Chemistry HW 9 - ekk86 hw09 Holcombe (53570) 1 This...

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ekk86 – hw09 – Holcombe – (53570) 1 This print-out should have 25 questions. Multiple-choice questions may continue on the next column or page – fnd all choices beFore answering. You must use the Table oF Stan- dard Reduction Potentials on Dr. Mc- Cord’s course website For all prob- lems in which values are not given. http://courses.cm.utexas.edu/pmccord/ ch302/help/red-table.html 001 10.0 points In the halF-reaction Cr 2 O 2 - 7 Cr 3+ what is the change in the oxidation state oF Cr? (Note that oxygen retains an oxidation state oF - 2). 1. - 6 to +3 2. - 8 to +3 3. +8 to +3 4. +4 to +3 5. +6 to +3 correct 6. - 4 to +3 Explanation: Oxidation numbers are per atom. The oxi- dation number oF a monatomic ion is equal to the charge on the ion. Chromium in Cr 3+ has an oxidation number oF +3. The sum oF ox- idation numbers in a polyatomic ion is equal to the charge on the ion. We set the sum oF the oxidation numbers in Cr 2 O 2 - 7 equal to - 2 and solve For the oxidation number oF Cr: 2 x + 7( - 2) = - 2 Cr 2 O 2 - 7 : x = +6 002 10.0 points Balance the reaction SnCl 2 +K 2 Cr 2 O 7 + HCl CrCl 3 + KCl + SnCl 4 +H 2 O . What is the coe±cient oF K 2 Cr 2 O 7 in the balanced equation? Correct answer: 1. Explanation: The balanced equation is 3 SnCl 2 2 Cr 2 O 7 + 14 HCl 2 CrCl 3 + 2 KCl + 3 SnCl 4 + 7 H 2 O . 003 (part 1 oF 2) 10.0 points Using oxidation and reduction halF- reactions, balance the skeletal equation O 3 (aq) + Br - (aq) O 2 (g) + BrO - 3 (aq) oF the action oF ozone on bromide ions. The reaction takes place in a basic solution. What is the smallest possible integer coe±cient oF BrO - 3 in the combined balanced equation? Correct answer: 1. Explanation: Reduction halF-reaction: O 3 (g) O 2 (g) Balance O: O 3 (g) O 2 (g) + H 2 O( ± ) Balance H: 2H 2 O( ± ) + O 3 (g) O 2 (g) + H 2 O( ± ) + 2 OH - (aq) Cancel H 2 O and balance the charge: H 2 O( ± ) + O 3 (g)+2 e - O 2 (g) + 2 OH - (aq) Oxidation halF-reaction: Br - (aq) BrO - 3 (aq) Balance O: 3H 2 O( ± ) + Br - (aq) BrO - 3 (aq) Balance H: 6 OH - (aq) + 3 H 2 O( ± ) + Br - (aq) BrO - 3 (aq) + 6 H 2 O( ± ) Cancel H 2 O and balance the charge: 6 OH - (aq) + Br - (aq) BrO - 3 (aq) + 3 H 2 O( ± ) + 6 e - Balance the e - : 3 [H 2 O( ± ) + O 3 (g) + 2 e - O 2 (g) + 2 OH - (aq)] 6 OH - (aq) + Br - (aq) BrO - 3 (aq) + 3 H 2 O( ± )+ 6 e - Add the halF-reactions: 3O 3 (g) + Br - (aq) 2 (g) + BrO - 3 (aq)
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ekk86 – hw09 – Holcombe – (53570) 2 004 (part 2 of 2) 10.0 points Identify the reducing agent in this reaction.
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This note was uploaded on 11/01/2011 for the course CH 302 taught by Professor Holcombe during the Spring '07 term at University of Texas at Austin.

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Chemistry HW 9 - ekk86 hw09 Holcombe (53570) 1 This...

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