chem1a-Spring96-mt3-Chandler -exam

chem1a-Spring96-mt3-Chandler -exam - Name:...

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Unformatted text preview: Name: _______________________ Page 2 of 10. Section 1: Multiple Choice. 12 questions, 3 points each. Instructions: For the following questions, circle the answer on the exam sheet and bubble in the correct answer on your Scantron sheet. There is only one correct answer per problem, so for each question fill in one bubble on your Scantron form. 1.) You are taking test version A. Please fill in bubble "A" on your Scantron sheet. 2.) Given a constant CO2(g) pressure of 1 atm over a vessel of water, which of the following will not increase the equilibrium concentration of CO2(aq) in the water? Note: CO2(g) dissolving in water is an exothermic process. A) B) C) D) E) Decrease the temperature of the water. Increase the amount of water. Increase the pH. Increase the partial pressure of CO2(g) from 1 atm to 2 atm. All will increase the concentration. 3.) Which of the following pairs will undergo a spontaneous oxidation/reduction reaction? A) Zn(s), Cd2+ B) Ag(s), Cd2+ D) Zn2+, Au+ E) Ag(s), Ag+ C) Li+, Br- 4.) For which process or reaction is ∆S° expected to be positive? A) I2(g) ------> I2(s) B) C) D) E) CO2(g) + 2 H2O(λ) ------> H3O+(aq) + HCO3-(aq) CH3OH(g) + 3/2 O2(g) ------> CO2(g) + 2 H2O(g) O2(g) + SO (g) ------> SO3(g) 2 Mg(s) + CO2(g) ------> 2 MgO(s) + C(s) 5.) A Zn | Zn2+ || Co2+ | Co galvanic cell is constructed in which the standard cell voltage, ∆ε°, is 0.48 V. What is the free energy change at 25 °C (in kJ) per mole of Zn lost at the anode, if all concentrations remain at 1.0 M throughout the process? A) -6053 B) -92.6 C) -46.3 D) 46.3 6.) Which of the following cannot occur at the same time? A) B) C) D) E) ∆Ssys > 0 and ∆Ssurr < 0 ∆Ssys = 0 and ∆Ssurr > 0 ∆Ssys < 0 and ∆Ssurr > 0 ∆Ssys > 0 and ∆Ssurr > 0 All of these can occur at the same time. E) 92.6 Name: _______________________ Page 3 of 10. 7-10. In the next four problems, choose which of the following five graphs best describes the behaviors listed below. 7.) Energy of a photon as a function of its wavelength. 8.) Mass of Mg(OH)2(s) precipitate (Ksp=5.6x10-12) in water as a function of moles of strong acid added. 9.) ln(K) as a function of 1 for an endothermic reaction. T 10.) ∆ε° for the cell Zn|Zn2+||Ag+|Ag as a function of the mass of the Ag electrode used. Assume that [Zn2+] = [Ag+] = 1.0 M. 11-12. Answer the next two problems using the following five choices: A) B) C) D) E) Spontaneous at all temperatures. Spontaneous at no temperature. Spontaneous at low temperatures, but not at high temperatures. Spontaneous at high temperatures, but not at low temperatures. Spontaneous only at 100° C. 11.) 2 H2(g) + O2(g) ------> 2 H2O(g). (∆H°<0; ∆S°<0) 12.) C6H12O6(s) + 6 O2(g) ------> 6 H2O(g) + 6 CO2(g). (∆H°<0; ∆S°>0) 13.) For the vaporization of bromine, Br2(λ) Br2(g), ∆H°=31 kJ•mol-1 and ∆S°=93 J•mol-1•K-1. Assuming that ∆H° and ∆S° are invariant with temperature, what is the boiling point of Br2(λ)? A) 298 K B) 300 K C) 333 K D) 373 K E) Can't determine. Page 4 of 10. Name: _______________________ Section 2: What's Wrong. 4 questions, 6 points each. For this section, in no more than twenty words per response, explain what is wrong with the following pictures. Note: only the first 20 words of each answer will be read! 1.) A galvanic cell which utilizes the potential difference between Au and Ag: 2.) For the photoionization of a metal: 3.) For the addition of AgCl(s) to 100 mL of water: 4.) For the following two systems of gas molecules: Page 5 of 10. Name: _______________________ Section 3: Short Answer. 5 questions, 65 points total. Answer the following five short answer questions. Partial credit will be given, so show your work whenever possible. Your final answers must be written in the boxes provided. 1.) (12 points) Up to 0.0432 grams of silver dichromate (Ag2Cr2O7; MW=432 g•mol-1) will dissolve in 1.00 L of water at 25° C to form Ag+ and Cr2O72-. a.) Calculate the solubility product constant, Ksp, for silver dichromate at 25° C. b.) Suppose AgNO3(s), which dissociates completely, is added to the above solution until the Ag+ concentration reaches 0.10 M. How many grams of Ag2Cr2O7(s) precipitate will form? Assume the volume remains 1.00 L. Name: _______________________ Page 6 of 10. 