Name Student Number CHEMISTRY 140 FINAL EXAM December 10, 2002 Numerical answers must be given with appropriate units and significant figures. Please place all answers in the space provided for the question. IF more space is required, write on the back of the question sheet and indicate this in the answer space. Marks for each question are as indicated in [] brackets. There are 145 marks available and 180 minutes (3 hours) to complete the exam. It is likely that the exam will be graded out of 130 (approximately) so it is possible to not do one question and not be penalized unnecessarily. The actual 100% mark will depend on class performance. Read the examination completely before you start writing. Leave sufficient time at the end of the exam to review your answers. If multiple answers are given for a question the FIRST answer will be graded. Do not use red ink on the exam. Red ink will not be graded.. PRINT YOUR FULL NAME NEATLY ON THE PAPER NOW. Question #1 / 10 Marks Question #7 /10 Marks Question #2 / 15 Marks Question #8 / 10 Marks Question #3 / 15 Marks Question #9 / 15 Marks Question #4 / 12 Marks Question #10 / 15 Marks Question #5 / 10 Marks Question #11 / 23 Marks Question #6 / 15 Marks Total / 145 Marks
2 1. Sodium thiosulfate, Na 2 S 2 O 3, is used as a fixer in black-and-white photography. Assume you have a bottle of sodium thiosulfate and want to determine its purity. The thiosulfate ion can be oxidized with I 2 according to the equation: I 2 (aq) + 2 S 2 O 3 2- (aq) 2 I - (aq) + S 4 O 6 2- (aq) This reaction occurs rapidly and quantitatively, and a simple method exists for observing when the reaction has reached the equivalence point. It can be used, therefore, as a method of analysis by titration. If you use 40.21 ml of 0.246 M I 2 in a titration, what is the weight percent of Na 2 S 2 O 3 in a 3.232 g sample of impure material? 2. Suppose you burn exactly 1500 g of benzoic acid, C 6 H 5 CO 2 H, in a combustion calorimeter and find that the temperature of the calorimeter increases from 22.50 to 31.69 C. The calorimeter contains 775 g of water, and the bomb has a heat capacity of 893 kj/k. How much heat is evolved per mole of benzoic acid? [15 marks] 3. You have a 550 ml tank of gas with a pressure of 1.56 atm at 24.0 C. You thought the gas was pure carbon monoxide but later found it was contaminated by small quantities of gaseous CO 2 and O 2. Analysis shows that the tank pressure is 1.34 atm (at 24.0 C) if the CO 2 is removed. Another experiment shows that 0.0870 g of O 2 can be removed chemically. What are the masses of CO and CO 2 in the tank? [15 marks] 4. Write Lewis structures and sketch and identify the molecular shapes for these molecules. Show formal charges if they are present. Estimate the direction of the molecular dipole in each case. [12 marks] SF 4 ClF 5
3 5. Calcium carbide, CaC 2, contains the acetylide ion, C 2-2. General molecular orbital energy level diagrams are provided below. Label the first molecular orbital energy level diagram for the ion using the 1s, 2s and 2 p orbitals as the basis set. How many net σ and π bonds does the C 2- ion have? What is the carbon-carbon bond order? How has the bond order changed on adding electrons to C 2 to obtain C 2-2- 2? Is the C 2 ion paramagnetic or diamagnetic? Names of orbitals C 2 C 2 2- How has bond order changed? Net σ bonds in C 2 2- = Net π bonds in C 2 2- = Bond order in C 2 2- = Is C 2 2- diamagnetic or paramagnetic?
4 6. Iridium (Atomic number 77) has a face centered unit cell and a density of 22.56 g/cm 3. Use this information to calculate the radius of an atom of iridium. [15 marks] 7 Given the bond-dissociation energies: nitrogen-to-oxygen bond in NO, 631 kj/mol; H-H in H 2, 436 kj/mol; N-H in NH 3, 389 kj/mol; O-H in H 2 O, 463 kj/mol; estimate H for the reaction: 2 NO(g) + 5 H 2 (g) 2 NH 3 (g) + 2 H 2 O(g) 8. Using VSEPR theory, predict the geometric shape of the carbonate ion (CO 3 2- ). Describe the shape in words and show formal charge and bond order. 9. Allene, H 2 CCCH 2, is a hydrocarbon with three carbon atoms attached to one another and the four hydrogens located on the teminal atoms as shown in the way the molecule is written. (The following page is provided for your rough work. [15 marks] a) Write a lewis structure for allene. Is there a reasonable alternative resonance structure for this molecule? What is the geometric shape around each carbon? [5] b) Propose a hybridization scheme for the carbon atoms in the molecule. Sketch the molecule showing the hybrid orbitals. Label the orbitals to make your diagram clear. [5] c) Based on your hybridization scheme and your knowledge of geometry around each hybridized carbon, predict whether the H-C-H plane at one end of the molecule is coplanar with the other H-C-H plane, or whether they are orthogonal to one another. Explain your answer briefly. [5]
