TA > TB are placed in thermal contact (the two samples are otherwise isolated from their surroundings), the entropy of the ‘hot’ material decreases, but this does not violate the second law. b. In a reversible process, the entropy change of a closed system is always zero. c. The entropy of an isolated system must always increase in a spontaneous process. d. The Gibbs free energy of a system is minimized at equilibrium under conditions of constant temperature and pressure. Under other conditions, its behavior is unspecified. 4. Entropy change of the universe in a heating process. We heat 1 mole of a material (at an initial temperature Ti) by placing it in thermal contact with a large heat reservoir that has a higher temperature Tf at constant pressure. Assume the heat capacity of this material is a constant approximately independent of temperature, and that the heat reservoir is so large that its temperature is unaffected (remains constant) during moderate heat transfers. a. What is the entropy change in the heat reservoir caused by this process? b. Show that the entropy change of the universe for this process is: ∆Suniverse=CPx−ln 1+x()[] …where x =−Tf− Ti()Tf. c. Does this spontaneous heating process obey the second law? Explain why or why not in 1-2 sentences. 3.012 PS 4 4 of 65. Diffusion experiments obey the second law. Consider the diffusion experiment sketched schematically below. The system is composed of two subsystems separated by a permeable membrane. The left subsysytem is initially composed of a pure liquid A, while the right subsystem is initially a second pure liquid B. The membrane allows free transfer of A and B molecules between the two subsystems. The system is held at constant temperature and pressure, and is closed to its surroundings. If the two liquids have similar modes of bonding, they may be completely miscible and mix to form an ideal solution. The chemical potential of a component i in an ideal solution is: µi=µi.o+RTlnXi where µI,o is a constant, R is the gas constant (8.3144 J/mole K), T is the absolute temperature, and Xi is the mole fraction of component i in the system: Xi=ninjj=1C∑ The sum is over all of the C components of the system. The total moles of molecules of each type in the system is fixed: nAL+nAR=NAnBL+ nBR= NB (The superscripts L and R refer to the left and right subsystems, respectively). NA and NB are constants. Left subsystemInitially pure liquid ARight subsystemIntially pure liquid BMembrane allows Aand B to passbetween subsystems 3.012 PS 4 5 of 6a. We showed several examples in lecture of how the condition dSuniverse = 0 can be used to define requirements for equilibrium (e.g., uniform T, uniform P, etc.). We further introduced the Gibbs free energy, which provides a simple criterion for equilibrium under conditions of constant temperature and pressure: dGsystem = 0. Using dGsystem = 0 and the approach used in lecture for demonstrating thermal and mechanical equilibrium, show that for chemical equilibrium in the simple system described above, the following conditions must be met: µAL=µARµBL=µBR (Treat each subsystem as a separate phase- the superscript L denotes the left subsystem, the superscript R denotes the right subsystem). b. Suppose the total number of molecules on either side of the membrane is initially equal and must remain equal (e.g., due to the liquids being approximately incompressible and having similar molecular volumes): nAL L+ nB= NLnAR+ nBR= NRNL= NR= NA= NB In addition, the chemical potential constants µi,o for each species can be taken as equal on either side of the membrane: µA,oL=µA,oRµB,oL=µB,oR Given these constraints, calculate the values of nAL, nAR, n , and at equilibrium in terms of the given constants if A and B form an ideal solution on either side of the membrane. BLnBRnAL+ nBL= NLnAR+ nBR= NRNL=NR dnAL=−dnBLdnAR=−dnBR 3.012 PS 4 6 of
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