2.) (12 points) Consider a monatomic ideal gas at a volume of 2 L which is held at constant temperature. The gas is reversibly compressed using a piston until it reaches a volume of 1 L. For each quantity below, indicate (by checking the box) whether it is =0, >0, or <0 for the overall isothermal compression process. Quantity ∆Psys ∆Vsys ∆Tsys ∆nsys w q ∆Esys ∆Esurr ∆Etot ∆Ssys ∆Ssurr ∆Stot =0 >0 <0 Page 7 of 10. Name: _______________________ 3.) (15 points) For a lecture demo, Lonnie creates a galvanic cell by placing a cadmium electrode in a 1.0 M solution of cadmium nitrate (Cd(NO3)2) and a titanium electrode in a 1.0 M solution of titanium nitrate (Ti(NO3)3), both at 25° C. To complete the circuit, the solutions are connected by a salt bridge and the electrodes are connected by a wire. a.) Lonnie measures the voltage across the cell and determines it to be 1.60 V. He also notices that the titanium electrode is growing larger. What is the standard reduction potential, ε°, for Ti3+ [Ti3+ + 3e- -------> Ti(s)]? b.) Into which solution are the negative ions from the salt bridge flowing? Explain your answer. c.) At the Tuesday lecture, Head TA Dave Laws tries to set up the galvanic cell, but accidentally spills a large amount of distilled water into one of the solutions. This causes the voltage measured to be higher than 1.60 V. Which solution did Dave accidentally spill the water into? Explain your answer. Name: _______________________ Page 8 of 10. 4.) (11 points) A student is doing a research project on the thermodynamics of dissolving sucrose, C12H22O11, in water according to the equation: C12H22O11(s) C12H22O11(aq) In her first experiment, the student takes 34.2 g of sucrose (MW=342 g•mol-1) and completely dissolves it in 1.00 L of water inside a calorimeter which is initially at 25.0 °C. Once the sucrose is dissolved, she measures the temperature to be 24.5 °C. a.) What is the enthalpy change, ∆H, in kJ•mol-1 for this process? Remember that for a calorimeter, you can use the equation: q = -mCp∆T, where the specific heat for water, Cp = 4.184 J•g-1•K-1. b.) How many grams of sucrose would the student have to dissolve in 1.00 L of water in order to lower the temperature from 25.0 °C to 23.5 °C? c.) Based on your answer to part (a), would you predict the K for the dissolution of sucrose in 50 °C water to be larger, smaller, or the same as the K for the dissolution of sucrose in 25 °C water? Explain your answer in 20 words or less. Name: _______________________ Page 9 of 10. 5.) (15 points) Two metals, metal A and metal B, are irradiated with green light at 500 nm. Metal A ejects an electron with a speed of 5.0x105 m•s-1, while metal B ejects an electron with a speed of 3.4x105 m•s-1. a.) Which metal, A or B, has the larger work function, Φ? Explain your answer. b.) If you wanted to eject an electron from metal B with exactly twice (2x) the kinetic energy as in part a.), which of the following light sources might be used? Circle your answer and explain your reasoning in 20 words or less. Light Source: Operating Wavelengths: Krypton-Fluorine laser: ≤250 nm Pulsed dye laser Argon ion laser: 250 nm to 500 nm 500 nm to 1100 nm c.) If metal A is silver, which of the following could be metal B? Circle your answer and explain your reasoning in 20 words or less. Hint: think of ejecting an electron as an oxidation process. Gold Lithium Page 10 of 10. Name: _______________________ Half Reaction Au+ + e- -----> Au(s) Br2(λ) + 2e- -----> 2 BrAg+ + e- -----> Ag(s) 2 H+ + 2e- -----> H2(g) Cd2+ + 2e- -----> Cd(s) Zn2+ + 2e- -----> Zn(s) Mg2+ + 2e- -----> Mg(s) Li+ + e- -----> Li(s) ε° (V) 1.68 1.07 0.80 0.00 -0.40 -.076 -2.38 -3.05 Possibly Useful Information S = kBlnΩ Absolute T(K) = T(°C) + 273.15 ∆Hsys = q = nCp∆T at constant pressure. PV = nRT ∆G = ∆H - T∆S O ∆ HO = ∆ H O form products − ∆ H reac tan ts ∆G° = -RTlnK R = 0.0821 L • atm • mol-1 • K-1 ∆G = ∆G° + RTlnQ R = 8.31 J • mol-1 • K-1 ∆ Ssys = nRln V2 V1 Chemistry is fun. hc λ ∆Stot = ∆Ssys + ∆Ssurr E= ∆Etot = ∆Esys + ∆Esurr Eelectron = Ephoton - Φ ∆E = q + w E kinetic = w = -Pext∆V ∆H = ∆E + ∆(PV) q ∆ Ssurr = − rev T ln K = − ∆G° = -nF∆ε°, where F=96,485 C • mol-1 mv2 2 ∆ H° 1 ∆ S° + R T R ...
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