HW08-key - Chemistry 111 Homework #8 Due: November 2,...

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Chemistry 111 Homework #8 Due: November 2, 2005(Wednesday) 1. The units of the rate constant can be established if we know the order of the reaction and the way the rate was measured (whether it was M/min, or M/s, or, atm/min, or…). True. 2. The units of the actual rate for the reaction are totally independent of the order for the reaction. True. The units of rate are always concentration per unit time… No exceptions. 3. Consider the following first order reaction 2 A Æ B + C. The rate of the reaction increases as more B and C are formed. False. The rate of the reaction decreases as more and more of B and C are formed, because the concentration of A is decreasing as the reaction progresses. 4. In the reaction in question 3 , the time required for one-half of substance A to react is dependent on the quantity of A present initially. False. The time required for one half of substance A to react – the half-life – is independent of the initial concentration because it is first order. 5. In the reaction in question 3, a plot of the inverse of the [A] versus time is linear. False. It would be linear if we were plotting the natural logarithm of the concentration of A versus time instead. 6. The units of the rate constant could be inverse minutes (min -1 ) for a the reaction in question 3 . True! The units of first order reactions are inverse time, with time in any units. 7. A certain second order reaction has a rate constant of 0.0321 M -1 min -1 at 27°C. At 57°C, the rate constant is measured to be 0.525 M -1 min -1 . Determine the activation energy of this reaction. a) 15.3 kJ/mol b) 1.19 kJ/mol c) 33.3 kJ/mol d) 757 J/mol e) 76.7 kJ/mol f) None of these. Calculate the activation energy with Arrhenius equation, which is:
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Chemistry 111 2 Homework #8 / Fall 2005 Answer Key ln k 2 k 1 = Ea R 1 T 2 1 T 1 In this equation, the temperatures must always be in Kelvin (as in every other equation we have ever used this year). Therefore, direct substitution yields: ln 0.525 0.0321 = Ea 8.314 J/Kmol 1 330K 1 300 K Ea = 76.7 kJ/mol . 8. A certain chemical reaction has an activation energy of 87.6 kJ/mol. At what temperature will the rate of this reaction be quadrupled, relative to 27°C? a) 306 K b) 315 K c) 312 K d) 308 K e) 301 K f) None of these. This is a very similar problem to the one before, except this time we know the activation energy and the initial temperature. We want the temperature at which the rate is quadrupled (where the ratio of the two rate constants is four). ln k 2 k 1 = Ea R 1 T 2 1 T 1 ln 4 () = 87,600J/mol 8.314J/molK × 1 T 2 1 300K T 2 = 312 K . 9. The following table lists the numerical value for the rate constant,
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This note was uploaded on 02/08/2010 for the course CHEM 111 taught by Professor Kenney during the Spring '08 term at Case Western.

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HW08-key - Chemistry 111 Homework #8 Due: November 2,...

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