CHEM 118 – Spring 2014 George A. Papadantonakis WORKSHEET

WORKSHEET 4 (Chemical Kinetics)
CHEM 118 – Spring 2014
George A. Papadantonakis
1. Define reaction rate. Distinguish between the initial rate, average rate and instantaneous rate of a
chemical reaction. Which of these rates is usually faster? The initial rate is the rate used by convention.
Give a possible explanation as to why.
2. For the reaction: 2ClO2 + 2 OH- → ClO3- + ClO2- + H2O the rate was found to be first order in [OH-]
and second order in [ClO2].
a. Write the rate law for this reaction
b. What happens to the rate if [ClO2] is doubled?
c. What happens to the rate if the volume is doubled at constant pH?
d. What happens to the rate if the pH is decreased by one unit?
e. Assume that k is known. If 0.00100 M ClO2 and 1.00 M OH- are mixed at t = 0 give the expression
for [ClO2] as a function of t, k, [ClO2]0 and [OH-]0.
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3. For the following two reactions with the given time and concentration data perform a linear
regression on the data to determine the order of each reaction. Obtain the value of the rate constant, k
from your regression analysis.
a) NH3 → NH2 + H
b) 2N2O → 2N2 + O2
Time (h) [NH3] (M) Time (min) [N2O] (M)
0
8.00 x 10-7
15.0
0.0835
-7
25
6.75 x 10
30.0
0.0680
50
5.84 x 10-7
80.0
0.0350
-7
75
5.15 x 10
120.0
0.0220
4. The decomposition of CH3CHO (g), CH3CHO (g) → CH4 (g) + CO (g) is second order with a rate constant
of 0.105 M-1 s-1 at 490oC. If the concentration of CH3CHO (g) is 0.012 M initially, then what will be the
concentration 5.0 min. later?
5. The elementary chemical reaction
O (g) + ClO (g) → Cl( g) + O2 (g)
is made pseudo-first order in oxygen atoms by using a large excess of ClO radicals. The rate constant for
the reaction is 3.5 X 10-11 cm3 molecule-1s-1. If the initial concentration of ClO is 1.0 X 1011 molecules/cm3,
how long will it take for the oxygen atoms to decrease by 90% of their initial concentration?
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6. A certain reaction has the following general form: aA → bB
At a particular temperature and [A]0 = 2.80 x 10-3 M, concentration versus time data were collected for
this reaction, and a plot of 1/[A]t versus time resulted in a straight line with a slope value of 3.60 X 10-2 L
mol-1 s-1.
a. Write the rate law and determine the value of the rate constant for this reaction.
b. Calculate the half-life for this reaction.
c. How much time is required for the concentration of A to decrease to 7.00 X 10-4 M?
7. The experimental rate law was found to be: rate = k [NO]2[O2] for the reaction
2 NO (g) + O2 (g) → 2 NO2 (g)
The following mechanisms have been proposed for this reaction. Determine which of the mechanisms
are consistent with the experimental expression and which are not by finding the rate law for each
mechanism. Of those which are consistent, indicate which is more probable.
k1
rate = k1 [NO]2 [O2]
a. 2 NO + O2 → 2 NO2
b. NO + O2 → NO2 + O
O + NO → NO2
c. NO + O2  NO3
NO3 + NO → 2 NO2
k1 slow
rate = k1 [NO] [O2]
k2 fast
k1, k -1 both fast
rate = k2K1 [NO]2 [O2]
k2 slow
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d. 2 NO → N2O2
k1 slow
N2O2 + O2 → N2O4
k2 fast
N2O4 → 2 NO2
k3 fast
e. 2 NO  N2O2
N2O2 + O2 → 2 NO2
f. NO + NO → N2 + O2
N2 + 2 O2 → 2 NO2
rate = k1 [NO]2
k1, k -1 both fast
rate = k2K1 [NO]2[O2]
k2 slow
k1, k -1 both fast
rate = k2K1 [NO]2 [O2]
k2 slow
8. The following mechanism has been proposed to account for the rate law of the decomposition of
ozone to O2 (g):
k1

→ O2 + O + M
O3 + M ←

k−1
2
O + O3 
→ 2O2
k
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Apply the steady-state hypothesis to the concentration of atomic oxygen, and derive the law for the
decomposition of ozone. (M stands for an atom or molecule that can exchange kinetic energy with the
particles undergoing the chemical reaction.)
9. From the following temperature dependence o a rate constant, determine the activation energy for
the reaction.
T (K)
338
328
318
308
298
273
k (s-1)
4.87 x 10-3
1.50 x 10-3
4.98 x 10-4
1.35 x 10-4
3.46 x 10-5
7.87 x 10-7
10. The activation energy for the reaction C4H8 (g) → 2C2H4 (g) is 262 kJ mol-1. At 600 K the rate constant
is 6.07 x 10-8 s-1.
a) Predict the order of the reaction
b) What is the rate constant at 800 K?
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11. The reaction H(g) +D2(g) → HD(g) + D(g) shows the exchange of isotopes of hydrogen of mass
number 1 (H) and 2 (D, deuterium). Data of the rate constants found at different temperatures were
used to construct an Arrhenius plot and the following equation of a line was found:
y = - 4359.214 x + 23.9637
a. Find the activation energy and the frequency factor for this reaction.
b. What is the value of the rate constant at 273.15K?
12. How can a non-spontaneous reaction be faster than a spontaneous reaction?
13. At temperatures above 600K the reaction CO (g) + NO2 (g) → CO2 (g) + NO (g) is believed to occur in
one step with a forward activation energy of 134 kJ/mol. Determine the reverse activation energy if ∆H
of reaction is -226 kJ/mol. Draw an energy level diagram for the reaction and carefully draw the
activated complex.
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14. The gas phase overall reaction of HCl + CH3CHCH2 → CH3CHClCH3
has the experimental rate expression, rate = k obs [HCl]3[CH3CHCH2]
A proposed mechanism for this reaction is
HCl + HCl  H2Cl2
(fast equilibrium)
HCl + CH3CHCH2  CH3CHClCH3* (fast, equilibrium, the asterisk, * denotes an excited, but not
yet a product)
CH3CHClCH3* + H2Cl2 → CH3CHClCH3 + 2 HCl (slow)
a. Does this proposed mechanism give the overall reaction?
b. Is it consistent with the experimental rate law?
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c. Is it physically reasonable?
d. Draw the reaction coordinate diagram for this proposed mechanism. You may assume that the
overall reaction is exothermic.
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