HW 9 Solutions

# HW 9 Solutions - Wright, Jacqueline Homework 9 Due: Nov 14...

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Wright, Jacqueline – Homework 9 – Due: Nov 14 2007, 11:00 pm – Inst: James Holcombe 1 This print-out should have 27 questions. Multiple-choice questions may continue on the next column or page – fnd all choices beFore answering. The due time is Central time. 001 (part 1 oF 1) 10 points Balance the redox reaction C + K 2 Cr 2 O 7 + H 2 SO 4 CO 2 + K 2 SO 4 + Cr 2 (SO 4 ) 3 + H 2 O . What is the coe±cient oF H 2 SO 4 in the balanced equation? Correct answer: 8 . Explanation: Balance the halF reactions: 0 C -→ +4 C O 2 + 4 e - 6 e - + +6 Cr 2 - 2 O 7 2 - -→ +3 2Cr Equate the e - : 3 h 0 C -→ +4 C O 2 + 4 e - i 2 h 6 e - + +6 Cr 2 - 2 O 7 2 - -→ +3 2Cr i Add the balanced halF reactions: 3 0 C +2 +6 Cr 2 - 2 O 7 2 - -→ +4 3CO 2 + +3 4Cr Neutralize the remaining charges with ex- isting counterions in acid 3C + 2K 2 Cr 2 O 7 + 8H 2 SO 4 3CO 2 + 2K 2 SO 4 + 2Cr 2 (SO 4 ) 3 + 8H 2 O . 002 (part 1 oF 1) 10 points Balance the redox reaction HNO 3 + H 3 PO 3 NO + H 3 PO 4 + H 2 O . What is the coe±cient oF H 3 PO 3 in the bal- anced equation? Correct answer: 3 . Explanation: Balance the halF reactions: +3 P -→ +5 P +2 e - +5 N +3 e - -→ +2 N Equate the e - : 3 h +3 P -→ +5 P + 2 e - i 2 h +5 N + 3 e - -→ +2 N i Add the balanced halF reactions: 3 +3 P +2 +5 N -→ 3 +5 P +2 +2 N Neutralize the remaining charges with ex- isting counterions in acid 2HNO 3 + 3H 3 PO 3 2NO + 3H 3 PO 4 + H 2 O . 003 (part 1 oF 1) 10 points Zinc Silver 1 . 56 V V Voltmeter e - e - 1 M Zn 2+ (aq) 1 M Ag + (aq) Salt bridge to carry ions In this electrochemical cell, what is the cathode? 1. the solid silver electrode correct 2. the Ag + (aq) ions in the 1 M solution 3. the Zn 2+ (aq) ions in the 1 M solution 4. the solid zinc electrode Explanation: Zn(s) + 2Ag + (aq) Zn 2+ (aq) + 2Ag(s) Reduction occurs at the cathode. In this cell the reduction halF reaction is Ag + (aq) + e - Ag(s) .

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Wright, Jacqueline – Homework 9 – Due: Nov 14 2007, 11:00 pm – Inst: James Holcombe 2 Ag + cations are attracted to the solid Ag electrode where they are reduced to Ag(s). 004 (part 1 of 3) 10 points Consider the mercury cell below. Zinc anode Steel cathode HgO, KOH and ZnO combined in an aqueous paste What is the electrolyte? 1. ZnO 2. HgO 3. KOH correct 4. steel Explanation: 005 (part 2 of 3) 10 points What is the oxidizing agent? 1. Hg( ) 2. HgO(s) correct 3. Zn(s) 4. ZnO(s) Explanation: 006 (part 3 of 3) 10 points What are the reactants for the reaction? 1. Zn, ZnO 2. Zn, KOH 3. HgO, KOH 4. HgO, Zn correct 5. Zn, Hg 6. HgO, ZnO Explanation: The balanced reaction is HgO(s) + Zn(s) Hg( ) + ZnO(s) 007 (part 1 of 1) 10 points For the reaction Cu(s) + Ag + (aq) Cu + (aq) + Ag(s) 1. Cu is oxidized at the anode and Ag is reduced at the cathode. correct 2. Cu is reduced at the cathode and Ag is oxidized at the anode. 3.
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## This note was uploaded on 12/08/2008 for the course CH 302 taught by Professor Holcombe during the Spring '07 term at University of Texas at Austin.

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HW 9 Solutions - Wright, Jacqueline Homework 9 Due: Nov 14...

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