Chemistry 12 Dr. Kline 28 September 2005 Name

Transcription

1 Test 1 first letter of last name Chemistry 12 Dr. Kline 28 September 2005 Name This test consists of a combination of multiple choice and other questions. There should be a total of 24 questions on 8 pages; please check to make sure that they are all here. You may use the equation/information sheet and any periodic table provided (on page 9). Do not use your own tables, scratch paper or other information. Do not share your calculator. Please turn off all cell phones, pagers and other communication devices; if they make noise, they will be confiscated. Shared calculators will be confiscated. Multiple Choice Questions Each of the following 20 questions is worth three points, for a total of 60 points. It is not necessary to show work for these questions; however, you may write on the exam. Answer each question by circling the letter(s) corresponding to the correct choice(s). Unless it is stated otherwise in a given problem, there is only one correct answer for each question. In questions where it states that more than one answer is possible, there may be one or more correct answers: please circle all of the correct answers. 1. Suppose that the average rate of appearance of D in the homogeneous gas phase reaction 3A + 2B C + 4D is mole/l s. What is the average rate of disappearance of A over the same time period? a mole/l s b mole/l s c mole/l s d mole/l s e mole/l s 2. Which of the following are possible units for the reaction A + 2B C + D that has the rate law, rate = k[a][b]? a. mole/l s b. mole 2 /L 2 s c. L/mole s d. L 2 /mole 2 s e. none the above 3. With respect to the reaction and rate law in Question #2, which of the following is a possible mechanism? a. A + B E (slow) E + B C + D b. A + B F (slow) F C + D c. 2B G (slow) G C + D d. 2B H (slow) A + H C + D e. none of them Chem 12 Test 1A online page 1 of 1

2 4. Consider the homogeneous gas phase second-order decomposition reaction A B + C. When the initial concentration of A is M, the first half-life is 44.0 s at 28 C. What is the time for the second half-life at the same temperature? a s b s c. 132 s d. 176 s e s f s 5. The reaction 2H 2 O 2 2H 2 O + O 2 has the following mechanism: H 2 O 2 + I H 2 O + IO H 2 O 2 + IO H 2 O + O 2 + I What is the catalyst in this reaction? a. H 2 O b. I c. IO d. H 2 O 2 e. there is none 6. Consider the decomposition reaction A B + C that is first-order in A. Suppose that 35% of the original amount of A has decomposed after 112 minutes at 25 C. What is the value of the rate constant, k, at this temperature (k values given have time units of minutes)? a b c d e f What is the half-life of the reaction in question #6 above at the same temperature? a. 56 minutes b. 150 minutes c. 86 minutes d. 74 minutes e. 180 minutes 8. How long will it take for 65% of the original amount of A to decompose at the same temperature (again, for the reaction in question #6)? a. 46 minutes b. 110 minutes c. 280 minutes d. 210 minutes e. 60 minutes 9. A certain reaction (not the reaction in question #6) experiences a doubling of its rate when the temperature is increased from 45 C to 55 C. What is the activation energy of this reaction? a. 50. kj/mole b. 60. kj/mole c. 68 kj/mole d. 73 kj/mole e. it is impossible to tell Chem 12 Test 1A online page 2 of 2

3 10. Suppose that the homogeneous gas phase reaction A B + C is allowed to continue at a constant temperature until 12.5% of the original amount of A remains. How many half-lives have transpired? a. two b. three c. four d. five e. it is impossible to tell without knowing the order of the reaction 11. Which of the following is/are true of a reaction at equilibrium? More than one answer may be correct. a. Q = K b. K = 1 c. k f = k r (f= forward; r = reverse) d. none of them 12. For which of the following reactions is K P = K C? More than one answer may be correct. a. 4NO(g) + O 2 (g) 2N 2 O 3 (g) b. S 2 (g) + C(s) CS 2 (g) c. 3O 2 (g) 2O 3 (g) d. none of them 13. Suppose that a reaction has K C =1100 at 25 C and K C =4500 at 55 C. Which of the following is true? a. the reaction is exothermic b. the reaction is endothermic c. it is impossible to tell whether the reaction is endothermic or exothermic 14. Suppose that a certain reaction has K C = and the current set of conditions correspond to Q C = Which way will this reaction shift to establish equilibrium? a. to the left b. to the right c. neither left nor right because it is already at equilibrium d. it is impossible to tell 15. What is the value of K P for the homogeneous gas phase reaction 2A + B C + 3D at 500K based on the following data (reactions below are also homogeneous gas phase reactions): A + B C + D K P =260 at 500K 2D A K P =120 at 500K a. K P = 31,000 b. K P = 380 c. K P = 0.46 d. K P = 2.2 e. K P = 140 Chem 12 Test 1A online page 3 of 3

