Name Chemistry 122-XXX Prof. Mines Practice Exam 1b (From an Exam 1 that was originally 100 points) (Exam from many years ago!) COMMENTS: This exam was from quite a long time ago. I provide it primarily to help students get a sense of the format of my exams, and to give you some examples of problems that I ve created so you can see how they compare to problems from your textbook, problem sets, etc. This exam covers (not necessarily all the) content from problem sets 2, 3, and 4. A few problems involved content that I no longer cover and have been removed. The problems that remain are as applicable now as they were then. That said, there are a number of sub-topics that we have covered this term that are not represented by a question on this exam.* For example, I think I cover a lot more about integrated rate laws, the Arrhenius equation, and Collision Theory now than is represented on this practice exam (these are PS2 topics). I m sure there are others as well. You are still responsible for those sub-topics, and there may well be questions on your exam 1b that reflect those topics. So the bottom line is this: use your problem sets to determine all possible content for this exam do not rely solely on this practice exam! This exam is just one of many items you should use to help you prepare for your Exam 1. *See the subheadings on all current problem set sheets (see online version of PS4 if your PS4 sheet does not have the subtopics listed there) to get an idea of how I characterize the subtopics in the course. Obviously, there are many sub-subtopics within these headings that are not listed, but at least this gives you a start. This exam consists of 10 (now 7) questions (many with multiple parts) on six (now four) pages. 1) Please briefly scan through the whole exam to make sure that you have all the pages (and so that you can make the best use of your time). 2) Do not spend too much time on any one problem in the first half of the test period. 3) Do questions that you feel you can do quickly first and come back later to questions you are unsure about. 4) Take notice of the point value of the problems to help you judge your time and efforts (see chart below!). 5) Make sure to report all calculated values to the proper precision and show your work (unless you are told otherwise). 6) Remember, don t panic if you don t immediately see the answer; allow yourself time to analyze each problem! Good luck! Problem Type Received/Possible 1 Mult. Choice /12 8 2 T/F /8 3 /8 4 Free /21 8 Response /6 9 /9 10 /10 TOTAL /70
Page 1 1. Circle the correct answer (12 8 pts total) (2 points each) (a) The rate constant for a reaction will change if: i) a catalyst is added ii) the concentration of reactants is increased iii) T is lowered iv) all of the above v) (i) and (iii) only (b) A system at equilibrium will shift from its equilibrium position if: i) a catalyst is added ii) the concentration of reactants is increased iii) T is lowered iv) all of the above v) (ii) and (iii) only (c) For the elementary reaction represented by A B, one can determine the rate of reaction when a specific concentration of A is present by: i) measuring the slope of a plot of [A] vs. time at the appropriate place. ii) using the rate law, if the rate constant is known. iii) measuring the time it takes for a certain (small) amount of A to disappear. iv) all of the above. v) (ii) and (iii) only use for (f) (d) REMOVED BECAUSE TOPIC NO LONGER TAUGHT (e) Removed; Exam 1a (f) Given the equation AB(s) A + (aq) + B - (aq), if the beaker to the right represents a system in which Q < K, which picture best represents what the system might look like after some time passes? A + (aq) B - (aq) solid salt AB (i) (ii) (iii) (iv) 2. Write T (True) or F (False) for the following statements. If the statement is false correct it by changing, dropping, or adding a few words (8 pts total; 2 pts each) a) The half life for a first order reaction becomes one half of its original value during the course of a reaction. b) One can write an expression for the equilibrium constant for any reaction whose balanced chemical equation is known, but one cannot always write the rate law.
Page 2 c) The mechanism of a reaction is the series of elementary or non-elementary steps that occur as the reactants are turned into products. d) When a system comes to equilibrium, no more reaction occurs in the forward or reverse directions. Free Response Questions 3. (8 pts) Consider the elementary process A B, for which the half-life is 10.8 yr. If you have a solution in which [A] = 0.670 M, what will the concentration be after 16.0 yrs?
Page 3 4. (21 pts) Consider the following equation: CCl 4 (g) C(s) + 2 Cl 2 (g) for which K c = 0.013 at 427 C. (a) (4 pts) Write the equilibrium constant expression. (b) (5 pts) If some CCl 4 is put into a sealed container at 427 C., do you expect that the majority of it will be converted into products before equilibrium is established? Explain briefly. (c) (4 pts) If a system containing CCl 4 (g), C(s), and Cl 2 (g) were at equilibrium and the volume of the container were doubled, would the system shift, and if so, in which direction would net reaction occur? Explain briefly. (d) (6 pts) If 3.2 moles of CCl 4 is put into a 1.0 L container at 427 C, set up a mathematical equation that would allow you to determine the concentrations at equilibrium. Solve this in the NEXT part. (e) (2 pts) Use your equation in (d) to solve for the concentrations of Cl 2 and CCl 4 at equilibrium. Comment on whether or not your answers are consistent with your prediction in (b). (NOTE: This part is only worth 2 points, so you might consider doing this after you have finished the rest of the exam.)
Page 4 5. (10 pts) and 6. (8 pts) OMITTED (no longer taught) 7. (8 pts) Exam 1a 8. (6 pts) K c = 4.1 at 300 C for the following equation: PCl 3 (g) + Cl 2 (g) PCl 5 (g) If a 2.0 L box at 300 C were filled with 2.1 moles of PCl 3, 1.3 moles of Cl 2, and 7.2 moles of PCl 5, would any net reaction occur in the forward or reverse direction? Justify your answer. 9. (9 pts) The observed rate law for the decomposition of ozone into oxygen is rather odd (see below; please don t let it bother you!). Two possible mechanisms are shown below. 2 [O3] Overall Equation: 2 O 3 (g) 3 O 2 (g) Observed Rate Law: Rate = k = k[o 3 ] 2 [O 2 ] -1 [O ] Proposal #1 Proposal #2 k 1 k 1 O 3 O 2 + O (slow) O 3 O 2 + O (fast, equilibrium) O + O 3 k2 k 2 O 2 + O 2 (fast) ` O + O 3 O 2 + O 2 (slow) Is one mechanism a better candidate than the other for representing what actually happens when this reaction occurs? Back up your answer with some work! k -1 2 NOTE: I no longer cover mechanisms with a fast equilibrium as their first step, so you won t see a mechanism like proposal #2 on your Exam 1. But I left this problem here because the question asked in this problem is still a good example of a kind of question I d ask on mechanisms. (Interestingly, Proposal 2 is highlighted in both Example 13.9 in Tro and in the video made by Tro in one of the Items on PS1in Mastering!) 10. (10 pts) (a) Which reaction (forward or reverse) has the greater activation energy? (b) Draw a reaction progress diagram to the right that is qualitatively consistent with (not quantitatively!) the figure on the left. Ask me if you are not sure what I mean here. # p a r t i c l e s T 1 T 2 KE E a,f E a,r (Potential) Energy (kj) Products Progress of Reaction (c) Is the reaction endothermic or exothermic? (d) If T is increased in this case, what, if anything, happens to K, and which direction (if any) would a system at equilibrium shift if T were raised?