Unformatted text preview: Chemistry 1A, Fall 2010
Midterm Exam #4 November 30, 2010 (90 min, closed book) Name:__________________________________ SID:___________________________________ GSI Name:________________ The test consists of 27 multiple choice questions. Circle the BEST answer and bubble the choice on your scantron form. Bubble the test form (A or B) on your scan tron form. The last page is blank so you can use it as scratch paper. Useful Equations and constants: Page Points 2‐7 75 75 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 × 1023 particles/mol log log Question Multiple Choice Total IR Red Color and Wavelength of Light 800 Wavelength (nm) 400 600 Green Blue 200
UV Page 1 of 8 Fiat Lux (Let there be light) Daylight 4000 3000 2000 1000 0 1000 0 1000 200 400 600 800 Wavelength (nm) 1000 3000 2000 Incandescent Light Bulb: 1. 2. 200 400 600 800 Wavelength (nm) The lambda max ( max) of the sun is approximately A) 200 nm B) 300 nm C) 400 nm D) 500 nm 3. The incandescent light bulb is at________. A) a lower temperature than the sun because the emitted light on average has a longer wavelength B) a higher temperature than the sun because the emitted light on average has a shorter wavelength C) the same temperature as the sun because when you look at the sun and a light bulb they are the same color An incandescent light bulb contains a filament made of tungsten metal. As the spectrum shows, when the tungsten is heated it emits many different wavelengths of light. Why is this? A) Metals contain a sea of electrons so electrons of different energies are ejected. B) Only photoelectrons with energy greater than the threshold energy are ejected. C) Blackbody radiation is the result of random motions of atoms and electrons. D) The wavelengths are different but the energy is the same. Imagine you have a white object and view it outside illuminated by the sun, then inside illuminated by incandescent lights exhibiting the emission spectrum above. How does the object appear when it reflects sunlight or incandescent light? A) The white object appears the same in both situations. B) The white object appears more red under the sun. C) The white object appears more yellow indoors. D) The white object appears more blue indoors. Page 2 of 8 4. Fluorescent Lights Fluorescent lights emit light through a multistep energy transfer process between electrons, mercury (Hg) gas, and a substance called a phosphor. When the light bulb is plugged in, a stream of fast moving electrons collide with the mercury atoms. This causes the electrons in the mercury atoms to move to a higher, excited energy state. When the excited electrons return to the ground state, photons of certain energies are emitted. Ultraviolet photons excite electrons in the white phosphor, which is coated on the inside wall of the glass bulb. The excited electrons in the phosphor transfer a small amount of energy to vibrations, and a large amount of energy is emitted as visible light. Mercury Vapor Lamp 4000 3000 2000 1000 4000 3000 2000 1000 0 Fluorescent Light (with mercury) 5. 6. 7. 8. 0 100 300 500 700 900 Wavelength (nm) 300 400 500 600 700 800 Wavelength (nm) What color is the light emitted at 436 nm? A) red B) yellow C) green D) blue What is the frequency of the photons emitted at 436 nm? A) 3.00 × 108 m sec‐1 B) 6.88 × 105 sec‐1 C) 6.88 × 1014 sec‐1 What is the change in energy (E) in joules for the transition in question 6? B) 4.56 × 10‐28 J C) 3.00 × 108 J A) 4.56 × 10‐19 J Why are the emission lines in the spectrum for the mercury lamp narrow? A) Our eyes can only detect a few different wavelengths of light. B) The energy levels are quantized. C) Mercury atoms can absorb and emit any amount of energy. D) Only electrons of certain energies are ejected. Why are there no photons emitted with wavelengths <400 nm for the fluorescent light? A) Our eyes cannot detect high energy light. B) The energy is transferred to the phosphor causing it to emit visible light. C) Wavelengths of light <400 nm do not have enough energy to eject electrons. D) Mercury atoms do not emit ultraviolet light. Page 3 of 8 9. 10. 11. Electrical energy is converted to light energy in the fluorescent bulb. The process begins with free electrons colliding with mercury atoms. What happens to an electron with energy that does not match the separation of energy levels in mercury? A) Only a portion of the kinetic energy of the electron is transferred to the Hg atom and the electron continues moving but more slowly. B) Nothing. If the energy of the electron does not match the E between the electronic energy levels of Hg atoms then the energy is not absorbed. C) All the electrons are absorbed since the Hg is a black body. D) Some of the energy is absorbed and the excess increases the kinetic energy of the mercury atoms. Why can you touch a fluorescent bulb? A) The bulb is cool because blue light is being emitted. B) The bulb is cool because the mercury atoms are moving slowly. C) The bulb is cool because glass is an insulator. D) The bulb is cool because not much infrared radiation is emitted. Sunscreens and Beer’s law 12. Extinction Coefficient Based on the data, estimate the extinction coefficient () of SPF 50 sunscreen? A) 8000 L g‐1 cm‐1 B) 10000 L g‐1 cm‐1 C) 12000 L g‐1 cm‐1 D) 14000 L g‐1 cm‐1 Sunscreen Absorption at 310 nm
