Unformatted text preview: Chem 105b Exam 3 Thursday March 22, 2007 Professors Krylov and Bradforth Questions 1-5 6 7 8 9 10 Total Please Sign Below: Points 42 12 12 12 7 15 100 Print Name Last, First Last 4 digits SID # TA's Name Score Grader I certify that I have observed all the rules of Academic Integrity while taking this examination. Signature: ________________________________________________ INSTRUCTIONS 1. You must show your work to receive credit. You must show your complete working on each of problems 5-10 to receive full credit. 2. If necessary, please continue your solutions on the back of the preceding page (facing you). 3. YOU MUST Use black ink. 4. There are 10 problems on 10 pages. Please count them before you begin. Chemistry 105 B
Professors Krylov/ Bradforth Midterm 3
Spring 2007 YOU MUST SHOW YOUR COMPLETE WORKING ON EACH OF PROBLEMS 5-10 TO RECEIVE FULL CREDIT. STATE AND JUSTIFY ANY APPROXIMATIONS YOU MAKE. Useful Physical Constants and Equations
TC = TK - 273.15 R = 8.314 J K mol
-1 -1 Kw = 1.0 x 10-14 at 25 oC pH = pKa + log10( [A ] / [HA] ) Thermodynamics Suniv = Ssys + Ssurr Ssurr = - H /T G = H - T S G reaction = np Gf (products) - nr Gf (reactants) S reaction = np S (products) - nr S (reactants) G = G + RT ln (Q) At equilibrium, G = 0 therefore, G = - RT ln (K) and ln K = - Ho / RT + So / R
o o o o o o o o 1 1. Multiple choice (8 points). Circle the correct answer(s): A. Which of the following statements are true: a) b) c) d) e) the entropy of the universe is increasing for any spontaneous process. the entropy of the universe is constant for any equilibrium process. the absolute entropy of a substance decreases with increasing temperature. a machine converting heat into work is never 100% efficient. all exothermic processes are spontaneous. B. Which of the following is not/are not a state function? a) b) c) d) e) f) g) h) q G H E P w S T 2. Short answer (8 points). Determine the sign of Ssys, Ssurr, and Suniv for an ice cube melting in water at T=0C and T=10C: H2O (ice)
H2O (liquid) T 0C 10C Ssys Ssurr Suniv 2 3. Multiple choice (6 points). Considering the molecular structure of the following acids and bases, assign the relative acid and base strengths: H H N CH3 H H N H 1
H O Cl 2
H O I 3
(a) 1 is a stronger base than 2, and 3 is a stronger acid than 4 (b) 1 is a stronger base than 2, and 4 is a stronger acid than 3 (c) 2 is a stronger base than 1, and 4 is a stronger acid than 3 (d) 2 is a stronger base than 1, and 3 is a stronger acid than 4 4. Multiple choice (10 points). Consider the dissociation of hydrogen:
2H(g) A. For this reaction, the following is true (circle the correct answer): a) H >0, S>0 b) H>0, d) H<0, S<0 S<0 c) H<0, S>0 B. One would expect that this reaction (circle the correct answer): a) will be spontaneous at any temperature. b) will be spontaneous at high temperatures. c) will be spontaneous at low temperatures. d) will not be spontaneous at any temperature. e) will never happen. 4 3 5. LONG ANSWER: Show all your working to receive partial/full credit (10 points).
Sulfur undergoes a phase change from the rhombic crystal structure to the monoclinic crystal form at temperatures above 95C: Srhombic Smonoclinic A. Predict the signs of H and S for the process Srhombic answer. Smonoclinic. Explain your Answer: B. Which form of sulfur has the more ordered crystalline structure? Explain. Answer: 4 6. LONG ANSWER: Show all your working to receive partial/full credit (12 points). Consider the gas phase reaction
NO + (1/2)O2
NO2 for which H = 57.0 kJ/mol and K = 1.5 106 at 25C. a) Calculate G at 25C for this reaction. Answer: Go= b) Calculate S at 25C for this reaction. Answer: So=
5 7. LONG ANSWER: Show all your working to receive partial/full credit (12 points). Consider the following reaction at 25C: 2NO2(g) N2O4(g) for which Ho=-58.03 kJ/mol and So=-176.6 J K-1 mol-1.
a) Calculate the value of K at 25C. Answer: K=
b) Assuming H and S are temperature-independent, calculate the value of K at 100C. Answer: K= 6 8. LONG ANSWER: Show all your working to receive partial/full credit (12 points). At 699 K, G = 23.25 kJ/mol for the reaction H2(g) + I2(g) 2HI(g). Is the reaction spontaneous as written if the reagents are both supplied at 10.0 atm pressure and the product is at 1.00 atm pressure? Answer: 7 9. LONG ANSWER: Show all your working to receive partial/full credit (7 points). Consider a solution that contains both C5H5N and C5H5NH+. The pKb of the base pyridine, C5H5N, is 8.77. What is the ratio [C5H5N ] /[ C5H5NH+] if the solution has a pH = 5.00? Answer: [C5H5N ] /[ C5H5NH+]= 8 10. LONG ANSWER: Show all your working to receive partial/full credit (15 points). Calculate the solubility of Mg(OH)2 (Ksp = 8.9 10-12) in each of the following: a) pure water Answer: solubility=
b) 1M NaOH Answer: solubility= 9 c) A pH 10 buffered solution Answer: solubility=
c) Write chemical equations to show what happens when Mg(OH)2 is added to NH3 solution (Hint: Mg2+ forms a complex ion with NH3 giving Mg(NH3)62+.) Without performing a calculation, what can you then say about the solubility in ammonia solution compared to in water? 10 ...
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