4 Suppose that the substances in the following equilibrium reaction are mixed at a certain temperature and the reaction is allowed to come to equilibrium before each of the following perturbations is applied. Assume any factor that is not mentioned is not changed. Assume any substance mentioned is in the same phase as it appears in the reaction equation. This equilibrium reaction applies to questions SiCl 4 (s) + 2H 2 O(g) SiO 2 (s) + 4HCl(g) H = -140 kj 16. What is the effect of adding a small amount of HCl on the number of moles of SiO 2 at equilibrium? 17. What is the effect of increasing the volume of the container on the number of moles of HCl at equilibrium? 18. What is the effect of decreasing the temperature on the number of moles of H 2 O at equilibrium? 19. What is the effect of removing some HCl on the concentration of SiO 2 at equilibrium? 20. Suppose that the reaction CaSO 3 (s) CaO(s) + SO 2 (g) is at equilibrium. The volume of the reaction container is then increased. Which of the following will happen? More than one answer may be correct. a. the moles of SO 2 at equilibrium will increase b. the partial pressure of SO 2 at equilibrium will increase c. the concentration of SO 2 at equilibrium will increase d. none of the above will happen Chem 12 Test 1A online page 4 of 4

5 Problems and Questions Each of the following is worth the indicated number of points, for a total of 40 points. Please show work with units to receive credit. Also, be sure to provide all of the information requested for each question points The homogeneous gas phase reaction A + 2B 2C + D was studied at 25 C and the following data were obtained. Expt. (A), M (B), M Rate, M/s a. Determine the specific rate law for the reaction. b. Calculate the value of k, including units, at 25 C. c. Calculate the rate of the reaction when the initial concentrations of A and B are M and M, respectively points Consider the data and reaction below. Note that there are two sets of pressure data for the reaction run at two different temperatures. Assume that the reaction starts with only reactant present at time zero. Be sure to read carefully the questions that follow. 2N 2 O 5 (g) 4NO 2 (g) + O 2 (g) Time, s P T, T = 318 K P T, T = 338 K a. Calculate the average rate of the reaction in atm/s over the first 300. seconds of the reaction at 338 K. b. Calculate the average rate of disappearance of N 2 O 5 in atm/s over the same time period at the same temperature. c. Briefly summarize how you could calculate the activation energy of the reaction from the data given. Or, if you don t think that it s possible to determine this, briefly explain why points Consider the homogeneous gas phase reaction SO 2 + NO 2 SO 3 + NO that has K = 3.75 at some temperature. Suppose that 2.00 moles of SO 2 and 3.00 moles of NO 2 are injected into a 10.0-L container at the specified temperature and the reaction is allowed to come to equilibrium. Calculate the concentrations of all four species at equilibrium points Consider the homogeneous gas phase reaction 3A + B 2C + 4D. Suppose that 3.00 moles of A, 2.00 moles of B, 1.00 moles of C, and 4.00 moles of D are combined in a 20.0-L container at 125 C and the reaction is allowed to come to equilibrium. The concentration of D at equilibrium is determined to be M. a. Calculate the concentrations of A, B, and C at equilibrium. b. Calculate the value of K C (with units) for this reaction at the specified temperature. c. Calculate the value of K P (with units) for this reaction at the specified temperature. Chem 12 Test 1A online page 5 of 5

6 TEST #1 Information You may (carefully) remove this sheet from the test. Some Conversions and Constants K = C atm = 760 torr = 760 mmhg energy R = 8.31 J/mole K pressure-volume R = L atm/mole K = 62.4 L torr/mole K Quadratic Equation For an equation of the type ax 2 + bx + c = 0 x = "b ± b2 " 4ac 2a Ideal Gas Equation PV = nrt (R = L atm/mole K = 62.4 L torr/mole K) Kinetics For a reaction of the type A B + C Zero-order First-order Second-order Rate Law Rate = k Rate = k(a) Rate = k(a) 2 Integrated (A) = -kt + (A) 0 ln(a) = -kt + ln(a) 0 1/(A) = kt + 1/(A) 0 Rate Law Half-life t 1/2 = (A) 0 /2k t 1/2 = 0.693/k t 1/2 = 1/k(A) 0 k = Ae -Ea/RT ln(k) = (-E a /RT) + ln(a) ; R as below; A = constant for a given reaction " ln k 2 $ # k1% = Ea " T 2 &T 1$ R # T 2T 1 % J where R = 8.31 mole K (temperature term in above eqn also equal to (1/T 1 1/T 2 ) Equilibrium K p = K c (RT) ng (R = L atm/mole K = 62.4 L torr/mole K) Chem 12 Test 1A online page 6 of 6