Chem11 Exam 3 Final

Name:________________________________ Section (circle): 1 2 3 4 5 Chemistry 11, Fall 2007 Cumulative Final Exam December 17, 2007 9:00 AM – 12:00 PM As always, full credit will not be given unless you have written down the reasoning or calculations you used to obtain the correct answer. Work on the back of pages will not be graded! Pay attention to significant figures. Please check now that your exam has seventeen pages (including this one). A periodic table and a list of formulas and electronegativities are attached at the back of the exam. If you finish early, just leave your completed exam on the front desk. If you have a question, someone will be checking in once every 30 minutes or so. You have three hours to complete this exam. While you are waiting to start, you may answer the extra credit question below: It is against the honor code at Amherst College to either give or receive help on this exam. The work you turn in must be yours and yours alone. Xtra credit: What is Professor Patricia B. O’Hara’s middle name? (circle one) 2 pts a) Barbara b) Bennington c) Bernadette d) Beryl Xtra credit question 2; What is YOUR favorite element and why? (2 pts)           Question Points XC1 02 XC2 02 I 25 II 30 III 15 IV 25 V 25 VI 25 VII 15 Total 164 Score     2 I. Hybridization: (25 points) American Chemical Society Molecule of the Week (June 11, 2007) “Gallic Acid, or 3,4,5trihydroxybenzoic acid, is produced by plants such as sumac, tea leaves, oak bark, and witch hazel; it provides the astringent property and anti-hemorrhoid activity of witch hazel. Derivatives are used in the production of azo dyes and photographic developers. It gets its name from gallnuts, which are the swelling of plant tissue in response to parasitic infection.” Assume that hydrogens are removed from all four OH groups of gallic acid. Redraw the molecule in the space provided above and use it to answer the following questions. (Don’t forget to include implicit hydrogens and lone pairs of electrons!) a) Mark sp3 hybridized atoms with a * sign, sp2 hybridized atoms with a # sign and sp hybridized atoms with a @ sign. (2.5 points) b) There are _________ sp2 hybridized atoms for which the geometry is ________ and the bond angles are _______. (5 points) c) Consider the dipole moments of all bonds in this molecule. Circle the most electropositive carbon atom in the molecule in the original gallic acid structure. (2.5 points) 3 d) Do you think this molecule has resonance structures? If yes, draw one possible structure. (5 points) e) Draw all of the sp2 hybridized orbitals in the molecule and their overlap to show the σ bonds. Is this molecule planar? (5 points) f) Draw all unhybridized pz orbitals of the molecule to show where the electrons would be delocalized. Would this delocalization have an effect on the bond lengths of the two CO bonds on the upper right? (5 points) 4 II. Molecular Orbital Model (30 points) The 2007 Nobel Prize in Chemistry was awarded to German chemist Gerhardt Ertl for his studies of chemical processes on solid surfaces. One of the reactions he studied is the oxidation of carbon monoxide on a Pt surface shown in the electron micrograph image at right as occurring in cyclical waves. CO itself is an odorless, colorless gas that is associated with incomplete combustion and leads to many human deaths each year. There is much commercial interest in creating devices such as catalytic converters to convert the carbon monoxide to carbon dioxide using molecular oxygen. 1. Write a balanced chemical equation for this process, indicating the species oxidized and reduced. (2 points) 2. Draw the Lewis structures for carbon monoxide, molecular oxygen, and carbon dioxide (including formal charge) and use it to answer the following questions. (5 points) CO O2 CO2 a. What are the predicted bond orders from these Lewis Structures: (i.e. single, double, triple)? ____________ _____________ _______________ b. According to the Lewis structures, which of the molecules above (if any) are predicted to be paramagnetic? __________ c. According to the Lewis structures, which of the molecules above will have the longest bond? _____________ 5 3. In order to construct molecular orbitals for homomuclear diatomics such as C2, O2, and the heteronuclear molecule CO, one must combine atomic orbitals to make molecular orbitals. a) What are the electron configurations (using σ, σ*, π ,π*nomenclature) and bond orders for these three diatomic molecules? (3 points) b) C2 shows sp mixing and O2 does not. Construct the energy level diagrams for C2 and O2 in the two rectangular boxes below, showing atomic orbitals energy levels on the left and right side of each rectangle and the molecular orbital