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CHEMISTRY 121 1 Department of Chemistry University of British Columbia Problem Set 1 Answer Review This problem set is a review of basic chemistry which forms the background for the material covered in this course. 1. What is a reducing agent? Reduction = gain of electrons. Oxidation = loss of electrons. A reducing agent loses electrons to cause another species to be reduced. The reducing agent itself is oxidized. 2. What is the oxidation state of phosphorus in Mg 2 [P 2 O 7 ]? The answer is: +5. In this compound, O has an oxidation number of -2, Mg has an oxidation number of +2. The sum of the oxidation numbers in a compound must equal the overall charge of 0, so 2 × (Ox. No. P) + 7 × (-2) + 2 × (+2) = 0 Therefore, Ox. No. P = +5 3. Balance the following reaction in basic solution: __Zn (s) + __[NO 3 ] - (aq) __[ZnO 2 ] 2- (aq) + __NH 3(aq) 7 OH + 4 Zn (s) + [NO 3 ] (aq) 4 [ZnO 2 ] 2– (aq) + NH 3(aq) + 2H 2 O To balance the half-reactions in basic solution: a. Balance atoms other than oxygen and hydrogen; i.e., in this case, Zn and N. b. Add H 2 O to balance oxygen. c. Add H + to balance hydrogen. d. Add the same number of OH ions as H + ions to both sides of the equation. e. Where possible group H + and OH together to form H 2 O. Subtract H 2 O on both sides. f. With all atoms balanced, add electrons to balance net electrical charges. The balanced half-reactions are (in aqueous solution): Zn(s) + 4OH ZnO 2 2– + 2H 2 O + 2e NO 3– + 6H 2 O + 8e NH 3 + 9OH The first half-reaction is multiplied by 4 (to make the number of electrons transferred equal for both reactions) and the two equations added. Common species on both sides are subtracted to yield the balanced equation. 4. What is the oxidation state of nitrogen in: (a) the nitrate ion? (b) the nitrite ion? (a). The answer is: +5
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CHEMISTRY 121 2 Department of Chemistry University of British Columbia The nitrate ion is NO 3 . The sum of the oxidation numbers equals the net charge of the ion: -1 = (Ox. No. N) + 3(-2) Ox. No. N = +5 (b). The answer is: +3 The nitrite ion is NO 2 . -1 = (Ox. No. N) + 2(-2) Ox. No. N = +3 5. What is the oxidation state of phosphorus in Na[H 2 PO 2 ]? The answer is: +1 -1 = 2(+1) + (Ox. No. P) + 2(-2) Ox. No. P = +1. 6. Give a balanced half reaction for the reduction of nitrate to nitric oxide in acidic solution. Nitrate is: NO 3 . Nitric oxide is: NO. The balanced half reaction is: NO 3 + 4H + + 3e NO + 2H 2 O To balance half-reactions in acidic solution: a. Balance atoms other than oxygen and hydrogen; i.e., in this case, N. b. Add H 2 O to balance oxygen. c. Add H + to balance hydrogen. d. With all atoms balanced, add electrons to balance net electrical charges. 7.
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This note was uploaded on 11/03/2009 for the course ECON 210 taught by Professor James during the Spring '09 term at The University of British Columbia.

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