CHEMISTRY 101 Hour Exam III December 7, 2006 Adams/Le Name KEY Signature T.A./Section Success means having the courage, the determination, and the will to become the person you believe you were meant to be. George Sheehan This exam contains 17 questions on 6 numbered pages. Check now to make sure you have a complete exam. You have one hour and fifteen minutes to complete the exam. Determine the best answer to the first 15 questions and enter these on the special answer sheet. Also, circle your responses in this exam booklet. Show all of your work and provide complete answers to questions 16 and 17. 1-15 (30 pts.) 16 (12 pts.) 17 (18 pts.) Total (60 pts) Useful Information: 1.000 L = 1000.0 ml Always assume ideal behavior for gases (unless explicitly told otherwise). PV = nrt R = 0.08206 L atm/mol K K = C + 273 STP = standard temperature and pressure = 0 C and 1.00 atm N A = 6.022 x 10 23 Solubility Rules: 1. Most nitrate salts are soluble. 2. Most salts of sodium, potassium, and ammonium cations are soluble. 3. Most chloride salts are soluble. Exceptions: silver(i), lead(ii), and mercury(i) chloride. 4. Most sulfate salts are soluble. Exceptions: calcium, barium, and lead (II) sulfate. 5. Most hydroxide salts can be considered insoluble. Soluble ones: sodium, potassium, and calcium hydroxide. 6. Consider sulfide, carbonate, and phosphate salts to be insoluble. Soluble ones: sodium and potassium.
Hour Exam III Page No. 1 1. Which scientist is still credited today for determining how an electron moves around the nucleus of an atom? a) Ernest Rutherford b) J.J. Thomson c) John Dalton d) Niels Bohr e) No scientist is credited for this discovery because we do not know exactly how electrons travel in an atom. 2. Which of the following elements has the largest ionization energy? a) P b) Al c) d) Ba e) K 3. Rank the following from smallest to largest atomic radius: Ar, S 2, Ca 2+, K +,. a) Ca 2+ < K + < Ar < < S 2 b) Ar < K + < Ca 2+ < S 2 < c) Ar < < S 2 < Ca 2+ < K + d) K + < Ca 2+ < Ar < S 2 < e) S 2 < < Ar < K + < Ca 2+ 4. For the titration of 3.00 M hydrochloric acid with 2.00 M sodium hydroxide, what volume of sodium hydroxide would be required to react with 1.00 L of hydrochloric acid to reach the endpoint? a) 0.667 L b) 1.50 L c) 2.00 L d) 3.00 L e) 6.00 L --------------------------------------------------------------------------------------------------------------------- Consider the following compounds to answer questions 5 8. HCN NH 4 + SF 6 N 2 5. How many of the compounds above contain a central atom that obeys the octet rule? a) 0 b) 1 c) 2 d) 3 e) 4 6. How many of the compounds above exhibit resonance? a) 0 b) 1 c) 2 d) 3 e) 4 7. Which compound exhibits London dispersion forces? a) HCN b) NH 4 + c) SF 6 d) N 2 e) All of the above exhibit London dispersion forces.
