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HW04-solutions

# HW04-solutions - kamalska(mk23835 HW04 Quigley(104001 This...

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kamalska (mk23835) – HW04 – Quigley – (104001) 1 This print-out should have 30 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0 points The reaction 2 A B + C has a K c of 0.2. The reaction is commenced with initial con- centrations [A] = 0 . 2 M, [B] = 0 . 2 M and [C] = 0 . 2 M. In what direction does the reac- tion occur? 1. The reaction shifts to the left to reach equilibrium. correct 2. The reaction is already at equilibrium. 3. Cannot be determined without knowing the temperature. 4. The reaction shifts to the right to reach equilibrium. Explanation: K eq = 0 . 2 [A] = [B] = [C] = 0.2 M Q = [B][C] [A] 2 = (0 . 2 M)(0 . 2 M) (0 . 2 M) 2 = 1 . 0 > K eq = 0 . 2 Therefore equilibrium will shift to the left. 002 10.0 points K c = 2 . 6 × 10 8 at 825 K for the reaction 2 H 2 (g) + S 2 (g) 2 H 2 S(g) The equilibrium concentration of H 2 is 0.0020 M and that of S 2 is 0.0010 M. What is the equilibrium concentration of H 2 S? 1. 10 M 2. 0.10 M 3. 0.0010 M 4. 1.02 M correct Explanation: K c = 2 . 6 × 10 8 [H 2 ] eq = 0 . 0020 M [S 2 ] eq = 0 . 0010 M 2 H 2 (g) + S 2 2 H 2 S K c = [H 2 S] 2 [H 2 ] 2 [S 2 ] [H 2 S] = radicalBig K c [H 2 ] 2 [S 2 ] = radicalBig (2 . 6 × 10 8 ) (0 . 0020 M) 2 (0 . 0010 M) = 1 . 0 M 003 10.0 points In the reaction N 2 O 4 (g) 2 NO 2 (g) , at equilibrium, which of the following rela- tions applies? [N 2 O 4 ] 0 and [NO 2 ] 0 are the initial concentrations of N 2 O 4 and NO 2 , re- spectively, and [N 2 O 4 ] and [NO 2 ] are the equi- librium concentrations. 1. - 2([N 2 O 4 ] 0 - [N 2 O 4 ]) = [NO 2 ] 0 - [NO 2 ] correct 2. [N 2 O 4 ] 0 - [N 2 O 4 ] = 2[NO 2 ] 3. - ([N 2 O 4 ] 0 - [N 2 O 4 ]) = [NO 2 ] 4. [N 2 O 4 ] = 2 ([N 2 O 4 ] 0 - [NO 2 ]) 5. [N 2 O 4 ] 0 - [N 2 O 4 ] = 2([NO 2 ] 0 - [NO 2 ]) Explanation: N 2 O 4 (g) 2 NO 2 (g) ini [N 2 O 4 ] 0 [NO 2 ] 0 eq [N 2 O 4 ] [NO 2 ] Δ [N 2 O 4 ] - [N 2 O 4 ] 0 [NO 2 ] - [NO 2 ] 0 = - x = 2 x Therefore, - 2([N 2 O 4 ] - [N 2 O 4 ] 0 ) = [NO 2 ] - [NO 2 ] 0 or - 2([N 2 O 4 ] 0 - [N 2 O 4 ]) = [NO 2 ] 0 - [NO 2 ] 004 10.0 points At 600 C, the equilibrium constant for the reaction 2 HgO(s) 2 Hg( ) + O 2 (g)

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kamalska (mk23835) – HW04 – Quigley – (104001) 2 is 2.8. Calculate the equilibrium constant for the reaction 1 2 O 2 (g) + Hg( ) HgO(s) . 1. 0.36 2. - 1 . 7 3. 0.60 correct 4. 1.1 5. 1.7 Explanation: 005 10.0 points Suppose the system 2 X(g) + Y(g) Z is at equilibrium and the concentration of X is 0.60 M. Sufficient X is now added to raise its concentration temporarily to 1.0 M. Which of the following would most likely represent the concentration of X when equilibrium has been reestablished? 1. 1.20 M 2. 1.00 M 3. 0.60 M 4. 0.80 M correct 5. 0.40 M Explanation: By increasing the concentration of X the equilibrium of the system is disturbed, and according to Le Chatelier’s principle (the sys- tem will respond in a way which reduces the stress), some of the extra X will react in order to restore equilibrium. The new equilibrium concentration of X will lie between the con- centration before extra X was added and the concentration before equilibrium was reestab- lished. 006 10.0 points Consider the following reactions at 25 C: reaction K c 2 NO(g) N 2 (g) + O 2 (g) 1 × 10 30 2 H 2 O(g) 2 H 2 (g) + O 2 (g) 5 × 10 82 2 CO(g) + O 2 (g) 2 CO 2 (g) 3 × 10 91 Which compound is most likely to dissociate and give O 2 (g) at 25 C?
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