Kinetics: the branch of chemistry that deals with rates of reactions-chemistry & engineering.

Usually expressed in terms of reactant:

I. Reaction Rate

rate =

Usually expressed in terms of reactant

:

^Rate:

2N₂O₅ 4 NO₂ O₂

Rate = -conc./ time = - [N₂O₅] / t

 Bellow you will find 3 sets of practice questions!!!!

Kinetics Practice Questions

Answer each question on your own paper. The answers have been provided below. Remember, you can only learn from your mistakes! Better to make those mistakes here than on your tests.

Use these two graphs to answer question 1.

Hint:

ANSWERS

AP CHEMISTRY REVIEW PROBLEMS

FOR EXAM  ON KINETICS

Answers to REVIEW problems 1-14 are at the end of this document.

Equations to know:

Ln[A[/[A]₀ = -kt

T_1/2 = ln2/k

k=Ae-Ea/RT

1. The reaction of hydrogen and iodine monochloride is represented by this equation:

                                    H_2(g) + 2ICl_(g) ---> 2HCl_(g) + I_2(g)

This reaction is first order in H_2(g) and also first order in ICl_(g). Which of these proposed mechanisms can be consistent with the given information about this reaction?

            Mechanism 1               H_2(g) + 2ICl_(g) ---> 2HCl_(g) + I_2(g)

            Mechanism 2               H_2(g) + ICl_(g) ---> HCl_(g) + HI_(g)   (slow)

                                                HI_(g) + ICl_(g) ---> HCl_(g) + I_2(g)     (fast)

            (A) 1 only                     (B) 2 only                  (C) Both 1 and 2        (D) Neither 1 nor 2

2. Nitramide, N₂H₂O₂, decomposes slowly in aqueous solution. This decomposition is believed to occur according to the reaction mechanism shown below. Which of the rate laws for the decomposition of nitramide is consistent with this mechanism.

            Step 1) N₂H₂O₂ <-> N₂HO₂^- + H⁺                 (fast equilibrium)

            Step 2) N₂HO₂^- ---> N₂O + OH^-                   (slow)

            Step 3) H⁺ + OH- ---> H₂O                            (fast)

(A) Rate = k [N₂H₂O₂]

(B) Rate = k [N₂H₂O₂] [H⁺]

(C) Rate = (k [N₂H₂O₂]) / [H⁺])

(D) Rate = (k [N₂H₂O₂]) / [N₂HO₂-]

(E) Rate = k [N₂H₂O₂] [OH^-]

3. CH₃Cl_(g) + H₂O_(g) ---> CH₃OH_(g) + HCl_(g)

When this reaction was studied, these data were obtained.

                        Initial Concentration, M           Initial Rate, M/s                     

                        (CH₃Cl)           (H₂O)

Exp. 1              0.100               0.100                           0.182

Exp.2               0.200               0.200                           1.45

Exp.3               0.200               0.400                           5.81

a) Based on these data, what are the orders of the two reactants in this reaction?

b) Write the rate law for the reaction

c) Calculate the value for the rate constant for the reaction

4. 2N₂O_5(g) ---> 4NO_2(g) + O_2(g)

What is the ratio of the rate of decomposition of N₂O₅ to the rate of the formation of NO₂?

                        (A) 1:2                        (B) 2:1             (C) 1:4             (D) 4:1

5. The reaction between chloroform, CHCl_3(g), and chlorine, Cl_2(g), to form carbon tetrachloride CCl_4(g) and HCl_(g) is believed to occur by this series of steps.

            (Step 1)   Cl₂ <---> Cl_(g) + Cl_(g)

            (Step 2)   CHCl_3(g) + Cl_(g) ---> CCl_3(g) + HCl_(g)

            (Step 3)   CCl_3(g) + Cl_(g) ---> CCl_4(g)

a) If this reaction is first order in CHCl₃ and half order in Cl₂, which statement about the relative rates of steps 1, 2, and 3 is correct?

            (I) Step 1 is the slowest.

            (II) Steps 1 and 2 must both be slow.

            (III) Step 2 must be the slower than step 1.

            (IV) Step 3 must be the slowest

b) Identify the intermediate and the catalyst in the reaction (if they exist).

6. Draw a graph that corresponds to the change in concentration of a reactant that is a first order reaction?

7. How does an increase in temperature affect the rates of the forward and reverse reactions for an

exothermic and an endothermic reaction?

8. The reaction between KMnO₄ and H₂C₂O₄ can be followed by monitoring the disappearance of the

purple color of the MnO₄^- ion. These data were obtained for the reaction carried out at a constant

temperature of 25C.