14000 12000 10000 8000 6000 4000 2000 0 0 10 20 SPF 30 40 50 13. 14. How much UV light is trans‐ mitted through a coating of 0.0010 cm SPF 20 sunscreen that contains 5 × 10‐5 g/mL of the active ingredient? A) 20% B) 30% C) 40% D) 50% SPF 25 sunscreen costs approximately $0.75 an ounce while SPF 50 costs $1.50 an ounce. Is it worth it to pay twice as much for SPF 50? A) Yes. You can use half the amount of SPF 50 because it absorbs twice as much. B) No. It is cheaper to put on a 20% thicker layer of SPF 25 for the same effect. C) No. SPF 25 is just as effective for lower cost. Page 4 of 8 Chlorofluorocarbon Molecules Chlorofluorocarbon molecules have a number of uses, especially in refrigerators and spray cans such as deodorants and air fresheners. These gaseous molecules are not very reactive and remain in the atmosphere for more than 10,000 years. This is of concern because these molecules absorb infrared light, and thereby contribute to global warming. Use the infrared data in the table to answer the questions below. Bond O−H C−H C−F C−Cl C−Br C=O 15. 16. 17. Bond length 96 pm 109 pm 135 pm 177 pm 194 pm 120 pm Bond energy 366 kJ/mol 413 kJ/mol 488 kJ/mol 330 kJ/mol 288 kJ/mol 799 kJ/mol IR stretching frequency 3500−3200 cm-1 3000−2850 cm-1 1400−1000 cm-1 850−550 cm-1 An estimate of the C‐Br stretching frequency is A) 690‐515 cm‐1 B) 1300‐1000 cm‐1 C) 1820‐1670 cm‐1 An estimate of the C=O (C double bond O) stretching frequency is A) 690‐515 cm‐1 B) 1300‐1000 cm‐1 C) 1820‐1670 cm‐1 Infrared Spectrum The infrared transmission spectrum shown below is most likely due to: A) CCl3F B) CH2F2 C) CF4 D) A mixture of HF and CF4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 3500 2500 1500 500 18. Absorption of infrared radiation contributes to global warming because: A) the speed of infrared photons causes molecules to vibrate B) an increase in vibrational energy of the molecules in the atmosphere raises the temperature C) the long wavelength of infrared radiation is transmitted over long distances D) infrared photons can transfer part of their energy to excite vibrations Page 5 of 8 19. Properties of Atoms Use the ionization energy data below to answer the following questions. 2nd IE 3rd IE 1st IE Potassium, K 419 kJ/mol 3051 kJ/mol 4412 kJ/mol Scandium, Sc 631 kJ/mol 1235 kJ/mol 2398 kJ/mol Nickel, Ni 737 kJ/mol 1753 kJ/mol 3395 kJ/mol Zinc, Zn 906 kJ/mol 1733kJ/mol 3833 kJ/mol 20. Which atom requires the least amount of energy to form a +3 ion? A) K B) Sc C) Ni D) Zn 21. The second electron in nickel is easier to remove compared with the second electron in potassium because A) there are two electrons in the 4s orbital in nickel B) energy is released when potassium forms a +1 ion C) the effective nuclear charge on the valence electrons in nickel is larger D) the effective nuclear charge on the valence electron in potassium is larger 22. The electron attachment energy of Cl is ‐349 kJ/mol. Is energy required or released when one electron is transferred from K(g) to Cl(g)? A) Energy is released. Both atoms have noble gas configurations. B) Energy is released. Chlorine has a higher effective nuclear charge. C) Energy is required. It takes energy to put an electron on chlorine. D) Energy is required. It takes energy to remove an electron from potassium. Deuterium is a hydrogen atom with 1 proton and 1 neutron in the nucleus. The O‐D stretching frequency is about 2600 cm‐1 while the O‐H stretch is about 3500 cm‐1. The best explanation for the decrease in the frequency of the stretching frequency is that: A) the bond is longer because deuterium atoms are larger B) the bond is stronger because the deuterium nucleus attracts electrons more strongly C) the mass is larger so the energy of the vibration is smaller D) the spin of the deuterium atom is different from that of the hydrogen atom Page 6 of 8 Atomic Orbital Energies Use the orbital energies in the table to answer the questions below. 1s lithium, Li beryllium, Be boron, B carbon, C nitrogen, N 23. 24. 25. 26. 27. −6512 kJ/mol −12419 kJ/mol −20191 kJ/mol −29721 kJ/mol −41011 kJ/mol 2s −526 kJ/mol −814 kJ/mol −1287 kJ/mol −1838 kJ/mol −2468 kJ/mol −211 kJ/mol −342 kJ/mol −473 kJ/mol 2p The 2s orbital on lithium has A) 0 nodes B) 1 node C) 2 nodes D) 3 nodes The photoelectron spectrum of beryllium has A) 1 peak B) 2 peaks C) 3 peaks Boron atoms can absorb light with energy A) 211 kJ/mol B) 220 kJ/mol C) both energies D) 4 peaks D) neither energy How much energy is required to remove 2 electrons from carbon? A) 342 kJ/mol B) 684 kJ/mol C) >684 kJ/mol D) between 342 and 684 kJ/mol Imagine that an electron in lithium is excited to the 3p orbital. The energy level of the excited electron is ‐144 kJ/mol. What is the energy required to excite an electron from the 2s to the 3p orbital? A) 382 kJ/mol B) 450 kJ/mol C) both energies D) neither energy Page 7 of 8 (This page is intentionally left blank to serve as scratch paper.) Page 8 of 8 ...
View Full Document
This note was uploaded on 02/13/2011 for the course CHEM 1A taught by Professor Nitsche during the Spring '08 term at Berkeley.
- Spring '08