a. rate = k[no] 2 b. rate = k([no][o 2 ] c. rate = k[no 2 ] 2 [NO] -2 [O 2 ] -1/2 d. rate = k[no] 2 [O 2 ] 2 e. rate = k([no][o 2 ]) 2

Transcription

1 General Chemistry III 1046 E Exam 1 1. Cyclobutane, C 4 H 8, decomposes as shown: C 4 H 8 (g)! 2 C 2 H 4 (g). In the course of a study of this reaction, the rate of consumption of C 4 H 8 at a certain point was 4.50 x 10-4 M/sec. What is the rate at which C 2 H 4 (g) is being generated at this point, also in M/sec? a x 10-4 M/sec b x 10-4 M/sec c x 10-4 M/sec d x 10-4 M/sec e x 10-3 M/sec 2. The units in which the rate of a chemical reaction in solution is measured is (could be) a. L 2 mol -1 s -1 b. mol L -1 s -1 c. mol s L -1 d. sec L -1 mol If a reaction involving a single reactant is first order with a rate constant of 4.50 x 10-2 s -1, how much time is required for 75.0% initial concentration of reactant to be used up? a seconds b seconds c seconds d seconds e seconds 4. A reaction has the rate law, rate = k[a][b] 2. Which one of the following will cause the greatest increase in the reaction rate? a. decreasing the temperature without changing the concentrations b. doubling the concentration of B c. quadrupling the concentration of A d. tripling the concentration of B e. doubling the concentration of A 5. The reaction, 2 NO(g) + O 2 (g)!2 NO 2 (g), was found to be first order in each of the two reactants and second order overall. The rate law should therefore be written as a. rate = k[no] 2 b. rate = k([no][o 2 ] c. rate = k[no 2 ] 2 [NO] -2 [O 2 ] -1/2 d. rate = k[no] 2 [O 2 ] 2 e. rate = k([no][o 2 ]) 2 6. For the reaction, 3 B + C!2E + 2 F, some initial rate measurements were carried out and data for three runs are shown below run # [B] [C] rate, M/sec The rate law, therefore, is a. rate = k[b] 3 [C] b. rate = k[b][c] c. rate = k[b] 2 [C] 2 d. rate = k[b] 2 [C] e. rate = k[b][c] 2 7. The reaction, 2 A 2 X 4 (g)! 2 A 2 X 3 (g) + X 2 (g), the reaction was found to be first order. The rate law, therefore, should be a. rate = k([a 2 X 3 ] 2 [X 2 ])/[A 2 X 4 ] 2 b. rate = k[a 2 X 4 ] 2 c. rate = k([a 2 X 3 ][X 2 ])/[A 2 X 4 ] d. rate = k[a 2 X 4 ] e. rate = k[a 2 X 4 ] 2 /([A 2 X 3 ] 2 [X 2 ]) 8. For a first order reaction with a single reactant, after seconds, 10.0% of the reactant 1

2 remains. The rate constant for the reaction is therefore a s -1 b s -1 c. 100 s -1 d s -1 e s The rate constant for a first order decomposition reaction is min -1. What is the half-life of the reaction? a min b sec c min d min 10. In a first order reaction, what fraction of the material will remain after 4 half lives? a. 1/16 b. 1/8 c. 1/4 d. 1/3 11. The initial concentration of a reactant in a first order reaction is molar. What will be its concentration after 3 half lives? a M b M c M d M e M 12. For the reaction, A! B + C, the rate law is Rate = k[a]. If it takes 80.0 seconds for 70.0% of a 10.0 gram sample of A to be transformed into products, what is the value of the rate constant? a s -1 b s -1 c s -1 d s -1 e s The reaction of substance A with substance C was carefully studied under conditions where the [C] remained essentially constant. The graphs of [A] vs time and that of ln[a] vs time both gave curves, but the graph of 1/[A] vs time gave a straight line. The reaction, therefore, is a. zero order wrt A. b. one-half order wrt A. c. first order wrt A. d. second order wrt A. e. third order wrt A. 14. A reaction is first order overall. For a given sample, if its initial rate is M/sec and 25.0 days later its rate dropped to 6.25 x 10-4 M/sec, what is its half-life? a days b days c days d. 5.0 days e days 15. For a one step reaction, the activation energy for the forward reaction is 40.0 kj/mol, and the enthalpy of reaction is kj/mol. Which statement below is true? a. The activation energy of the forward reaction would be affected to a greater extent than the activation energy of the reverse reaction by addition of a catalyst. b. The value for the enthalpy of reaction would be decreased by addition of a catalyst. c. The reaction is endothermic. d. The reverse reaction is slower than the forward reaction (smaller rate constant). e. The reaction rate would be decreased by an increase in temperature. 16. The equation, A + 2 B! C + D describes an elementary reaction, which takes place in a single step. The rate law must therefore be a. rate = k[a][b] 2 2

