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Unformatted text preview: Version 068 – Exam 4 – Mccord – (52450) 1 This print-out should have 30 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0 points Which type of widely used battery is NOT rechargeable? 1. lithium-ion 2. lead-acid (storage batteries) 3. alkaline correct 4. nickel-cadmium (NiCad) Explanation: Alkaline batteries were not designed to be rechargeable and do not do so efficiently, al- though there are some websites that disagree. 002 10.0 points What is the potential in an electrolysis cell formed from the two half reactions below if the sodium ion concentration is 10 − 3 M and the aluminum ion concentration is 1M? Half reaction E ◦ Na +-→ Na- 2 . 7 V Al 3+-→ Al- 1 . 7 V 1.-0.82V 2. 1.0V 3.-1.18V correct 4. 0.82V 5.-1.0V 6. 1.18V Explanation: E ◦ =- 1 . V E cell = E ◦ cell- . 05916 N e log Q =- 1 .- . 05916 3 log [Al 3+ ] [Na + ] 3 =- 1 .- . 05916 3 × 9 =- 1 . 18 003 10.0 points Suppose for some reaction the rate constant doubles in going from T = 50 ◦ C to 70 ◦ C. What is E a for this reaction? 1. 16 kJ/mol 2. 8 kJ/mol 3. 32 kJ/mol correct 4. 24 kJ/mol 5. 40 kJ/mol Explanation: T 1 = 50 ◦ C + 273 = 323 K k 2 = 2 k 1 T 2 = 70 ◦ C + 273 = 343 K ln k 2 k 1 = E a R parenleftbigg 1 T 1- 1 T 2 parenrightbigg E a = R ln k 2 k 1 1 T 1- 1 T 2 = (8 . 314 J / mol · K) ln 2 1 323 K- 1 343 K = 33 . 5 kJ / mol 004 10.0 points Why can the lead-acid batteries used in cars generate electricity for several years before running down ( i.e. , the battery is dead)? 1. The electrons they generate and not the chemical components provide most all of the electricity. 2. The mechanical motion of the engine drives an alternator that generates electric- ity to recharge the battery. correct Version 068 – Exam 4 – Mccord – (52450) 2 3. A lead-acid battery is so large that it holds large quantities of the chemicals whose electrochemical interaction creates the elec- tricity. 4. These batteries are used only to generate the electricity that makes the first spark plug “spark.” Therefore, they are hardly used at all when a car is started, and never when it is driven. Explanation: Lead-acid batteries are secondary cells. The battery is used when a car is started, which is why it recharges via the alternator when you drive, but it is also sometimes used when your A/C compressor cycles. 005 10.0 points When a ruthenium chloride solution was elec- trolyzed for 500 s with a 120 mA current, 31 mg of ruthenium was deposited. What is the oxidation number of ruthenium in ruthe- nium chloride? 1. 4 2. 5 3. 2 correct 4. 1 5. 3 6. 6 Explanation: t = 500 s m = 31 mg = 0 . 031 g I = 120 mA = 0 . 12 A = 0 . 12 C / s F = 96485 C Ru n + (aq) + ne − → Ru(s) The amount of Ru is n Ru = (0 . 031 g Ru) parenleftbigg 1 mol Ru 101 . 07 g Ru parenrightbigg = 0 . 000306718 mol Ru ....
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This note was uploaded on 06/15/2010 for the course CH 302 taught by Professor Holcombe during the Spring '07 term at University of Texas at Austin.

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exam4_pdf - Version 068 – Exam 4 – Mccord – (52450) 1...

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