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2009 Exam 3 Key - Name Section Number Chemistry 1314 Honors...

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Name Section Number Chemistry 1314 Honors Midterm Exam #3 November 4, 2009 This exam is closed book, notes, papers, etc. You may use a calculator. If you need a physical constant or unit conversion factor that is not provided, please ask. This exam must represent your own work. Scores: 1. a. / 8 b. / 6 c. / 6 2. / 20 3. / 20 4. a. / 9 b. / 6 c. / 5 5. a. / 5 b. / 5 c. / 5 d. / 5 Total: / 100 NOTE: No answers based on octets being particularly stable will be accepted on any question.
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CHEM 1314 H Midterm Exam #3, 2009 Page 2 1. The photoelectric effect refers to the observed ejection of electrons from the surface of a metal exposed to radiation. The experimenter can vary the frequency and the intensity of the light, and then can measure the kinetic energy and the number (flux) of ejected electrons. a. Sketch the observed dependences of (i) electron kinetic energy on frequency of light, (ii) electron kinetic energy on intensity of light, (iii) electron flux on frequency of light, and (iv) electron flux on intensity of light. Note any important conditions of the other variables. b. Explain briefly how these observations can be understood in terms of the nature of radiation. From (i) we can conclude that the energy of the light increases proportionally with the frequency ( i.e. , E=h ν ). From the existence of a threshold frequency in (i) and (iii), we conclude that the energy must not be transferred cumulatively to the metallic electrons, and thus must come in packets, or “photons.” Slope: 1 point each Threshold for i and iii: 1 point each v > v 0 on ii and iv: 1 point each Energy proportional to frequency: 3 points Particle nature of light: 3 points
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CHEM 1314 H Midterm Exam #3, 2009 Page 3 c. Describe qualitatively the spectrum of radiation emitted by hot hydrogen atoms. Assuming the nature of radiation from part (b) above, explain how this spectrum demonstrates the existence of quantum energy levels in hydrogen. 2. Explain each of the following observations in terms of the properties and energies of the occupied orbitals of the given atom. (There are two observations in each part; explain both of them.) a. The ionization energy of a potassium atom is less than that of a calcium atom, whereas the ionization energy of a potassium ion, K + , is larger than the ionization energy of a calcium ion, Ca + . b. Within a group, the atomic radii tend to increase with increasing atomic number, but within a period, the atomic radii tend to decrease with increasing atomic number.
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