3 b. rate = k([c][d])/([a][b] 2 ) c. rate = k[a]/[c] d. rate = k[a] 2 e. rate = k[a][b] 17. The reaction: A + 3 B! D + F was studied carefully and the following mechanism was finally determined. A + B! C (fast) C + B! D + E (slow) E + B! F (very fast) The rate law for the reaction would therefore be a. rate = k[a][b] 2 b. rate = k[c][b] c. rate = k[a][b] 3 d. rate = k[a][b] 18. The decomposition of hydrogen peroxide in solution was studied in laboratory, and the following mechanism proposed based on the experimental data. Which one of the following statements is false? H 2 O 2 + I -! H 2 O + IO - (slow) H 2 O 2 + IO -! H 2 O + O 2 +I - (fast) a. The reaction is first order wrt H 2 O 2. b. The reaction is first order wrt I -. c. I - is a catalyst. d. IO - is an intermediate. e. The reaction is first order wrt IO A catalyst alters the rate of a reaction by a. providing an alternate pathway which has a different activation energy b. changing the products formed in the reaction c. changing the frequency of collisions between molecules d. changing the enthalpy of reaction for the reaction 20. Which one of the following statements concerning the rate of a chemical reaction is false? a. It will be very rapid if the activation energy is large. b. It will be slow if one or more of the steps is slow. c. It may be inhibited sometimes by certain catalytic agents. d. It is dependent on temperature. e. It often increases when the concentrations of one of the reactants is increased. 21. Given the reaction, c aa + bb! dd + ee where C is a catalyst. If we try, rate =k[a] q [B] r [C] s for a generic rate law statement, which one of the statements below is false? a. The exponents q, r, and s are often integers. b. The exponent s must be determined experimentally. c. The exponents q and r are equal to the coefficients a and b, respectively. d. The overall order of the reaction is q + r + s. e. The symbol k represents the rate constant. 22. All of the following statements about chemical equilibrium are true except: a. At equilibrium, the reactant and the product concentrations show no further change with time. b. A true chemical equilibrium can only be attained starting from the reactant side of the equation. c. At equilibrium, the forward reaction rate equals the reverse reaction rate. d. The same equilibrium state can be attained starting either from the reactant or product side of the equation. e. At equilibrium, the reactant and product concentrations are constant. 23. In a study of the system, H 2 (g) + I 2 (g)! 2 HI(g), at 699 K, several different reaction mixes, described below, were prepared, placed in a

