Name: Grading key page 1/10

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1 Name: Grading key page 1/10 Chemistry 125/126, Exam 1 Tuesday, November 9, 2010 Welcome to the first hourly exam for Chemistry 125/126. This exam contains (page 10). A few notes about the exam. To receive credit show your calculations; you will not receive credit for numerical answers alone. The exam should take about l hour, however, you may use up to 1.5 hours to complete the exam. Save any questions GSIs have been told not to answer questions during the exam. Your graded exam will be available starting tomorrow. If, after checking the exam key, you feel there was a mistake in grading, please turn your exam in to Prof. Kerner (mailbox 1500o chem.) with a statement describing the mistake. Regrades must be turned in by noon, Monday, Nov. 22, Course Information Section GSI Section GSI Section GSI 111 M AM Hung Ki Yoon 151 TPM Shi Jin 249 Th AM Christina Franzese 113 M AM Kristen Ko 211 W AM Wen Wen 251 Th AM Ben Coupland 115 M AM Mallory Van Dongen 213 W AM Akiko Kochi 311 Th PM Mahmoud El Azzouny 117 M AM Deidra Gerlach 215 W AM Kevin Hartman 313 Th PM Akiko Kochi 119 M AM Lauren Soblosky 217 W AM Deidra Gerlach 315 Th PM Catherine Mocny 121 M AM Christina Franzese 219 W AM Russell Bornschein 317 Th PM Neranga Abeyasinghe 123 M AM Shuwen Sun 221 W AM Jeanne Hankett 319 Th PM Wencheng Ge 125 T AM Wendi Hale 223 W AM Joshua Skodack 321 Th PM Ping Guo 127 T AM Erik Guetschow 225 W PM Wendi Hale 323 Th PM Shuai Niu 129 T AM Neranga Abeyasinghe 227 W PM Andy Crawford 325 F AM Wen Wen 131 T AM Yuwei Liu 229 W PM Ken Fletcher 327 F AM Charity Haynes 133 T AM Shuwen Sun 231 W PM Yuwei Liu 329 F AM Catherine Mocny 135 T AM Molly Soper 233 W PM Lauren Soblosky 333 F AM Deidra Gerlach 137 T AM Shuai Niu 235 W PM Molly Soper 335 F AM Joshua Skodack 139 T PM Mahmoud El Azzouny 237 W PM Ben Coupland 339 F AM Kristin Ko 141 T PM Charity Haynes 239 Th AM Hung Ki Yoon 341 F AM Andy Crawford 143 T PM Ken Fletcher 241 Th AM Erik Guetsc how 343 F AM Kevin Hartman 145 T PM Ying Zhou 243 Th AM Mallory Van Dongen 345 F AM Russell Borenschein 147 T PM Wencheng Ge 245 Th AM Shi Jin 347 F AM Ying Zhou 149 T PM Ping Guo 247 Th AM Russell Bornschein 349 F AM Jeanne Hankett Page Points Score 15 Name: 2 06 GSI: 3 07 Section: Bonus (+03) Total 75 (+03) Page 1/10

2 Name: Grading key page 2/10 Question 1 (15 points) asks you to make predictions based on your knowledge of periodic trends and the relationship of structure to properties and reactivity. Periodic tables are provided at the back of this exam for your reference. A. Predict the comparative solubility in water of LiCl, CdCl 2 and HgCl 2. Most soluble least soluble LiCl > CdCl 2 > HgCl2 for correct order or zero B. Predict the relative oxidizing agent strength of the metal ions K +, Zn 2+, and Ca 2+. Strongest Zn2+ > Ca2+ > K + for correct order or zero weakest oxidizing agent C. You individually add the solid chloride salts of strontium ( Sr 2+ ), chromium III ( Cr 3+ ), and tin II ( Sn 2+ ), to water. Predict if the resulting solution is colored or colorless. Circle any salt that is colored in aqueous solution. SrCl 2 CrCl3 SnCl 2 + for circling CrCl3; zero points if any other salt circled. No negative points. D. Correctly complete (circle your choice) each sentence: (+ 2 points for each correct choice) #1. Bromine, Br, is more electronegative than ( Cl Ni ). #2. Salts of Sr 2+ are likely to be more water soluble than salts of ( Ca 2+ Ba2+ ) containing the same anion. #3. The halide ( At - Cl - ) is a better reducing agent than Br -. only 6 points Page 2/10

