Chem 126 Exam 1, Sep 30, 2019 last name first name (Middle) Print Name:___________________│________________│____| Instructor: Circle one: Sarkar Srinivas/Balasubramanian Naughton Section #:___________Student #: _________ DIRECTIONS (PUT AWAY YOUR CELL PHONE. IF YOU ARE CAUGHT WITH A CELL PHONE OR PDA OUTSIDE DURING THE EXAM, YOU WILL BE EJECTED FROM THE EXAM IMMEDIATELY AND GIVEN A 0) 1. First fill in the information at the top of this paper. Print your last name as it appears on the class roster, then print your first name and middle initial. Then read and sign the honor code statement below. Finally fill in the requested information on the SCANTRON ANSWER SHEET according to the following scheme: NAME: Print LAST name, then first (print clearly) SUBJECT: Course and section DATE: 09/30/19 PERIOD: Your Student Number (we give you this) 2. The last page of this exam is a periodic table. 3. This exam consists of two parts, A and B, for a total of 125 points. PART A has 19 multiple choice questions worth five points each for a total of 95 points. Each question has only one correct answer, and no credit is lost if you choose a wrong answer. Therefore, leave no question unanswered, even if you simply must guess. 4. PART B consists of open ended problems worth 30 points. Show all work, including how units cancel to give the final answer, along with appropriate sig. figs. Write your final answers in the spaces provided. INTEGRITY CODE STATEMENT: On my honor I pledge that I have not violated the provisions of the NJIT student integrity code as specified by http://www.njit.edu/academics/pdf/academic-integrity-code.pdf Signature: ————————— ___________ DO NOT WRITE BELOW THIS LINE _____________________ PART A: ______________ (95) PART B: #1 ______________ (15) #2 ______________ (15) TOTAL SCORE: 1 Chem 126 Exam 1, Sep 30, 2019 Part A: 19 questions worth 95 points 1. The average rate of decomposition of NH3 in the reaction 2NH3 (g) →N2(g) + 3H2(g) is 7.25 x 10-3 atm/s over a certain interval of time. What is the rate of appearance of H2 (g)? a. 10.9 x 10-3 atm/s b. 7.25 x 10-3 atm/s c. 4.83 x 10-3 atm/s d. 191 x 10-3 atm/s e. 17.7 x 10-3 atm/s 2. For a given reaction: 2A + B2 → 2 AB ; Rate = k[A]2[B2]1, what are the units for the rate constant, k? a. M s b. M-1 s-1 c. M-2 s-1 d. M-3 s-1 e. M-1 s 3. The rate of a reaction A-> products was determined to be 0.50 mol/hr. Using dimensional analysis, express this in molecules per second. a. 1.3 x 10-3 molecules/s b. 2.4 x 10-17 molecules/s c. 1.1 x 1027 molecules/s d. 8.36 x 1019 molecules/s e. 3.0 x 1023 molecules/s 4. For a given reaction, Z→ products, the rate law was determined to be rate = k[Z]3. If the rate is 5.0 M/min when the [Z] is 2.0 M, by what factor will the rate increase when the [Z] = 6.0 M? a. Rate is 2X the original rate b. Rate is 3X the original rate c. Rate is 4X the original rate d. Rate is 9X the original rate e. Rate is 27X the original rate 2 Chem 126 Exam 1, Sep 30, 2019 5. The rate of the reaction, 2A2 + 2B -->2A2B is given by Rate = k[A2]2. Calculate the value of the rate constant if the rate of the reaction is 0.60 M/min at [A] = 0.20M. a. 30 M-1.min-1 b. 15 M-1.min-1 c. 120 M-1.min-1 d. 1.0 M-1.min-1 e. 18.0 M-1.min-1 6. The balanced equation for the reaction of bromate ion with bromide ion in acidic solution is given by: BrO3– + 5Br– + 6H+ → 3Br2 + 3 H2O. If the rate of disappearance of the bromide ion ( Br–) is 0.26 M/sec, what is the rate of appearance of Br2? a. 0.26 M/sec b. 0.17 M/sec c. 0.39 M/sec d. 0.09 M/sec e. 1.3 M/sec 7. Consider the reaction: 2A + 5B → products. Which of the following is the correct expression to describe the rate of the reaction (t is time)? a. d[A]/dt b. –d[B]/5dt c. 2d[A]dt d. 2d[A]/5d[B] e. d[A]/dt – d[B]/dt 8. The reaction 2A + 5B → products is second order in A and first order in B. What is the rate law for this reaction? a. rate = k[ A]1[B]2 b. rate = k[ A]2[B]1 c. rate = rate = k[A]5[B]2 d. rate = k[ A]3[B]2 e. rate = k[ A]2[B] 3 Chem 126 Exam 1, Sep 30, 2019 9. At a given temperature, a first-order reaction has a rate constant of 3.4 x 10–3 s–1. WHat is the time required for the reaction to be 44% complete? a. 4.0 min b. 1.8 min c. 20 min d. 2.8 min e. 19 min 10. Ni decays by a first-order process via the emission of a beta particle. The 63 Ni isotope has a half-life 63 of 100. years. How much of the original sample of Ni will remain in 300 years? a. ½ b. ¼ c. ⅛ d. ⅓ e. ⅔ 63 11. Which of the following correctly describes R in the reaction mechanism shown? Step 1: A2 + B2 → R + C (fast) Step 2: A2 + R → C (slow) a. a reactant b. an intermediate c. a catalyst d. a product e. none of the above 12. The following mechanism if proposed for the decomposition of nitrous oxide (N2O) to N2 and O2 Which rate law is consistent with the mechanism given? 