t11 reaction kinetics 19-26

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TUTORIAL 11: REACTION KINETICS

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TOPIC 11.1 : REACTION RATE

1. (a) Define reaction rate.

(b) Write the differential rate equation for the following reactions.

i. I(aq) + OCl(aq) Cl(aq) + OI(aq)

ii. 3O2(g) 2O3(g)

iii. 4NH3(g) + 5O2(g) 4NO(g) + 6H2O(g)

2. Consider the reaction:

N2(g) + 3H2(g) 2NH3(g)

Suppose that at a particular moment during the reaction, molecule of hydrogen is

reacting at the rate of 0.074 M s1. Calculate the rate of

(a) formation of ammonia.

(b) depletion of nitrogen.

3. (a) Explain the following terms.

i. rate law

ii. rate constant

iii. half-life

(b) Consider the reaction:

A product

i. Write the rate law .

ii. Determine the unit of rate constant.

iii. Sketch the graphs of rate of reaction versus concentration of A

for zero, first and second order reactions.

4. The conversion of cyclopropane to propene in the gas phase is a first order reaction

with a rate constant of 6.7 10-4

s1

at 500oC.

CH2

CH2

CH2

CH2

CH CH3

(a) If the initial concentration of cyclopropane was 0.25 M, calculate the

concentration after 8.8 min.

(b) How long will it take for the concentration of cyclopropane to decrease from

0.25 M to 0.15 M?

(c) How long will it take to convert 74% of the starting material to propene?


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5. The reaction of nitric oxide with hydrogen at 1280C is

2NO(g) + 2H2(g) N2(g) + 2H2O(l)

The following data was collected at this temperature :

Experiment [NO]/M [H2]/M Initial rate /M s1

1 5.00 103

2.00 103

1.25 105

2 10.00 103

2.00 103

5.00 105

3 10.00 103

4.00 103

10.00 105

Based on the data, determine

i. the rate law.

ii the rate constant.

6. The data below were obtained from the following reaction at 27oC .

CH3CH(Cl)CH3 + NaOH CH3CH(OH)CH3 + NaCl

Expt. [CH3CH(Cl)CH3]/M [NaOH]/M Reaction rate/M min1

1 0.15 0.25 3.0 10-3

2 0.15 0.50 6.0 10-3

3 0.45 0.25 9.0 10-3

(a) What is the order with respect to each reactant?

(b) Write the rate equation.

7. The decomposition of dinitrogen pentoxide with the rate constant of 5.1 104 s

1at

45oC is given below:

2N2O5(g) 4NO2(g) + O2(g)

(a) Calculate the concentration of N2O5 after 3.2 min if the initial concentration is 0.25 M .

(b) If the initial concentration of N2O5 is 0.35 M, calculate the time needed for

the concentration to be reduced

i. to 0.08 M.

ii. by 62%.


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8. Iodine atoms combine to form molecular iodine in the gas phase:

I(g) + I(g) I2(g)

The rate constant for the above reaction is 7.0 109

M1

s1

at 23C.

(a) If the initial concentration of iodine atoms is 0.086 M, calculate the

concentration after 2.0 min.

(b) Calculate the half-life of iodine atoms if the initial concentration is

i. 0.42 M

ii. 0.60 M

9. The data listed in the table below were obtained from the following decomposition :

A products

Time/min [A]/M ln [A]]A[

1

0 1.00 0.00 1.00

5 0.63 -0.46 1.60

10 0.46 -0.78 2.20

15 0.36 -1.02 2.80

25 0.25 -1.39 4.00

(a) Establish the order of the reaction by graphical method.

(b) Determine the rate constant, k.

(c) Determine the half-life, t1/2, if [A]0 = 1.00 M.

TOPIC 11.2: COLLISION THEORY AND TRANSITION STATE THEORY

1. What is meant by activation energy?


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TOPIC 11.3 : FACTORS AFFECTING REACTION RATE

1. Explain the effect of temperature on reaction rate based on Maxwell-Boltzmann

distribution curve.

2. The following equation shows the decomposition of HI(g):

2HI(g)Pt

H2(g) + I2(g) H = -ve

(a) What is the function of platinum?

(b) Draw and label the potential energy profile for the reaction with and without

platinum.