3 Name: Grading key page 3/10 Question 2 (6 points) deals with your preparation of solutions of cobalt chloride A. How many grams of CoCl 2 6H 2 O do you weigh out to prepare 200 ml of 0.10M cobalt (II) chloride? CoCl2 + 6H2O = (35.5) + 6(2+16) = g/mol (1 point for correct formula) weight; work must be shown) *237.93g x 0.10mol x 200mL = 4.76 grams to weigh out. ( *1 pt. for correct set-up) 1 mol 1000mL *no double penalty i.e. set-up ok if formula wt is same as calculated above. ( 1 point for correct answer) grams of CoCl 2 6H 2 O = 4.76 B. How many mmol of Cl - (aq) are in the resulting solution? 200mL x 0.10M CoCl2 = mmol CoCl2 ( 1 point for correct process) = 20 mmol CoCl2 ( 1 point for correct calculation and reference to CoCl 2. For example if answer indicates 20 mmol Cl- that is incorrect) 2 x 20mmol = 40 mmol Cl- (1 point for recognizing stoichiometry) mmol Cl - = 40 mmol Page 3/10

4 Name: Grading key page 4/10 Question 3. (18 points) involves using a spectrophotometer to determine the concentration of a transition ion in aqueous solution (symbolized Trans Ion). The graphs below show the ions' absorption spectrum (at a specific but undesignated concentration) and calibration graph at λ = 600nm). Absorption Spectrum Trans Ion Absorption Wavelength (nm) A. At what concentration (M) of Trans ion was the absorption spectrum obtained? Circle the Molarity of trans ion used to produce the absorption spectrum: or zero B. Your teammates dilutes the sample of Trans ion used to produce the above absorption spectrum. She adds 9.00 ml of deionized water to 3.00 ml of the trans ion sample and generates another absorption spectrum of the diluted sample from λ 400 to λ 700. Indicate if each statement below is correct or not regarding the spectrum for the diluted sample. Statement Correct Not Correct 1. The absorption spectrum for the diluted sample will have absorption values = 1/3 the values shown in the above spectrum. 2. The absorptivity coefficient, Ε, increases from λ 400 to λ 450, in the absorption spectrum for the diluted sample. X X 2 points or zero each total = 4 points Page 4/10

5 Name: Grading key page 5/10 C. What is the slope of the above calibration graph line (λ=600 nm) in units of absorbance/m? Slope may be determined using Beer s law or y/ x y/ x = abs/ M (or abs/ M)etc. = 0.25 abs/m - 1 point if inappropriate points are used to determine slope. If calculations are not shown not credit given slope = 2.5 abs/m (values abs.m ok) 2 points D. You produce a second calibration graph for Trans Ion at λ=450 nm. You use the same samples, sample holder, and spectrophotometer. Will the slope of the resulting calibration graph line be more, less, or the same as the above calibration line at λ=600 nm? Indicate (circle) if the slope of calibration graph line at λ=450 will be more than less than the same as the slope at λ=600 E. Use the Beer-Lambert Law and the above calibration graph to determine the unknown concentration (M) of a solution of Trans Ion that has an absorbance of 0.30 at λ = 600 nm. Show all calculations for credit. You will not get credit for numerical answers alone. Absλ600 = Elc 0.30abs/M = 2.5 M (2 pts. for correct set-up; no double penalty if slope copied from 3B answer is wrong - i.e. slope of 2.5 or slope value calculated under 3B is ok. 0.12M = c (1 pt. for correct calc. using slope from 3B) Trans Ion M = 0.12 M ( M ok) F. You determine that a diluted sample of Trans Ion has a concentration = 0.25 M. You diluted the sample by adding 5.0 ml of water to 1.0 ml of Trans Ion solution. What is the concentration (M) of the original undiluted solution of Trans Ion? Show your calculations below. Mi x Vi = Mf x Vf Mi x 1.0mL = 0.25 M x 6.0mL (1 pt for correct set up and 1 pt. for correct values) Mi = 0.25M x 6.0mL 1.0mL Mi = 1.5 M (1 pt. for correct calculation) Trans Ion M = 1.5 M Page 5/10

6 Name: Grading key page 6/10 Question 4 (18 points) deals with your teams analysis of the reaction between 10 ml of 0.10M copper (II) bromide and 10 ml of 0.10M rubidium iodide: CuBr2 (aq) + RbI (aq) precipitate and color change A. You hypothesize that "Cu 2+ is critical to formation of the precipitate and the color change". 1. What test reaction, by itself, could you use to directly prove if the hypothesis is true or false? Record the specific reagents you will combine to test the hypothesis: NaBr + RbI 1 pt for removing Cu 2+ from CuBr 2. There is no credit for the entire question if another ion is removed or the reagent formula is invalid. 1 pt for substituting an acceptable soluble species for Cu 2+ (acceptable instead of CuBr 2 are alkali chloride salts-- i.e. KBr or LiBr or RbBr...) 1pt. for reagent stoichiometry (i.e. 1 pt if incorrect (e.g. KBr 2 etc) 2. If your hypothesis is true what will you observe? Expected observations if the hypothesis is true: The products precipitate AND color change (no credit w/o reference to both) will NOT form or You observe no reaction occurring or zero B. You add different solvents to portions of the product mixture. Based on your studies of salt solubility in different solvents, indicate if the amount of precipitate will decrease, increase, or remain the same upon adding hexane or acetone. 1. Upon adding hexane to a sample of the product mixture, the amount of precipitate, increases decreases remains the same 2 points or zero 2. Upon adding acetone to a sample of the product mixture, the amount of precipitate, increases decreases remains the same C. You believe that copper (I) bromide or copper (II) iodide may be the precipitate. Indicate (circle) if each copper compound is white or a color other than white. Compound White or other color? (Circle) CuBr White other color CuI 2 White other color 2 points or zero each 4 points 2 points Page 6/10