2N2O ⇄ N4O2 (fast) N4O2 --> 2N2 + O2 (slow ) 2 a. Rate = k[ N2O] b. Rate = k[N2O4] c. Rate = k[N2O] d. Rate = k[N2O4]2 e. Rate = k[N2O] [N2O4] 4 Chem 126 Exam 1, Sep 30, 2019 13. During the decomposition of ozone, it was observed that chlorofluorocarbons, important sources of Cl radical in the upper atmosphere, lower the activation energy and increase the rate of decomposition of ozone. This suggests that Cl radical produced by chlorofluorocarbons is: a. an intermediate b. a catalyst c. a reactant d. a product e. an activated complex 14. A 1.0 mol sample of CaCO3 is allowed to decompose in a 2.0 L vessel at a certain temperature according to the reaction CaCO3 (s)) ⇄ CaO(s) + CO2(g) . At equilibrium, 0.40mol of CO2 is present. What is the value of Kc? a. 1.6 b. 0.40 c. 0.16 d. 0.20 e. 0.8 15. What is the expression for the equilibrium constant Kc for the reaction at 100°C? 2NO2Cl(aq) ⇄ 2NO2(aq) + Cl2(aq) . a. Kc=[NO]2[Cl2] b. Kc=[NO2Cl]2 c. Kc= [N O ] 12 [Cl ] 2 2 [N O2 Cl]2 d. Kc= [N O e. Kc= 2 2 ] [Cl 2 ] 2 [N O2 ] [Cl 2 ] [N O2 Cl]2 16. Consider the balanced equation: 3A + 2B → 3C + D. 5.0 mol A are mixed with 3.0 mol B at an elevated temperature. After 3.5 mol of A have reacted, how much B is left? a. 0.9 mol b. 2.3 mol c. 1.5 mol d. 1.4 mol e. 0.7 mol 5 Chem 126 Exam 1, Sep 30, 2019 17. Which of the following is true about a system at equilibrium? a. The rate of forward reaction and the rate of reverse reaction are unequal b. The concentration of reactants = concentration of products c. Equilibrium can only be attained in the forward direction (if you start with reactant) d. The concentration of the reactant and products are unchanged e. The equilibrium constant is the ratio of the rates of the reaction 18. Calculate Kp for H2O(g) + ½ O2(g)⇄ H2O2(g), given the following reactions: H2(g) + O2(g) ⇄ H2O2(g) 2H2(g) + O2(g) ⇄ 2H2O(g) a. b. c. d. e. f. Kp = 2.3 x 106 Kp = 1.8 x 1037 1.7 x 1030 4.1 x 1043 6.4 x 10-21 1.0 x 10-25 2.4 x 10-19 1.9 x 10-13 19. A 10.0-g sample of solid NH4Cl is heated in a 5.00-L container to 900°C. At equilibrium the pressure of NH3(g) is 1.51 atm. NH4Cl(s) ⇄ NH3(g) + HCl(g). Calculate Kp (Hint: write the equilibrium expression first). a. 1.51 b. 2.28 c. 3.02 d. 8.21 e. 9.65 6 Chem 126 Exam 1, Sep 30, 2019 Part B: (30 points) Answer all the questions. Show your work clearly and write answers in the space provided. Report answers with units where possible. 1A.(9 points) Determine (a) the rate equation and (b) the rate constant for the hypothetical reaction A + B → C given the following initial concentrations and initial rate data. Run # [A]0 (mol/L) [B]0 (mol/L) Initial Rate (mol/L·s) (1) 0.025 0.20 2.2 (2) 0.050 0.20 8.8 (3) 0.025 0.40 2.2 Order with respect to A: ____________________________ Order with respect to B: ____________________________ rate constant with units: ____________________________ Rate law: ____________________________ 7 Chem 126 Exam 1, Sep 30, 2019 1B.(6 points) Suppose the rate of reaction is second order for A--> B + C. What is the approximate concentration of A (in M) after 55 s if [A]0 = 0.80 M and k = 0.035 M-1s-1 2A. (5 points) Consider the following reaction: CS2(g) + 4H2(g) -->CH4(g) + 2H2S(g). The equilibrium constant K is about 0.31 at 900.°C. What is Kp at this temperature? 8 Chem 126 Exam 1, Sep 30, 2019 2B) (10 points) For the equilibrium system: CO2(g) + H2(g) ⇄ CO(g) + H2O(g) K equals 1.6 at 1260 K. If 0.15 mol each of CO2, H2 were placed in a 2.0 L flask, calculate the equilibrium concentrations of all the species. CO2(g) + H2(g) ⇄ CO(g) + H2O(g) Initial: Change: Equilibrium: Expression for K [CO2]eqbm__________________ [H2]eqbm__________________ [CO]eqbm__________________ [H2O]eqbm__________________ 9 Chem 126 Exam 1, Sep 30, 2019 10
