Exam4_HW12-15 - Version 046 – Exam 4 – Mccord –(52445...

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Unformatted text preview: Version 046 – Exam 4 – Mccord – (52445) 1 This print-out should have 30 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. McCord CH302 10am This exam is only for McCord’s MWF 10am CH302 class. 001 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. 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. 2. The electrons they generate and not the chemical components provide most all of the electricity. 3. A lead-acid battery is so large that it holds large quantities of the chemicals whose electrochemical interaction creates the elec- tricity. 4. The mechanical motion of the engine drives an alternator that generates electric- ity to recharge the battery. correct 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. 002 10.0 points Express the rate of the following reaction in terms of the [SF 6 ]. 2 CH 2 S + 12 F 2-→ C 2 F 4 + 4 HF + 2 SF 6 1. rate =- 2Δ[SF 6 ] Δ t 2. rate =- Δ[SF 6 ] 2Δ t 3. rate = Δ[SF 6 ] 2Δ t correct 4. rate = 2Δ[SF 6 ] Δ t 5. rate = Δ[SF 6 ] Δ t Explanation: rate =- [CH 2 S] 2Δ t =- [F 2 ] 12Δ t = [C 2 F 4 ] Δ t = [HF] 4Δ t = [SF 6 ] 2Δ t 003 10.0 points While electroplating a spoon with silver (molar mass = 107.9 g/mol) from an Ag 2+ so- lution, 19 . 3 × 10 3 C of charge is used. By how much has the mass of the spoon increased? 1. 107.9 g 2. 0 g 3. 5.39 g 4. 10.79 g correct 5. 21.58 g 6. 53.9 g Explanation: This is a 2 electron process. g = Q(molar mass) nF = (19 . 3 × 10 3 C)(107 . 9 g / mol) (2)(9 . 65 × 10 4 C / mol) = 10 . 79 g Version 046 – Exam 4 – Mccord – (52445) 2 004 10.0 points Consider the lead storage battery and the chemical reaction that drives it forward on discharge. There are various substances present as reactants and products. Which of the following substances is reduced as the battery discharges. 1. PbSO 4 (s) 2. PbCl 2 (s) 3. Pb-Ni(CO 3 ) 2 (s) 4. Pb (s) 5. PbO 2 (s) correct 6. H 2 SO 4 (aq) Explanation: PbO 2 has lead in the +4 oxidation state. This will accept 2 electrons and thus be re- duced to PbSO 4 as the battery discharges. Pb metal is oxidized to PbSO 4 . 005 10.0 points Which type of widely used battery is NOT rechargeable? 1. nickel-cadmium (NiCad) 2. lithium-ion 3. lead-acid (storage batteries) 4. alkaline correct Explanation: Alkaline batteries were not designed to be rechargeable and do not do so efficiently, al- though there are some websites that disagree....
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Exam4_HW12-15 - Version 046 – Exam 4 – Mccord –(52445...

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