7 Name: Grading key page 7/10 D. The Table below lists some species according to actual oxidizing agent strength. Oxidizing agent strength: Br 2 > I 2 > Cu + > Cu 2+ > Rb + Based solely on the above table, identify (circle) the equation below correctly describing the expected redox behavior of the copper species in a mixture of CuBr2(aq) and RbI(aq). a) Cu I - I 2 + Cu b) Cu I - I 2 + 2Cu + c) Cu2+ + I- no reaction d) 2Cu + I 2 2I - + 2Cu + 4 point or zero Question 5 (6 points) This question asks you to make predictions regarding redox reactions involving metals and metal ions (0.10 M chlorides) in aqueous given the reducing agent strength: Ni > Fe 2+ > Pd A. Complete the table below to indicate if a reaction occurs (YES) or does not occur (NO). NiCl 2 FeCl 3 PdCl 2 Ni Fe 2+ Pd NO NO YES NO YES YES 0.5 pt for each correct observation (i.e yes or no) for total of B. What are the relative oxidizing agent strengths of Ni 2+, Fe 3+, and Pd 2+? Strongest oxidizing agent Weakest oxidizing agent Pd2+ > Fe3+ > Ni2+ or zero Page 7/10

8 Name: Grading key page 8/10 Question 6 (12 points) deals with a study of potential redox reactions involving the metal ions thallium (Tl) and iron (Fe) and halides where the observations below were made. Note: Hexane was added to all product mixtures. Common oxidation states of the cations are: Tl + and Tl 3+ and Fe 2+ and Fe FeCl 3 (aq) + NaBr(aq) Product? yellow-brown colorless aqueous layer is yellow- brown; hexane layer is colorless. 2. TlCl 3 (aq) + NaBr(aq) Product? colorless colorless aqueous layer is colorless; hexane layer is yellow-orange. You carry out some further tests: 3. FeCl 2 (aq) + Br 2 (aq) Product? yellow-green yellow-orange aqueous layer is yellow-brown; hexane layer is colorless 4. NaCl (aq) + NaBr (aq) Product? colorless colorless aqueous layer is colorless; hexane layer is colorless. A. Based on test reaction 4, what can you conclude about reaction 2? What can you conclude about the TlCl 3(aq) + NaBr (aq) reaction from test 4? Tl3+ (+2 for correct species and charge; -1 if charge on Tl incorrect) is a reactant (or an Oxidizing agent) (+1) and critical for production of the yellow-orange hexane layer (+1) (or must be causing oxidation of Br - to Br 2 etc) 4 points B. Analyze test reaction 3 to complete the table below. Do not record spectators for product species. Reaction Oxidizing Agent Reducing Agent Net Reaction Product species FeCl2 Br- Fe3+ 3 Br2 (or Fe 2+ ) (both species must be correct = 2 pts) or FeBr 3. 2 point 2 point Wrong/no credit if answer includes spectator species 2 points C. From reactions 2 and 3 together, you can determine the relative strengths of Tl 3+ vs. Fe 3+ as oxidizing agents. Complete the table below. Oxidizing agent strength of Tl 3+ vs. Fe 3+ : Strongest Weakest Oxidizing Agent Tl3+ > Fe3+ 2 points Page 8/10

9 Name: Grading key page 9/10 BONUS QUESTION ( or zero) The bonus questions deals with the products of reaction 2 (question 6): TlCl 3 (aq) + NaBr (aq) Products? colorless colorless colorless aqueous layer contains a white precipitate; hexane layer is yellow-orange. Identify the white precipitate based on the following test observations: 1.Tl(NO 3 ) 3(aq) + NaBr (aq) colorless aqueous layer contains a white precipitate; hexane layer is yellow-orange. 2.TlCl 3(aq) + NaNO 3 (aq) colorless aqueous layer; hexane layer is colorless. Formula of the white precipitate from the TlCl 3(aq) + NaBr (aq) reaction? TlBr or zero Page 9/10