General Chemistry Exam 3 Chem 211 Section 03, Spring 2017

Transcription

1 General Chemistry Exam 3 Chem 211 Section 03, Spring 2017 Exam Information You have 50 minutes to complete this exam. Be sure to put your name on the next page. Please ask for clarification of questions if needed. Complete your answers legibly in the space provided and SHOW YOUR WORK. Full credit will only be given for problems that demonstrate how the question was solved. Remember, most calculations are not considered complete unless they have the correct number of significant figures and units. You may only use a simple scientific calculation (no graphing or programmable calculators), and all book materials must be put away. This information may be helpful on the exam: Unit conversions and constants: 1 cal = J T (K) = T ( o C) o F = o C ml = 1 cm 3 1 atm = torr 1 cal = J 1000 ml = 1 L 1 A = 1 C sec -1 1 J = 1 V C NA = x mol -1 Plank s Constant; h = x J sec -1 Speed of light; c = x 10 8 m s -1 Gas Constant; R = J mol -1 K -1 Faraday s constant; F = 96, C mol -1 Kw = 1.00 x (at 25 o C) Useful equations: q = s m T PV = nrt P T = i P i K p = K c RT M 1V 1 = M 2V 2 For the equation: ax 2 + bx + c = 0, x = b± b2 4ac 2a ph = -log[h 3O + ] ph + poh = Kw = 1.00 x ; Kw =Ka Kb poh = -log[oh ] pk a = -log(k a) Kw =[H 3O + ] [OH ] G o = H o - T S o G o = -RT ln(k) G = G o + RT ln(q) G o = -nf E o E o cell = E o red, cathode - E o red, anode ln(k) = - Ho R 1 T + So R E cell = E o - RT nf ln(q) SI Prefixes: f p femto pico n nano 10-9 m Micro 10-6 m milli 10-3 c centi 10-2 d deci 10-1 da deca 10 h hector 10 2 k kilo 10 3 M mega 10 6 G giga 10 9 T tera Feel free to remove this page from the exam packet while working. Page 1 of 8

2 Student Name: 1. (14 points) For each of the following reactions assign oxidation numbers to all atoms and identify the species oxidized, the species reduced, the oxidizing agent, and the reducing agent. a) CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O (g) b) 6 Fe 2+ (aq) + 14 H + (aq) + Cr 2 O 7 2- (aq) 6 Fe 3+ (aq) + 2 Cr 3+ (aq) + 7 H 2 O (l) 2. (12 points) Indicate if the following statements are true or false and briefly explain why. a) The standard entropy of a substance in the gas phase is always greater than its standard entropy in the liquid state. b) For the following process, the entropy of the substance increases: O2 (g, 200 kpa, 300 K) O2 (g, 100 kpa, 300 K). c) A nonspontaneous process can never be made to occur. Page 2 of 8

3 3. (16 points) Consider a cobalt-lead electrochemical cell. The negative electrode is a cobalt rod immersed in a 1.0 M Co(NO3)2 (aq) solution, and the positive electrode is a lead rod in a 1.0 M Pb(NO3)2 (aq) solution. a) Sketch this electrochemical cell using the diagram below. Include and label all required components of the electrochemical cell. Be sure to identify the following: the anode, the cathode, the half-reaction occurring in each half-cell, and the direction of electron flow. b) Determine the standard cell potential for this electrochemical cell. c) Write the net chemical reaction for this electrochemical cell. d) Write the line diagram for this electrochemical ell. Page 3 of 8

4 4. (10 points) Balance the following oxidation-reduction reactions. a) Cu 2+ (aq) + S 2 O 3 2- (aq) Cu + (aq) + S 4 O 6 2- (aq) (acidic) b) IO 3 - (aq) + I - (aq) I 3 - (aq) (acidic) 5. (10 points) A voltaic cell consists of a Zn/Zn 2+ half-cell and a Ni/Ni 2+ half-cell at 25 C. The initial concentrations of Ni 2+ and Zn 2+ are M and M, respectively. What is the initial cell potential of this voltaic cell? Page 4 of 8

5 6. (16 points) Calculate H o rxn, S o rxn, G o rxn and K for the following reaction: (see tabulated thermodynamic data at the end of the exam) 2 H 2 O 2 (l) + N 2 H 4 (l) N 2 (g) + 4 H 2 O (g) Predict the effect on the equilibrium constant when the temperature of the system decreases. Page 5 of 8

6 7. (12 points) An electrochemical cell is set up so that the following reaction occurs. At 25 o C, determine the value of G o rxn. 2 Ag (s) + F 2 (g) 2 Ag + (aq) + 2 F - (aq) 8. (10 points) Copper can be electroplated from a solution of Cu 2+ at the cathode of an electrolytic cell; how much time does it take to deposit g of copper metal when a current of 5.60A is applied? Page 6 of 8

7 Extra Credit - you can earn up to 5 points extra credit by answering following correctly: A critical reaction in the production of energy to do work or drive chemical reactions in biological systems is the hydrolysis of adenosine triphosphate, ATP, to adenosine diphosphate, ADP, as described 2- by the following: ATP (aq) + H 2 O (l) ADP (aq) + HPO 4 (aq) For this reaction, G o rxn = kj/mol at 37 o C and ph 7.0. Calculate the value of Grxn in a biological cell in which: [ATP] = 5.0 mm, [ADP] = 0.50 mm, and [HPO4 2- ] = 5.0 mm. Is the hydrolysis of ATP spontaneous under these conditions? Question Total EC Final % Points Score Page 7 of 8

8 Standard Reduction Potentials: (species are aqueous unless noted) Half Reaction (V) Half Reaction (V) F2 (g) + 2e 2F 2.87 O2(g) + 2H2O + 4e 4 OH 0.40 Ag 2+ + e Ag Cu e Cu (s) 0.34 Co 3+ + e Co Hg2Cl2 + 2e 2Hg(l) + 2Cl PbO 2 (s) +4H + +SO 42 +2e PbSO 4 (s)+2h 2O 1.69 Cu 2+ + e Cu e + 2H + + IO4 IO3 + H2O H + + 2e H2 (g) 0.00 Au e Au (s) 1.50 Pb e Pb (s) PbO2 (s)+ 4H + + 2e Pb H2O 1.46 Sn e Sn (s) C12(g) + 2e 2Cl 1.36 Ni e Ni (s) Cr2O H + + 6e 2Cr H2O 1.33 Co e Co (s) O2 (g) + 4H + + 4e 2H2O 1.23 Cd e Cd (s) IO3 + 6H + + 5e 1/2I2 + 3H2O 1.20 Cr 3+ + e Cr VO H + + e VO 2+ + H2O 1.00 Zn e Zn (s) AuCl4 + 3e Au (s) + 4Cl H2O + 2e H2 (g) + 2OH Hg e Hg H2 + 2e 2H Ag + + e Ag (s) 0.80 Mg e Mg (s) O2(g) + 2H + + 2e H2O Ca e Ca (s) Cu + + e Cu (s) 0.52 Li + + e Li (s) Tabulated Thermodynamic Data: Page 8 of 8