Hour Exam III Page No. 2 8. Which compound has bond angles of 120 around the central atom? a) HCN b) NH 4 + c) SF 6 d) N 2 e) At least two of the above have bond angles of 120. ---------------------------------------------------------------------------------------------------------------- 9. How many of the following statements are true concerning an electron in its ground state? I. The electron must be in its lowest-energy state. II. Energy must be applied to the electron in order to excite it. III. The electron must be located on the nucleus of an atom. IV. The electron can release energy to obtain a lower ground state. a) 0 b) 1 c) 2 d) 3 e) 4 10. Liquid water turns to ice. Is this process endothermic or exothermic? Choose the best answer. a) endothermic; The water absorbed heat and got colder, therefore forming ice. b) endothermic; Energy in the form of heat was given off by the water to become colder and form ice. c) exothermic; The water released energy, slowing the water molecules down to form solid ice. d) exothermic; Heat was absorbed by the water, moving its molecules faster to condense on an object and form ice. e) neither endothermic nor exothermic; There was no energy transfer in or out of the water to form ice. 11. Which of the following compounds has the highest boiling point? a) H 2 b) Mg 2 c) C 2 d) CH 4 e) SF 4 12. Choose the bond that is the most polar. a) N N b) C c) P S d) Sr e) Fe P
Hour Exam III Page No. 3 13. When an electron is excited in an atom or ion a) the electron moves further from the nucleus. b) only specific quantities of energy are released in order for the electron to return to its ground state. c) white light is never observed when the electron returns to its ground state. d) the electron is only excited to certain energy levels. e) All of the above statements are true when an electron is excited. 14. Which atom or ion has the smallest radius? a) 2+ b) + c) d) e) 2 15. Which of the following solutes will generally not dissolve in the specified solvent? Choose the best answer. (Assume all of the compounds are in the liquid state.) a) C 4 mixed with water (H 2 ) b) NH 3 mixed with water (H 2 ) c) CH 3 H mixed with water (H 2 ) d) N 2 mixed with methane (CH 4 ) e) C 2 mixed with methane (CH 4 )
Hour Exam III Page No. 4 4 points 16. a) State whether each electron configuration is possible or not possible. (The elements could be in a ground state or excited state.) Justify each answer. For those that are possible, state what the element is. For those that are not possible, give a corrected electron configuration that is possible. 1s 2 2s 2 2p 7 Not Possible; There can only be 6 electrons in the p orbitals. ne corrected electron configuration is 1s 2 2s 2 2p 6 3s 1 [Ne] 3s 2 3p 3 Possible; the element is phosphorus 1s 2 2s 2 2p 6 2d 7 1s 2 2s 1 2p 3 Not Possible; The d orbitals do not exist until the 3 rd energy level. ne corrected electron configuration is 1s 2 2s 2 2p 6 3s 2 3p 5 Possible; the element is carbon in the excited state 3 points b) Fill in the energy level diagram for the ground-state of nitrogen. Justify your placement of electrons in the energy level diagram. The electrons fill closest to the nucleus first (lower in energy). In the 2p orbital, the electrons spread out between the three different orientations because the electrons have negative charges so they repel each other. 5 points c) If nitrogen forms bonds with potassium, what type of bonds are these (ionic, polar covalent, or covalent)? What is the resultant electron configuration for each element when these bonds occur? Are these bonds polar or nonpolar? Why? The bonds are ionic. K + = 1s 2 2s 2 2p 6 3s 2 3p 6 N 3 = 1s 2 2s 2 2p 6 These bonds are polar because there is a large difference in electronegativity betweem potassium and nitrogen which creates an unequal electron distribution in the bond.
Hour Exam III Page No. 5 10 points 17. a) Fill in the table below with the appropriate information. For the Lewis structures, be sure to make your drawings clear and make your dots visible (or else you will not receive credit). In addition, make sure you give a brief justification as to why each molecule is polar or nonpolar overall. Formula Lewis Structure Geometry Molecular Shape Polar or Nonpolar? (with justification) Xe 4 Xe tetrahedral tetrahedral Non-polar (equal electron distribution throughout; dipoles cancel) NF 3 F N F F tetrahedral pyramid Polar (uneven electron distribution; dipoles do not cancel; more electron density towards fluorines) I 3 I bipyramid T-shape Polar (uneven electron distribution; dipoles do not cancel; more electron density towards chlorines) BH 3 H H B H planar planar Non-polar (equal electron distribution throughout; dipoles cancel) KrBr 4 Br Br Kr Br Br octahedral square planar Non-polar (equal electron distribution throughout; dipoles cancel)
Hour Exam III Page No. 6 b) Give an example of a linear molecule that is polar. Explain why it is polar even though the bond angle is 180, along with a complete Lewis structure of your molecule. 4 points Answers will vary. Make sure it is a valid Lewis structure that is actually linear and polar. The molecule is polar because there is uneven electron distribution throughout (the dipole(s) do not cancel due to the differences in electronegativity between the atoms). c) ne Lewis structure of C 4 is C According to the VSEPR model, the actual bond angles around the central atom are 109.5 (meaning C 4 is tetrahedral in shape). Why aren t the bond angles 90 as indicated in the Lewis structure above? (In other words, why isn t C 4 square planar in shape?) 4 points Electrons repel each other so the atoms arrange themselves around the carbon to maximum the distance between the chlorine atoms (where the electron pairs are) and minimize repulsions.