Test-exam 2

Test-exam 2 - Chemistry 100A Exam II Fall Quarter 2010...

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1 Chemistry 100A Exam II Fall Quarter, 2010 University of California, San Diego DIRECTIONS ********** Please turn off your cell phone!!! ********** READ THIS FIRST This test is designed to be taken with the provided Scantron . You must use a #2 lead pencil to take the test. There is only one correct answer to each question, and there is no penalty for incorrect responses. You should answer every question. DO NOT take the examination booklet with you when you leave the room. You may, however, write on the examination booklet. MARK YOUR SCANTRON SHEET Print your last name, first name, and middle name on the line next to NAME. On the line next to SUBJECT, enter A01, A02, A03, or A04. Write and bubble in your ID number below I.D. NUMBER. When bubbling, “A” must be replaced by “1”. Ignore TEST FORM and EXAM NUMBER. There are two columns on the front page. Use only the column that starts with 1. When you have selected your answer, blacken the corresponding space on the answer sheet. Make a heavy, full mark, but no stray marks. If you decide to change an answer, erase ANY unwanted mark carefully and completely. Any question for which more than one response has been blackened will be counted as wrong. Any question that is unanswered will also be counted as wrong. DO THIS AFTER THE TEST For the answer key, go to WebCT > Chem100A > Exams > Exam_II_Key. This is a 25-question exam. Each question is worth 4 points. Maximum possible points is 100 (=4 x 25)
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2 Electrochemistry and Vitamin C 1. The oxidation state of chromium in 24 KCrO is a. –6 b. –2 c. 0 d. +2 e. +6* 2. How many electrons are transferred in the following reaction? 32 2 2 21 2 1 0 5 6 ClO H I I Cl H O   a. 2 b. 5 c. 10* d. 12 e. 30 Ans. One way is to look at this problem is by examining the reaction, 2 ClO Cl . The oxidation state of Cl in 3 ClO is +5. The oxidation state of 2 Cl is zero. Thus, there is a change of 5 in oxidation state. Including the “2” in front of 2 ClO Cl , the total change is 10. 3. Let the following diagram be a Zn/Cr voltaic cell. Under standard condition which of the following is true? a. The electrons flow from the cathode to the anode. b. The electrons flow from the zinc electrode to the chromium electrode.* c. The potassium ions in the salt bridge flow to the zinc cell. d. The chromium metal, Cr(s), is oxidized. e. The zinc metal, Zn(s), is reduced. Ans. a. Wrong. For Votaic cell electrons flows from anode to cathode. b. Correct. From the table, Zn 2+ 2e - Zn(s) -0.76V while Cr 3+ + 3e - Cr(s) -0.74V. Therefore, zinc direction should be switched and zinc is the anode. Zn ZnSO 4 (aq) Cr(NO 3 ) 3 Voltmeter
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3 c. Wrong. The zinc electrode produces more and more positive ions. To keep Votaic cell electrically neutral, the chloride ions in the salt bridge will have to move to the zinc cell, not potassium ions.
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Test-exam 2 - Chemistry 100A Exam II Fall Quarter 2010...

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