energy level in the center. Add electrons. (6 pts) C2 O2 c) Describe what happens to the energies of the two lowest energy orbitals (σs and σs*) in C2 as a result of sp mixing. (4 points) 6 d) CO is observed to show sp mixing. Reconstruct its energy level diagram in the rectangular box at left, paying particular attention to the relative energy orderings of the C and O atomic orbitals. Insert electrons. At right, draw orbital pictures for the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) for CO. (6 pts) CO e) In the reaction studied by Ertl both CO and O2 must first bind noncovalently to a platinum metal surface. List two properties of each of the small molecules (CO and O2) that will affect their binding to the metal (Pt) surface. (4 pts) CO __________________________________ ___________________________________ O2 __________________________________ ___________________________________ 7 III. Transition Metal Chemistry (15 points) Spectrochemical Series: Large splitting: CN- > NO2- > en > NH3 > H2O > OH- > F- > Cl- > Br- > I- Small splitting 1. Draw the crystal field splitting of the d orbitals for each of the complex ions below and insert electrons. Only one of the complexes is paramagnetic. (6 points) [Fe(CN)6]-4 [Fe(H2O)6]+2 2. How does the splitting of the d orbitals affect the color of each of the complex ions above? (3 points) 3. Suggest a method that would help you to distinguish between cis-Pt(NH3)2Cl2 and transPt(NH3)2Cl2. (3 points) 4. Explain why Nb and Ta have approximately the same atomic radius DESPITE the fact that Nb is in the 5th period and Ta is in the 6th period. (3 points) 8 IV. Concentration and Mass Spectroscopy (25 pts) 1. Shown above are levels of lead and copper in the Town of Amherst Drinking Water. The second column reports the MCLG, or maximum contaminant level goal and the third column reports the highest level measured over the year. Convert the contaminant level reported in the third column from ppb or ppm to molarity (moles per liter). (5 points) 2. The Town of Amherst reported that drinking water in Amherst in 2006 contains nitrate ions, [NO3]-1, which contaminate drinking water from fertilizer run-off. a) Draw the Lewis structure for the nitrate ion, including formal charge on the atoms and draw resonance forms that are consistent with all three N-O bonds being the same length. (6 point) b) Next, the report cited that the analysis didn’t measure the nitrate directly, but instead measured the nitrogen in the nitrate, and that was reported as 2.5 ppm and cited as being within the safe limits. Calculate the weight percent of nitrogen in nitrate, and from this number, the ppm of nitrate itself in the drinking water. (8 points) 9 3. The town drinking water report also included the levels of chlorinated, fluorinated and brominated hydrocarbons, those nasty substances left over from pesticides and cleaning solvents. Below are the mass spec for three of the hydrocarbons: Identify which mass spec belongs with which substance using the knowledge that F has only one predominant isotope, Cl exists as two isotopes, 35Cl and 37Cl with natural abundances of 0.75 and 0.25 and the Br exists as two isotopes, 79Br and 81Br with roughly equal natural abundances. (6 points) Difluorobenzene C6H2F4 150.07 g/mole Dichlorobenzene C6H4Cl2 147.00 g/mole Bromobenzene C6H5Br 157.01 g/mole Identification____________ Why? Identification____________ Why? Identification: _______________ Why? 10 V. Gases (25 points) First: Take a deep breath, inhale, exhale. Again, inhale, exhale. Now go ahead: Partial Pressure of Various Components in the Air You’re Breathing Now. inhale exhale H2 O Variable 47 mmHg CO2 0.3 mmHg 40 mmHg O2 159 mmHg 105 mmHg N2 593 mmHg 568 mmHg Total 760 mmHg 760 mmHg 1. Calculate the mole fractions of carbon dioxide and oxygen in the inhaled and exhaled breath. (6 points) 2. Calculate the number of moles of each gas (excluding water) in one deep breath (about 12.5 l) at STP. Repeat these calculations for the exhaled breath. (6 points) 11 3. Calculate the rms speeds of the water vapor and carbon dioxide in your exhaled air at STP. (4 points) 4. Neither of these two gases is very ideal. Water condenses to liquid around 0o C and has lots of attractive forces due to its intrinsic dipole. Carbon dioxide condenses to a solid around –80oC and though it doesn’t have a dipole, is much larger than water. Shown below are the a and b coefficients for the two gases. Which gas is which and why? (4 points) a coefficient (atmL2mol/L2) b coefficient IDENTITY (L/mole) 3.61 0.0449 ________ 5.46 0.0305 ________ Why___________________________________________________________ _______________________________________________________________ 5. The