Question

O3 + O -->2O2)
Depletion of the ozone layer is believed to be caused by chlorine catalyzed O3 decomposition. For the uncatalyzed reaction Ea,f= 19 kJ/mol. For the following mechanism, Ea1, f =2.1 and Ea2,f =0.4 kJ/mol:
Proposed Mechanism:
Cl+O3--> ClO +O2
ClO+O--> Cl+O2

a.) What is the dH for the reaction?
I know the answer is -390 kJ/mol. I don't know where this answer comes from!

b.) What is the Ea, r for the uncatalyzed reaction?
I know the answer is 409 . likewise I don't know where this number comes from . I see added and subtracted numbers but I need an explaination.

c.) Which step in the above catalyzed reaction is faster ?
How would I know ?
d.) If dH* for the first reaction is -165 kJ/mol, sketch the energy profile for the reaction.
dH* kJ/mol O3= 142.3 ,Cl=121.7 , O=247.5
Sketching energy profiles for the reaction how ?
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Based on a graph of Energy vs. Reaction pathway.
a.) How do I determine the intermediates formed in the reaction ?

b.) How many transition states are there and how do I identify them from the graph?
(The graph has has three humps, )

c.) How many activation complexes are there ? How do I identify them from a graph?

d.) How do I determine which step is the fastest and which step is the slowest ? from a graph

e.)Is the reaction A-->B exotherminc or endothermic , how do I know based on the graph.

Answer 1

a)

b)This is the energy of activation for the reverse uncatalized reaction.

You can see in the graph, that going if the reaction goes from left to right the energy of activation is a littel hill, and the reaction goes downhill. If you go in the opposite direction the first you need to climb a big hill (the energy of activation for the reverse reaction). It is the positive DH+Ea,f.

c) is the second step, it has lower activation energy 0.4kJ/mol

d) Knowing te DH for the first reaction is -165 you can calculate the DH for the ClO:

Now you can do a graph like the one above, knowing the DH1=-165 and DH2=(121.7+0)-(99+247.5)=-224.8kJ/mol

From the graph I already identified:

a) compouns formed in the middle that are not part of the overall reaction: ClO (it appears as product of the first and reactant of the second)

b) In the graph each peak represents a transition state, the highest energy compound duirng all the reaction path.

c)Activation complexes, sometimes those complexes are confused with the transtition state. The activated complexes are all the different structures formed before the transition state, so there can be as much as steps posible during the uphill of the reaction (when you are climbing in the graph).

d) the fastest is the one with the lower activation energy, in the graphthe shortest hill to climb is the lower.

e) exothermic, in the graph when your products are below the reactants it is exothermic.