            [MnO₄^-]          [H₂C₂O₄]         Rate (M/min)

 0.015              0.020               0.60

 0.015              0.040               1.20

 0.030              0.020               2.40

a) Specify the order of this reaction with respect to [MnO₄^-] and [H₂C₂O₄]

 b) Write the rate law expression for the reaction and calculate the value for the rate constant.

9.   The reaction 2 N₂O₅ --> 4 NO₂ + O₂ is first order with respect to N₂O₅.

            (i) Draw a graph that represents the change in [N₂O₅] over time as the reaction proceeds.

            (ii) Describe how the graph in (i) could be used to find the reaction rate at a given time, t.

            (iii) Considering the rate law and the graph in (i), describe how the value of the rate constant, k, could be determined.

            (iv) If more N₂O₅ were added to the reaction mixture at constant temperature, what would be the effect on the rate constant, k? Explain.

10.       Cl_2(aq) + H₂S_(aq) ---> S_(s) + 2 H⁺_(aq) + 2 Cl^- _(aq)

The rate equation for this reaction is: rate = k[Cl₂][H₂S] Which of these mechanisms is (are) consistent with this rate equation?

I.          Cl₂ + H₂S ---> H⁺ + Cl^- + Cl⁺ + HS^- (slow)            

            Cl⁺ + HS^- ---> H⁺ + Cl^- + S                (fast)

II.        H₂S  <-> H⁺ + HS^-                 (fast equilibrium)             

            Cl₂ + HS^- ---> 2Cl^- + H⁺ + S              (slow)

            (A) I only        (B) II only       (C) Both I and II        (D) Neither I or II

11.  2 A(g) + B(g) <===> 2 C(g)

When the concentration of substance B in the reaction above is doubled, all other factors being held constant, it is found that the rate of the reaction remains unchanged. The most probable explanation for this observation is that

(A) the order of the reaction with respect to substance B is 1

(B) substance B is not involved in any of the steps in the mechanism of the reaction

(C) substance B is not involved in the rate-determining step of the mechanism, but is involved in subsequent steps

(D) substance B is probably a catalyst, and as such, its effect on the rate of the reaction does not depend on its concentration

(E) the reactant with the smallest coefficient in the balanced equation generally has little or no effecton the rate of the reaction

12.       H_2(g) + I_2(g) ---> 2 HI_(g)

For the exothermic reaction represented above, carried out at 298 K, the rate law is as follows.

                                 Rate = k [H₂] [I₂]

Predict the effect of each of the following changes on the initial rate of the reaction and explain your

prediction.

(a) Addition of hydrogen gas at constant temperature and volume.

(b) Increase in volume of the reaction vessel at constant temperature.

(c) Addition of a catalyst. In your explanation, include a diagram of potential energy versus reaction

coordinate.

(d) Increase in temperature. In your explanation, include a diagram showing the number of

molecules as a function of energy.

13.  2 ClO_2(g) + F_2(g) ---> 2 ClO₂F_(g)

The following results were obtained when the reaction represented above was studied at 25C.

Experiment                  [ClO₂]                         [F₂]                  Initial Rate M/sec

1                                  0.010                           0.010               2.4E-3

2                                  0.010                           0.040               9.6E-3

3                                  0.020                           0.020               9.6E-3

(a) Write the rate law expression for the reaction above.

(b) Calculate the numerical value of the rate constant and specify the units.

(c) In experiment 2, what is the initial rate of decrease of [F₂]?

(d) Which of the following reaction mechanisms is consistent with the rate law developed in (a)?

Justify your choice.

                        I.

       ClO₂ + F₂ <---> ClO₂F₂ (fast)

       ClO₂F₂ ---> ClO₂F + F (slow)

       ClO₂ + F ---> ClO₂F (fast)

                        II.

       F₂ ---> 2 F^- (slow)

       2 (ClO₂ + F^- ---> ClO₂F) (fast)

14. What is the activation energy for the reverse of this reaction? N₂O_4(g) ---> 2NO_2(g)  if the enthalpy change for the reverse reaction is DH = - 58.04 kJ/mole and the activation energy for the forward reaction is Ea = +61.24 kJ/mole.

     (A) -3.2 kJ

     (B) +3.2 kJ

     (C) -119.28 kJ

     (D) +119.28 kJ

15. Draw the potential energy profile for the reaction described in question #14, label the reactants, products, DH and Ea.

16. Sketch a graph that shows the pathway of reaction that is endothermic and has a high activation energy.

Miscellaneous Questions

17. Ozone decomposes according to the following balanced equation: 2 O₃ -> 3 O₂ 

The following mechanism was proposed

O₃ ->  O₂ + O (fast)

O₃ + O ->  2 O₂ (slow)

Select the rate law rate expression for this mechanism.