5 10. Given the following information: 1/2 N 2 (g) + 3/2 H 2 (g) NH 3 (g) H 1 NH 3 (g) + 5/4 O 2 (g) NO(g) + 3/2 H 2 O(l) H 2 H 2 (g) + 1/2 O 2 H 2 O(l) H 3 Write an algebraic expression for H for the following reaction, expressed in terms of H 1, H 2, and H 3. N 2 (g) + O 2 (g) 2 NO(g) H =? [15 marks] 11. Short answer questions. [18 marks] a) Calculate the quantity of heat, in kilojoules, required to raise the temperature of 9.25 L of water from 22.0 C to 37.0 C. [2] b) Identify the oxidizing and reducing agents in the following reaction: [2] 2 Fe 2+ (aq) + OCl - (aq) + H 2 O(l) 2 Fe 3+ (aq) + Cl - (aq) + 2 OH - (aq) c) Match each of the following molecules with one of the hybridization schemes. The central atoms are written first [4] a) sp PF 6 b) sp 2 COS c) sp 3 SiCl 4 d) sp 3 d - NO 3 e) sp 3 d 2 AsF 5 d) Use the spdf notation to write the complete electron configuration for antimony (Sb). Do not use the Noble gas core method, write it out completely. [2]
6 e) For each of the following pairs, circle the atom that has the larger size. [2] (i) Te or Br (ii) K or Ca (iii) Ca or Cs (iv) Al or Au f) In what type of material is the energy gap between the valence band and the conduction band the greatest: metal, semiconductor or insulator? Explain. [2] g) The structure of lithium sulfide is shown below. What is the chemical formula of the unit cell? [2] h) In the above structure, the structure can be described as some kind of lattice of S 2- with Li + filling some kind of holes. What kind of lattice and what kind of holes would be described? [2]
7 Periodic Table of the Elements 1A 59-140 Fall 199 8A 1 H 1.008 2A 3A 4A 5A 6A 7A 2 He 4.003 3 Li 6.941 4 Be 9.012 5 B 10.81 6 C 12.01 7 N 14.01 8 O 16.00 9 F 19.00 10 Ne 20.18 11 Na 22.99 12 Mg 24.31 3B 4B 5B 6B 7B 3B 1B 2B 13 Al 26.98 14 Si 28.09 15 P 30.97 16 S 32.07 17 Cl 35.45 18 Ar 39.95 19 K 39.09 20 Ca 40.08 21 Sc 44.96 22 Ti 47.88 23 V 50.94 24 Cr 52.00 25 Mn 54.94 26 Fe 55.85 27 Co 58.93 28 Ni 58.69 29 Cu 63.55 30 Zn 65.39 31 Ga 69.72 32 Ge 72.61 33 As 74.92 34 Se 78.96 35 Br 79.90 36 Kr 83.30 37 Rb 85.47 38 Sr 87.62 39 Y 88.91 40 Zr 91.22 41 Nb 92.91 42 Mo 95.94 43 Tc (98) 44 Ru 101.7 45 Rh 102.91 46 Pd 106.42 47 Ag 107.87 48 Cd 112.41 49 In 114.82 50 Sn 118.71 51 Sb 121.76 52 Te 127.60 53 I 126.90 54 Xe 131.29 55 Cs 132.91 56 Ba 137.33 57 La 138.91 72 Hf 178.49 73 Ta 180.95 74 W 183.85 75 Re 186.21 76 Os 190.20 77 Ir 192.22 78 Pt 195.08 79 Au 196.97 80 Hg 200.59 81 Tl 204.38 82 Pb 207.20 83 Bi 208.98 84 Po (209) 85 At (210) 86 Rn (222) 87 Fr (223) 88 Ra 226.03 89 Ac 227.03 104 Rf (261) 105 Ha (262) 106 Sg (263) 107 Ns (262) 108 Hs (265) 109 Mt (266) 110 111 Lanthanides 58 Ce 140.12 59 Pr 140.91 60 Nd 144.24 61 Pm (145) 62 Sm 150.36 63 Eu 151.97 64 Gd 157.25 65 Tb 158.93 66 Dy 162.50 67 Ho 164.93 68 Er 167.26 69 Tm 168.93 70 Yb 173.04 71 Lu 174.97 Actinides 90 Th 232.04 91 Pa 231.04 92 U 238.03 93 Np 237.05 94 Pu (244) 95 Am (243) 96 Cm (247) 97 Bk (247) 98 Cf (251) 99 Es (252) 100 Fm (257) 101 Md (258) 102 No (259) 103 Lr (260) electronic charge, e = 1.602 x 10-19 C Avogadro's number, N = 6.022 x 10 23 mol -1 Planck s constant, h = 6.6261 x 10-34 J s electron rest mass, m e = 9.109 x 10-28 g Faraday's constant, F = 9.649 x 10 4 C mol -1 Speed of light, c = 2.998x10 8 m s -1 proton rest mass, m p = 1.673 x 10-24 g Boltzmann constant, k B = 1.3805 x 10-23 J K -1 Specific heat of H 2 O(l), c H2 O = 4.184 J g -1 K -1 Density of H 2 O δ H2 O = 1.00 g/cm 3 Rhydburg constant, R H = Rhc = 2.179 x 10-18 J Gas constant, R = 8.314 J mol -1 K -1 Rhydburg constant, R=1.097 x 10 7 m -1 Gas constant, R = 0.08206 L atm mol -1 K -1 Atmospheric pressure 1 atm = 101.325 kpa 273.15 K is equivalent to 0 C = 760 mm Hg