Chem 4511
Problem Set 8
Due by Friday, November 11th @ 5pm Assume all gases to behave ideally and all temperatures to be 298K unless stated otherwise 1. The rate constant for the decomposition of a certain substance is 2.80x10-3 L mol-1 sec-1
at 30°C and 1.38x10-2 L mol-1 sec-1 at 50°C. Evaluate the Arrhenius parameters, A and
Ea, of the reaction.
We have 2 equations with 2 unknowns. Instead of solving for one of the unknowns it
is more elegant to set it up as a ratio.
;
1
2.80
1.38
10
10
1
303
8.314
1
1
323
64900
64.9
Plug this value of k in to find the value of A:
2.8
10
.
·
4.32
·
10
·
2. a )Write a two-step mechanism for the destruction of ozone that meets these
requirements:
• Cl atom acts as a catalyst
• ClO is an intermediate, and reacts with O
• The overall net reaction is: O3 + O Æ 2O2
b) Assuming that the first step in your mechanism is the slower of the two steps, draw
a potential energy diagram corresponding to this exothermic reaction.
Cl is a catalyst so it must get produced in the first step and must produce the
intermediate ClO:
O3 +Cl Æ ClO + O2
In the second step we must use up the intermediate and reform the catalyst
ClO + O Æ O2 + Cl
Overall:
Because there are two elementary steps
in the reaction, there should be two
humps in the reaction profile
corresponding to the two different
activation energies. The first hump
MAY be the larger one because we are
told that the first step is the slow step.
However, remember that there are
other reasons for a reaction rate to be
slow, including low concentrations.
O3 + O Æ 2O2 √
E Reaction Coordinate