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C125 KineticsKEYs

# C125 KineticsKEYs - Chem125 Winter2009 Dr.Neff Answers to...

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Chem 125 Winter 2009 Dr. Neff Answers to Kinetics Problems 1. The rate law for the reaction A + B C is first order in both [A] and [B]. The figure below represents samples with different concentrations of A (dark spheres) and B (open squares). In which sample will the reaction A + B C proceed most rapidly? Why? (i) (ii) (iii) The reaction will proceed fastest in (iii) due to the greater concentrations of reactants – there’s more reactants present, so more collisions, so greater rate of reaction. 2. Catalytic converters in automobiles combat air pollution by converting NO and CO into N 2 and CO 2 : 2 NO (g) + 2 CO (g) N 2 (g) + 2 CO 2 (g) a) How is the rate of appearance of N 2 related to the rate of disappearance of CO? Write an equation relating the two rates. In words – the rate of appearance of nitrogen is half as fast as the rate of disappearance of carbon monoxide. OR the rate of disappearance of carbon monoxide is twice as fast as the rate of appearance of nitrogen. b) How is the rate of appearance of CO 2 related to the rate of disappearance of NO? Write an equation relating the two rates. Carbon dioxide is made at the same rate as nitrogen monoxide is used up. c) How is the rate of disappearance of CO related to the rate of disappearance of NO? Write an equation relating the two rates. Carbon monoxide and nitrogen monoxide disappear at the same rate. 3. Two reactions in which there is a single reactant have nearly the same magnitude rate constant. One is first order, the other is second order. a) If the initial concentrations of the reactants are both 1.0 mM, which reaction will proceed at the higher rate? Since the concentrations are both 1 mM, then it doesn’t matter if its first order or second order, the rate will be the same. (1 mM) 1 = 1, (1 mM) 2 = 1. b) If the initial concentrations of the reactants are both 2.0 M, which reaction will proceed the higher rate? Here the second order reaction will go faster, since 2 2 = 4 > 2 1 = 2 4. Each of the following reactions is first order in the reactants and second order overall. Which reaction is fastest if the initial concentrations of the reactants are the same? All reactions are at 298 K. Reaction Rate constant k, cm 3 /molecules•s ClO 2 (g) + O 3 (g) ClO 3 (g) + O 2 (g) 3.0 x 10 -19 ClO 2 (g) + NO (g) NO 2 (g) + ClO (g) 3.4 x 10 -13 ClO (g) + NO (g) Cl (g) + NO 2 (g) 1.7 x 10 -11 With the largest k value, this reaction would be fastest, k directly proportional to rate ClO (g) + O 3 (g) ClO 2 (g) + O 2 (g) 1.5 x 10 -17 5. In the presence of water, the species NO and NO 2 react to form nitrous acid and (HNO 2 ) by the following reaction: NO (g) + NO 2 (g) + H 2 O (l) 2 HNO 2 (g) When the concentration of NO or NO 2 is doubled, the initial rate of the reaction doubles. When the concentration of water is doubled, the initial rate doesn’t change at all. What is the rate law for this

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