Tutorial - Chemical Kinetics

Chemical Kinetics

First – Order Reaction

1.       The conversion of cyclopropane to propene in the gas phase is a first – order reaction with a rate constant of 6.7 x 10^-4 s^-1 at 500 °C.

®     ­       CH3  -  CH  =CH2

a.       If the initial [ ] of cyclopropane was 0.25 M, what is the [ ] after 8.8 min?

b.      How long (in minutes) will it take for the concentration of cyclopropane to decrease from 0.25 M to 0.15 M?

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

Half - time

1.       The decomposition of ethane, C₂H₆ to methyl radicals is first order reaction with a rate constant of 5.36 x 10^-4 s^-1 at 700 °C:

C₂H₆ (g)   ­®   2CH₃

Calculate the half life of the reaction in minutes.

2.       Rate of reaction  A  +  B  ­®  Products is given as a functional of different initial [ ] of A and B. 

[A] (mol/L)	[B] (mol/L)	Initial rate (mol/L/min)
0.01	0.01	0.005
0.02	0.01	0.010
0.01	0.02	0.005

a.       Determine the order of reaction with respect to A and with respect to B.

b.      What is the half life of A in the reaction.

3.       Propane, C₃H₈, burns in oxygen to give carbon dioxide and water. If the concentrations of propane decrease at a rate of 0.30 Ms^-1, what is the rate at which the concentration of oxygen is decreasing? Determine also the rate of increase in concentration of all the products.

4.       Consider the reaction

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

Suppose that, at a particular moment during the reaction, molecular oxygen is reacting at the rate of 0.0024 M/s.

a)         At what rate is N₂O₅ being formed?

b)      At what rate is NO₂ reacting?

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

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

From the following data collected at this temperature, determine :

a)      The rate law

b)      The rate constant

c)       The rate of the reaction when [NO] = 12.0 x 10^-3 M and [H₂] = 6.0 x 10^-3 M

Experiment	[NO] (M)	[H2] (M)	Initial rate (M/s)
1	5.0 x 10-3	2.0 x 10-3	1.3 x 10-3
2	10.0 x 10-3	2.0 x 10-3	5.0 x 10-3
3	10.0 x 10-3	4.0 x 10-3	10.0 x 10-3

6.       The following reaction

H₂SeO₃    +    6I^-    +    4H⁺    ­®    Se    +    2I₃^-    +    3H₂O

Has the rate law: rate  =  k[H₂SeO₃ ] [I^-  ]³ [H⁺]²

Answer the following questions based on the rate law above.

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

b)      What is the overall order of the reaction?

c)       What are the units of the rate constant, k?

8.		The following data were collected for the reaction A  +  B  ­®  C  +  D Experiment Initial concentration of A (M) Initial concentration of B (M) Initial rate of formation of C (Ms-1) 1 010 0.10 1.2 x 10-3 2 0.10 0.20 2.4 x 10-3 3 0.20 0.10 4.8 x 10-3 Determine the rate order of reaction and calculate its rate constant. Following that, write the rate law for the reaction.
9.	(a)	Consider the reaction  A à  B. The rate of the reaction is 1.6x10-2 Ms-1 when the concentration of A is 0.35 M. Calculate the rate constant, k if the reaction is (i)                 First order in A (ii)               Second order in A
	(b)	(i)       Define the half-life of a reaction. (ii)     Write the equation relating the half life of a first order of reaction to the rate constant, k. (iii)   Determine the half life of a compound if 75 % of a given sample of the compound decomposes in 60 min. Assume first order kinetics.
	(c)	The rate constant for a second order reaction 2NO2 (g)  à  2NO (g)  +  O2 (g) is 0.54 M-1s-1 at 300 °C. How long (in second) would it take for the concentration of NO2 to decrease from 0.562 M to 0.128 M.