enrichment of carbon dioxide in the exhaled air is due to the metabolism (controlled combustion by the enzymes in your body) of pure sugar such as the gummy bears on the snack table in the front of the exam room. If each gummy bear weighs 5 g and is 40.0% carbon, how many breaths would it take you to exhale the carbon content of the gummy bear? (5 points) 12 V. Classical and Quantum Mechanics (25 points) A classic children’s story is about a fictional character, a young boy named Peter Pan, who can fly and has found out how to stay young forever. He lives in a land with other lost boys called Never Never land and he likes to spy on human families. He has a friend who is a fairy, whose name is Tinker Bell. Once when spying on a human family, he had to escape quickly and in so doing, he lost his shadow. How terrible is that? This question asks you to use quantum mechanics to understand Peter Pan’s shadow and how Peter Pan might be able to fly. Pan’s Shadow Photo credit to KL 1. In the box below, sketch the physical set-up by which shadows are cast by large objects like people. Locate the light source, the object (Peter, assume he is 1 m tall and 0.5 m wide), and the screen on which you would see the shadow. Why does Peter Pan’s shadow (or your own shadow) not show diffraction? (4 pts) 2. Another fictional character in this story is a fairy named Tinkerbell. She is extremely small, let’s imagine she is 6.6 x 10-7 m in diameter. What happens to her shadow when she becomes the object imaged (instead of Peter Pan) in your sketch above? (4 pts) 13 3. It is necessary in this story for Peter Pan to “fly” home. DeBroglie argues that P. Pan might be able to fly home if his wave length, λ, = distance to Peter’s home = 2.4 km. The boy has a mass of 22.0 kg, and flies with a velocity of 1.00 x 102 m/s. Calculate λ and see if DeBroglie can get P. Pan home. (6 points) λ= Will this get P. Pan home? Comment. 4. Heisenberg believes that P. Pan can get to his home 2.4 km away if the uncertainty in his position, ∆x, = distance home. He says P.Pan’s velocity (1.00 x 102 m/s) has an uncertainty of 0.01%. Calculate ∆x and see if Heisenberg can get P. Pan home. (6 points) ∆x = Will this get P.Pan home? Comment. 5. Maybe Tinkerbell can get Peter back home. Given the values above for velocity (1.00 x 102 m/s), the uncertainty in velocity of 0.01%), and uncertainty in the distance of 2.4 km, calculate the mass Tinkerbell would have to be in order to make Heisenberg correct. (5 points) m= Comment on the magnitude of this mass. 14 VII. Lab Questions (15 points): a) Write the complete electron configuration of an atom (not an ion) of cobalt. (1 point) b) What is the oxidation number of Co within CoCl2.6H2O? (1 point) c) What is the complete electron configuration of a Co ion in the oxidation state described in (b)? (1 point) d) Describe what happens in words when one mole of [Co(en)2Cl2]Cl is treated with excess silver nitrate. (2 points) e) Balance the equation: (3 points) ____CoCl2.6H2O + _____HCl + _____H2O2 + _____en → _____trans-[Co(en)2Cl2]Cl + _____H2O f) Draw the trans- and the cis-isomers of [Co(en)2Cl2]Cl. (you are expected to know the chemical formula of en, if you don’t, then perhaps knowing that it is ethylene diamine will help.) (6 points) g) The terms cis- and trans refer to what kind of isomerism? (1 points) 15 Assorted Equations, Constants, and Conversion Factors         [M]  =  moles/liter   pH  =  -­‐log[H+]   pH  +  pOH  =  14   M1V1  =  M2V2  =  #  moles   PV  =  nRT   Pa = χaPtotal χa = na/ntotal [Pobs + a(n/V)2](V-nb) = nRT           e = 1.60218 × 10–19 C Fundamental charge: NA = 6.022 × 1023 mol–1 1 J = 1 N m = 1 kg m2 s–2 1 kJ = 103 J E = hν       c = 2.9979 × 108 m s–1 R  =  0.08216  l·atm/(mole·K)         R  =  8.3145  J⋅mol-­‐1  K-­‐1 STP≡  1.000  atm,  273.15  K c = λν   deBroglie:  λ  =  h/mv     2 K.E. = ½(mev ) = hν − hν Bohr:  En  =  -­‐2.178  x  10-­‐18  J  (Z2/n2)         0   NA  =  6.022137  ×  1023   a0  =  0.52917725  ×  10-­‐10  m   π  =  3.14159     c = 2.9979 × 108 m/s   h = 6.626 × 10–34 J·s me = 9.10939 × 10–31 kg mp = 1.673 × 10–27 kg mn = 1.675 × 10–27 kg   1  atm  =  760  mmHg  =  760  torr 1 nm = 10-9 m     1 kg = 103 g ΔE  =  Ef  -­‐  Ei  =  -­‐2.178  x  10-­‐18  J  Z2  (1/nf2  -­‐  1/ni2)     Heisenberg  Uncertainty: ΔE(Δt)  ≥  h/4π Δx(mΔv)  ≥  h/4π Formal Charge = Group Number – [1/2 Nbonding electrons + Nnonbonding electrons] Bond Order = ½ [Bonding Electrons – Antibonding Electrons]     H 2.20 Li Be B C N O F 0.98 1.57 2.04 2.55 3.04 3.44 3.98 Na Mg Al Si P S Cl 0.93 1.31 1.61 1.90 2.19 2.58 3.16 K Ca Ga 0.82 1.00 1.81 As 2.18 Br 2.96 Rb Sr In 0.82 0.95 1.78 Sb 2.05 I 2.66 Cs Ba 0.79 0.89 Table I. Electronegativies of selected elements 16 17