4 liter container, and allowed to attain equilibrium at 699 K. Despite having different starting compositions, as shown, four of the five mixtures had the identical composition at equilibrium. Which one of the systems attains a different equilibrium composition than the others? a. System 1:0.100 moles of H 2, moles of I 2 and moles of HI b. System 2:0.175 moles of H 2, moles of I 2 and moles of HI c. System 3:0.400 moles of H 2, moles of I 2 and moles of HI d. System 4:0.150 moles of H 2, moles of I 2 and moles of HI e. System 5:0.300 moles of H 2, moles of I 2 and moles of HI Given the pair of reactions shown with the equilibrium constants, PCl 3 (g) + Cl 2 (g)! PCl 5 (g) K 1 2 NO(g) + Cl 2 (g)!2 NOCl(g) K 2 What is the equilibrium constant for the reaction, PCl 5 (g) + 2 NO(g)! PCl 3 (g) + 2 NOCl(g) a. K 1 K 2 b. K 2 /K 1 c. K 1 /K 2 d. (K 1 K 2 ) -1 e. K 2 - K The Kc for, R 2! D 2 is 6.8 x Which one of the following statements is true? a. The equilibrium concentration of D 2 is always greater than that of R 2. b. The equilibrium concentration of R 2 is always greater than that of D 2. c. Adding more R 2 will increase the value of the equilibrium constant. d. Adding a catalyst will increase the equilibrium concentration of D 2. e. Adding a catalyst will increase the value of the equilibrium constant. 26. For the reaction, 2 SO 2 (g) + O 2 (g) "! 2 SO 3 (g), at K the equilibrium constant, K c, has a value of Calculate the value of K p at the same temperature. a x 10 3 atm -1 b atm -1 c. 960 atm -1 d atm -1 e atm For a specific reaction, which of the following statements can be made about the equilibrium constant? a. It can change with temperature. b. It may be changed by addition of a catalyst. c. It increases if the concentration of one of the products is increased. d. It increases if the concentration of one of the products is decreased. e. It always remains the same. 28. For the reaction, 2 SO 2 (g) + O 2 (g) "!2 SO 3 (g), at K the equilibrium constant, Kc, has a value of 4.62.A system was charged to give these initial concentrations, [SO 3 ] = M [O 2 ] = M,[SO 2 ] = M. In which direction will it go? a. to the right or the left depending on the pressure b. to the left c. it will remain at the same concentrations d. to the right 29. Consider the following equilibrium, CO(g) + 3 H 2 (g) "!CH 4 (g) + H 2 O(g). The result of removing some CH 4 (g) and H 2 O(g) from the system is that 4

5 a. more CH 4 (g) and H 2 O(g) are produced to replace that which is removed b. K c decreases c. more CO(g) is produced d. more H 2 O(g) is consumed to restore the equilibrium e. more CH 4 is consumed to restore the equilibrium 30. The system, H 2 (g) + X 2 (g) "! 2 HX(g) has a value of 24.4 for the constant, K c.a system being tried in a 3.00 liter reactor was charged with moles of H 2, moles of X 2, and moles of HX. The catalyst was introduced using a remote unit, and the system was allowed to come to equilibrium. Which statement below describes the situation? a. The reaction goes to the right, Q < K. b. The reaction goes to the left, Q < K. c. The reaction goes to the right, Q > K. d. The reaction goes to the left, Q > K. e. It is not possible to predict in which direction the system will travel to arrive at equilibrium. 31. For the reaction system, 2 SO 2 (g) + O 2 (g) "! 2 SO 3 (g), the equilibrium concentrations are SO 3 : 0.120M; SO 2 : 0.860M; O 2 : 0.330M Calculate the value of K c for this reaction. a b c d e For the equilibrium, H 2 (g) + X 2 (g)"!2hx(g), K c = 24.4 at 300 K. A system made up from these components which is at equilibrium contains moles of H 2 and moles of HX in a 3.00 liter container. Calculate the number of moles of X 2 (g) present at equilibrium. a mol b mol c mol d mol e mol 33. Which of the following has Kc = Kp? a. SO2Cl2(g) SO2(g) + Cl2(g) b. C(s) + H2O(g) CO(g) + H2(g) c. 2 SO3(g) 2 SO2(g) + O2 (g) d. 2 HI(g) H2(g) + I2(g) 34. At equilibrium, what is equal? a. concentrations of products and reactants b. rate constants for the forward and reverse reactions c. the rate of the forward and reverse reaction d. the partial pressures of the reactants and products e. the reaction quotient and the rates of reactions 35. Ni (s) + 4CO (g) Ni(CO)4 (g) At 680 o C, this equilibrium mixture contained 0.20 moles of Ni(CO)4, 0.40 moles of CO, and 0.80 moles of Ni in a 2.00 L container. Calculate Kc for the reaction at 680 o C. a b c d CS2(g)+ 3Cl2 (g) "! S2Cl2 (g)+ccl4(g) When 0.80 mol of CS2 and 2.4 mol of Cl2 are placed in a 1.00 liter container and allowed to reach equilibrium, the mixture contains 0.60 mol CCl4. What is the equilibrium concentration of Cl2? a M b. 1.2 M c. 1.6 M d. 1.8 M e. 2.1 M 37. H2 (g) + Br2 (g) 2HBr (g) Calculate the value of the equilibrium constant, Kc, for this reaction if the following equilibrium 5

