chem1A-fa09-mt4-Stacy-soln - Chemistry 1A, Fall 2009...

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Unformatted text preview: Chemistry 1A, Fall 2009 Midterm Exam #4 December 3, 2009 (90 min, closed book) Name:_______________KEY________________ SID:___________________________________ GSI Name:________________ The test consists of 25 multiple choice questions. Circle the BEST answer and bubble the choice on your scantron form. Useful Equations and constants: Question Multiple Choice Total Page 2-6 Points 80 80 Score E = h = c Ekin (e-) = h − binding energy 1 eV = 1.6021×10-19 J 1 J = 1 kg·m2/s2 h = 6.63×10-34 J·s c = 3.0×108 m/s NA = 6.02 x 1023 particles/mol Name_____________________GSI__________ Consider the following elements: Ca, Fe, Ga, and Se 1. 2 The electron configuration for Fe is 62 25 8 A) [Ar] 3d 4s B) [Ar] 4s 3d C) [Ar] 3d 2. How many nodes does the 3d orbital have? 2 A) 2 B) 3 C) 4 3. Choose the best explanation for why the ionization energy of Ga is LESS than the ionization energy of Ca. A) The ionization energy decreases across a row due to increasing effective nuclear charge. B) The ionization energy increases across a row due to increasing effective nuclear charge. 2 C) The effective nuclear charge for Ga is smaller because there are more electrons in the n=3 shell. For which element is the total energy required to remove 3 electrons the greatest? 2 B) Ca A) Fe C) Ga How many electrons are in the n=3 level in Se? 2 C) 18 A) 6 B) 8 Which quantum numbers describe the outermost electron in Ga? 2 A) shell=4 B) shell=3 C) shell=3 subshell = p subshell = d subshell = p orientation=pz orientation=dxy orientation=px spin = +½ spin = −½ spin = +½ 4. 5. 6. Page 2 of 6 Name_____________________GSI__________ A fellow Chem 1A student takes a trip to an art museum and decides to review the concepts of light and color. What follows are his thoughts about what he is observing. Circle the sentence that is correct. 7. As I walk in the door, I notice that the museum lights are very dim. There is also a sign that says: “No flash photography in the museum.” I’ve heard that bright light makes the paint fade. A) I bet that happens because the light energy can cause vibrations in the paint pigment molecules, which cause the red pigments to get hotter than the blue pigments. 3 B) It seems that red paint fades more than blue because red paint absorbs shorter wavelength blue light. 2 C) High intensity light is effective in breaking chemical bonds. Here’s a sculpture done like a neon sign. The description says that when you put an electric current through the neon gas, it glows with a characteristic color A) This must be because electrons move from energy levels close to the nucleus to energy levels farther away as they emit the light. B) A characteristic energy is needed to ionize the atoms. You only see colors that are not absorbed. 3 C) The electric current moves an electron to an excited state. When the electron returns to the ground state it emits a specific wavelength of visible light. Another room has a collection of photographs. The sign says that they were taken indoors without a flash. White objects appear yellow. 3A) This is because the filament in the light bulb in the room was not hot enough to emit equal intensities of all visible light. B) This is because the light bulb in the room emitted light in the infrared region of the spectrum. 2C) This is because violet light is absorbed by the white objects. If you walk too near to the paintings, an alarm sounds. I notice that there is a beam of red light (700 nm) crossing in front of the paintings shining onto a photocell. The alarm sounds if you block the light from striking the photocell. What must be true of the light striking the photocell? A) Light of 500 nm will not eject electrons to complete the circuit. B) Light of 750 nm will eject electrons provided the intensity is high enough. C) More intense light of 700 nm will eject electrons with greater kinetic energy. 3 D) None are true. 8. 9. 