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H14_Kinetics 1 - krohn(tek335 H14 Kinetics 1 Mccord(52445...

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krohn (tek335) – H14: Kinetics 1 – Mccord – (52445) 1 This print-out should have 16 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0 points The rate of the reaction 2 O 3 3 O 2 is equal to 1. - 1 3 Δ[O 2 ] Δ t . 2. - 1 2 Δ[O 3 ] Δ t . correct 3. - Δ[O 3 ] Δ t . 4. - Δ[O 2 ] Δ t . 5. + 1 2 Δ[O 3 ] Δ t . Explanation: The rate of a chemical reaction can either be expressed as the rate of disappearance of a reactant (as in this case) or as the rate of appearance of a product. However, this rate is multiplied by the inverse of the coef- ficient of the species in question. The nega- tive sign here reminds us that as the reaction progresses, this species is consumed and de- creases in concentration. 002 10.0 points When the reaction 3 NO(g) N 2 O(g) + NO 2 (g) is proceeding under conditions such that 0.015 mol/L of N 2 O is being formed each second, the rate of the overall reaction is ? and the rate of change for NO is ? . 1. 0.015 M · s 1 ; +0.045 M · s 1 2. 0.030 M · s 1 ; - 0.005 M · s 1 3. 0.015 M · s 1 ; - 0.005 M · s 1 4. none of the other answers is correct 5. 0.015 M · s 1 ; - 0.045 M · s 1 correct Explanation: Notice that three moles of NO are required to make one mole of N 2 O in this reaction, so the rate of disappearance of NO (which has a negative sign) is three times the rate of appearance of N 2 O. 003 10.0 points Consider the irreversible reaction A + 2 B 3 C . Which of the following correctly expresses the rate of change of [B] ? 1. Δ[B] Δ t = - 1 2 (rate of rxn) 2. Δ[B] Δ t = +2 (rate of rxn) 3. Δ[B] Δ t = - (rate of rxn) 4. Δ[B] Δ t = + 1 2 (rate of rxn) 5. Δ[B] Δ t = - 2 (rate of rxn) correct 6. Δ[B] Δ t = +(rate of rxn) Explanation: As B is a reactant and consequently disap- pears during the reaction, its rate of change will be negative and the inverse of the coeffi- cient is used when writing the rate: (rate of rxn) = - 1 2 Δ[B] Δ t - 2 (rate of rxn) = Δ[B] Δ t 004 10.0 points Suppose a compound is involved in three different reactions denoted R 1 , R 2 , and R 3 . Tripling the concentration of this reactant in all three reactions causes the rates of reaction to increase by factors of 3, 9, and 1, respec- tively. What is the order of each reaction with respect to this reactant?
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krohn (tek335) – H14: Kinetics 1 – Mccord – (52445) 2 1. R 1 = 0; R 2 = 1; R 3 = 2 2. R 1 = 2; R 2 = 0; R 3 = 1 3. R 1 = 3; R 2 = 9; R 3 = 1 4. R 1 = 1; R 2 = 2; R 3 = 0 correct 5. R 1 = 1; R 2 = 3; R 3 = 0 Explanation: Tripling the concentration of the reactant in R 1 brings about a tripling (= 3 1 ) increase in the rate of the reaction, so the reaction is first order with respect to the reactant in this case. Likewise, tripling the concentration of the reactant in R 2 brings about a 9-fold (= 3 2 ) increase in the rate of the reaction, so the reaction is second order with respect to the reactant here.
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