6 concentrations are observed at 1297 K: [H2] = [Br2] = M and [HBr] = 7.4 M. a. 1.3 x 10 3 b. 1.4 x 10 5 c x 10 4 d. 1.4 x A 1.00 liter flask contained 0.24 mol NO2 at 700 K. which decomposed according to the following equation. When equilibrium was achieved, 0.14 mol NO was present. Calculate Kc. 2 NO2(g) 2 NO(g) + O2(g) a b c. 1.1 x 10-2 d. 5.7 x 10 3 e. 9.6 x For the reaction H2O (g) + Cl2O (g) 2 HOCl (g), the equilibrium constant is at 25 o C. Consider the following cases and for each of these assign one of the following qualifiers A. this is at equilibrium, no change in the concentrations needed B. this is not at equilibrium, the reaction will shift left to reach equilibrium C. this is not at equilibrium, the reaction will shift right to reach equilibrium a. A 1.0 L flask contains 1.00 mol HOCl, mol Cl2O and mol H2O b. A 2.0 L flask contains mol HOCl, mol Cl2O and 0.98 mol H2O 2. Consider the exothermic equilibrium H2O (g) + CO (g) H2(g) + CO2(g) Assign the qualifiers (no change, shift left, shift right) under the following circumstances for a system initially at equilibrium. (2,2,2) (1) gaseous carbon dioxide is added (2) The system is cooled down (3) The pressure is increased by decreasing the volume 3. A reaction follows the following elementary steps (2,1,2,2,3) I. O3 + N2O! O + N2O3 II. O+ CF2Cl2! CF2Cl + ClO III. ClO + O3! Cl + 2O2 IV. Cl + CF2Cl! CF2Cl2 V. N2O3 + O3! N2O2 + 2O2 a. What is the overall reaction? b. List catalysts, if any c. List intermediates d. If this reaction is an overall 3 rd order reaction, which step is the slow step? e. What is the rate law for this reaction? 6

7 Numerical questions, calculations are needed to be shown 1. The reaction 2 HCl(g) H2(g) +Cl2(g) is analyzed. At equilibrium, it was found that [H2]= M, [Cl2]= 0.23 M, [HCl]= 2.07 x10-4 M in 1.0 Liter container. To this system moles of Cl2 were added. What will be the final concentrations of each of these when the equilibrium is reestablished? (7) 2. The equilibrium, CO2 (g) + H2 (g) H2O (g) + CO (g) has a K of at 552 o C. The system initially had 5.0 moles CO2, 5.0 moles H2, 1.0 moles CO, 1.0 moles H2O, determine the equilibrium concentrations of each. (6) Write the expression for the equilibrium constant, K C, for the reaction, 4 Cr(s) + 3 CCl 4 (g)"! 4 CrCl 3 (g) + 4 C(s) The half-life of a second order reaction depends on the initial concentration of the reactants. (true/false) The final step in a multistep reaction mechanism is called the rate determining step. (True/false) 7