10. Page 3 of 6 Name_____________________GSI__________ Plastic sleeves made out of polypropylene can protect photographs from damage by light. To test how well the polypropylene sheet protector blocks harmful light, a Chem 1A student collected absorbance spectra using a UV/Vis spectrometer. The student cut several small strips of the plastic and measured 1-5 layers stacked on top of one another. The data she gathered are shown below. Polypropylene UV/Vis Spectra 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 200 400 600 800 1.6 1.4 1.2 5 Layers 4 Layers 3 Layers 2 Layers 1 Layer Absorbance 1.0 0.8 0.6 0.4 0.2 0.0 0 0.1 0.2 0.3 0.4 0.5 0.6 Polypropylene Sheet Protectors y = 2.7287x + 0.0167 R² = 0.9995 Absorbance Wavelength (nm) Plastic Sheet Thickness (mm) 11. When the absorbance at 273 nm is plotted versus thickness of the plastic sheet a linear graph is obtained (shown above right). What is the slope of the line for this plot? 3 B) C) A) The Chem 1A student found a cheaper supplier of polypropylene sheet protectors. She determined the sample absorbance at 273 nm to be 1.23. What is the thickness of the sheet? A) 3.73 mm 3 B) 0.44 mm C) 1.23 mm Based on the spectra shown above, what color are the sheet protectors? 3 A) colorless B) yellow C) white Ozone, O3, in the atmosphere filters ultraviolet light from the sun because light with wavelengths less than 300 nm breaks the oxygen-oxygen bond. What is the oxygenoxygen bond energy (kJ/mol) in ozone? A) 6.6 x 10-28 kJ/mol B) 6.6 x 10-19 kJ/mol 2 C) 318 kJ/mol 3 D) 399 kJ/mol E) 1.0×1015 kJ/mol 12. 13. 14. Page 4 of 6 Name_____________________GSI__________ The first 4 electron energy levels in hydrogen, H, and helium cation, He are provided in the table. Use the data to answer the questions below. Energy level n=1 n=2 n=3 n=4 15. Hydrogen, H -1312 kJ/mol -328 kJ/mol -146 kJ/mol -82 kJ/mol Helium cation, He+ -5248 kJ/mol -1312 kJ/mol -583 kJ/mol -328 kJ/mol C) + Which spectrum represents the photoelectron spectrum of He+? 4 A) B) 2000 4000 6000 2000 4000 6000 2000 4000 6000 16. Which spectrum represents the lowest energy absorption line of H (g) at room temperature? 4 A) B) C) 2000 4000 6000 2000 4000 6000 2000 4000 6000 17. 18. 4 How much energy is required to ionize He+? A) 5248 kJ/mol B) 1312 kJ/mol C) 3936 kJ/mol Which species releases the most energy when an electron attaches: H, He, He+? A) He because it is a noble gas. 3 B) H because the added electron will fill the 1s shell. 4 C) He+ because it has the highest effective nuclear charge. D) H because it is more electronegative than He and He+. What happens to the electron in the ground state of hydrogen atoms, H, if you shine light of energy 1061 kJ/mol on a sample? A) The electron has enough energy to move to the n=2 shell. The excess energy is released as kinetic energy. B) The electron does not have enough energy to move to the n=2 shell. Nothing happens. 4 C) The energy does not correspond to the energy difference between n=1 and n=2. The light is transmitted. D) The energy is large enough to ionize the electron from the n=2 shell. 19. Page 5 of 6 Name_____________________GSI__________ A scientist is trying to determine if the blue color of the methane flame is due to CH molecules. The molecular orbital diagram is shown below. The 2s orbitals and 2px and 2py are all non-bonding orbitals, n2s and n2p. It is only the σ and σ* orbitals that determine the bonding. * n2p n2s Carbon Orbitals CH Orbitals Hydrogen Orbitals 20. 21. What is the bond order for CH? A) 0 B) ½ The first excited state for CH is: A) n2s2 σ2 n2py1 B) n2s2 σ2 4 C) 1 D) 1 ½ 4 D) n2s2 σ1 n2py1 n2pz1 C) n2s2 σ1 n2py2 22. Light of energy ~3 eV is emitted from a methane flame. What is the wavelength and color of the light emitted? The visible range of light is from 700 nm to 420 nm. 4 3 A) 700 nm B) 700 nm C) 500 nm D) 420 nm red blue blue-green violet CH absorbs blue light when an electron moves from the σ bonding orbital to the n2p nonbonding orbital. Explain why HF does not absorb blue light. 4 A) An electron cannot move to the n2p nonbonding orbital because it is already filled. 3 B) Fluorine has a higher effective nuclear charge so the electron is attracted to fluorine more strongly. C) Fluorine is more electronegative so the energy of the σ bonding orbital is lower. D) HF satisfies the octet rule, but CH does not. Which molecule is NOT attracted strongly to a magnet? 4 − A) CH C) OH D) HF B) CH Which illustration best describes the location of an electron in the n2p nonbonding orbital in CH. 4D) A) B) C) C H 23. 24. 25. Page 6 of 6 ...
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This note was uploaded on 09/24/2010 for the course CHEM 22345 taught by Professor Stacy during the Spring '10 term at University of California, Berkeley.

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