(a) rate = k [O₃]² / [O₂]

(b) rate = k [O₃]² [O]

(c) rate = k [O₃]²

(d) rate = k [O₂]³

18. Concentration of reactant [A] versus time that produces a straight line for a first order reaction when plotted is:

(a) 1/[A]

(b) [A]

(c) ln[A]

(d) [A]²

19. Which statement about second order reactions is correct?

(a) Second order reactions require different reactants.

(b) Second order reactions are faster than first order reactions.

(c) Second order reactions are unaffected by changes in temperature.

(d) The half-life of a second order reaction depends on the initial reactant concentration.

20. A first order reaction has a rate constant of 0.0541 s^-1 at 25C. What is the half-life for this reaction?

(a) 18.5 s

(b) 12.8 s

(c) 0.0781 s

(d) 0.0375 s

21. The reaction between NO and I₂ is second-order in NO and first-order in I₂. What change occurs in the rate of the reaction if the concentration of each reactant is tripled?

(a) 3-fold increase

(b) 6-fold increase

(c) 18-fold increase

(d) 27-fold increase

22. For the rate equation, Rate = k[A][B]², what are the units for the rate constant, k, if the rate is given in mol*L^-1 sec^-1?

(a) L*mol*sec

(b) L*mol^-1*sec^-1

(c) L²*mol^-2*sec^-1

(d) L³*mol^-3*sec^-2

23. For the reaction: 2A  +  2B -> Product, the rate law is Rate = k[A][B]². Which mechanism is consistent with this information?

(A)    B + B <->  C

         C + A ->  Product (slow)

(B)    A + B -> C (slow)

         C + B -> product

(C)    A + A <-> C

         B + B -> D

         C + D -> Product (slow)

(D)    A + B <-> C

         B + C -> D (slow)

         D + A -> product

24.  Which straight line gives the activation energy for a reaction?

(a) rate constant vs T

(b) ln (rate constant) vs T

(c) rate constant vs T^-1

(d) ln (rate constant) vs T^-1

25.  The rate data given were obtained for the reaction, 2NO_(g)  +  2H_2(g)  ->  N_2(g)  +  2H₂O_(g)

What is the rate law for this reaction?

NO pressure (atm)        H₂ pressure (atm)                   Rate (atm*sec^-1)

0.375                            0.500                                      6.43 E-4

0.375                            0.250                                      3.15 E-4

0.188                            0.500                                      1.56 E-4

(A) Rate = kPNO

(B) Rate = kP²NO

(C) Rate = k(PNO)(P²H₂)

(D) Rate = k(P²NO)(PH₂)

26.  What is the rate law for the hypothetical reaction with the

mechanism shown?

                                                         2A     <->   intermediate 1              fast equilibrium

                                     intermediate 1 + B  ->   intermediate 2               slow

                                     intermediate 2 + B  ->   A₂B₂                             fast

(A) Rate = k[A]²

(B) Rate = [B] ²  

(C) Rate = k[A][B]

(D) Rate = k[A] ² [B]

27.  According to the Arrhenius equation: k = Ae-Ea/RT, a plot of lnk against 1/T yields

(A) Ea as the slope and A as the intercept

(B) Ea/R as the slope and A as the intercept

(C) Ea/R as the slope and ln A as the intercept

(D) -Ea/R as the slope and ln A as the intercept

Answers to REVIEW problems 1-14:

1. B

2. C

3. a) H₂O order = 2, CH₃Cl order = 1

c) k = 181.56 M^-2-sec^-1

4. A

5. Answer is (III)

step (I) is supposed to be an equilibrium, rate of step 2 = k[CHCl₃][Cl], from step 1 you get that

[Cl]² = k[Cl₂] so

[Cl] = sqrt k [Cl₂]^1/2

6. D

7. POORLY WORDED Question, should say what effect does increase in temp have on exo and endo thermic directions of reaction? If assume forward is exothermic rxn, then increase in temp increases the rate forward, for endothermic (reverse rxn), increase in temp increases the rate reverse more than for forward because majority of molecules have Ea in exo direction so change in number of molecules with Ea greater in endo direction. Bottom line increasing temp increases rate in both directions.

8. a) MnO₄^- order = 2, H₂C₂O₄ order = 1

b) 1.33E5 M^-2-sec^-1

9. (i) exp decrease

(ii) take dc/dt at any time t

(iii) plot ln[] vs time, calculate slope

(iv) none

10. A

11. C

12. increase, decrease, increase, increase

13. first order in both

b) k = 2.4 M^-1-sec^-1

c) initial rate divided by 2

d) I

14. B

 ANOTHER GREAT PRACTICE TEST