(c) Give another two factors that can influence the reaction rate and explain your

answer.

3. The rate constant of a reaction is 3.46 10-2

s1

at 298 K. Calculate the rate constant

at 350 K if the activation energy for the reaction is 50.2 kJ mol

1

.

4. The rate constant of a reaction at 463 K is 2.52 105

s1

and at 503 K is

6.30 104

s1

. Determine the activation energy for the reaction.

5. Rate constants, kfor decomposition of hydrogen iodide at different temperatures are

given in the table below :

Rate constant, k( mol1

dm3

s1

) Temperature (K)

3.75 10-9

500

6.65 10-6

600

1.15 10-3

700

7.75 10-2

800

(a) Write the Arrhenius equation.

(b) Determine the activation energy for the decomposition of hydrogen iodide

graphically from the above data.[The gas constant, R = 8.314 J K

1mol

1]


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OBJECTIVE QUESTIONS

1. The rate of formation of water from the reaction:

2MnO4(aq) + 6H

+(aq) + 5H2O2(aq) 2Mn

2+(aq) + 8H2O(l) + 5O2(g)

is 0.035 M s-1

. What is the rate of H2O2 being reacted?

A. 8.75 x 10-4 M s1B. 2.19 x 10-3 M s1C. 8.75 x 10-3 M s1D. 2.19 x 10-2 M s1

2. For the reaction:A + 2BC+ 2D, the initial rate, dt

][d

Ais 2.6 x 10

-2M s

1.

What is the value ofdt

][d

B?

A. 6.5 x 103 M s1B. 1.3 x 102 M s1C. 2.6 x 102 M s1D. 5.2 x 102 M s1

3. If a reaction is described as zero order with respect to reactantA, this means that

A. A is a catalyst in the reaction.B. A is not involved in the rate determining step.C. the value of the rate constant is independent ofA.D. the rate of reaction is inversely proportional to the concentration ofA.

4. The following data were measured for the reaction:

2NO(g) + Cl2(g) 2NOCl(g)

Reaction rate

(mol dm3

hr1

)

Concentration (mol dm3

)

NO Cl21.19

4.79

9.59

0.50

1.00

1.00

0.50

0.50

1.00

Choose the rate equation for the reaction.

A. rate = k[NOCl]2B. rate = k[NO][Cl2 ]C. rate = k[NO]

2

[Cl2]D. rate = k[NO][Cl2]0.5


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5. The initial concentration of an active substance in an aqueous solution of medicine is

5.0 x 103

mol dm3

. After 20 months, an analysis shows that its concentration

becomes 4.2 x 103

mol dm3

. Determine the decay duration of this medicine by

assuming the decomposition of the active substance is a first order reaction.

[Decay duration of the medicine is the time required as it decomposes 10% of the

initial concentration].

A. 9 monthsB. 12 monthsC. 18 monthsD. 23 months

6. The reactionA + 2B products was found to have the rate law, rate = k[A] [B]2.

Predict by what factor the rate of reaction will increase when the concentration ofB is

doubled and the concentration ofA remained unchanged.

A. 2B. 4C. 6D. 8

7. Half-life of a reaction increases with initial concentration if the reaction is

A. zero orderB.

first order

C. second orderD. none of the above

8. The half-life of a radioactive element is 50 minutes. How long will it take for the

element to decay by 87.5%?

A. 1.67 hoursB. 2.00 hoursC. 2.25 hoursD. 2.50 hours

9. If activation energy,Ea for a certain biological reaction is 50 kJ mol1, how many

times will the rate of the reaction increase when body temperature increases from

37C to 40C?

A. 1.00B. 1.15C. 1.20D. 2.00


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10. The rate of a reaction increases by a factor of 45 as the temperature rises from 25C

to 55C. Calculate the activation energy of the reaction.

A. 66 kJ mol1B. 103 kJ mol1C. 166 kJ mol1D. 203 kJ mol1

SUMMARY OF OPTIONS

A B C D

I only I and II only II and III only I, II and III

11. The reaction : Q products is believed to be of first order.

Which of the following graph(s) is/are correct?

I.

ln[Q]

t

II

[Q]

t

III

Rate

[Q]

t11 reaction kinetics 19-26

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