AAMC CBT5 question #6

[30somethingyearold]

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Hi all,

I just can't seem to understand the answer to the following question. I know it was answered on SDN awhile ago I'm hoping someone else's view on this might help me understand the answer.

Question 6:
Reaction 3: F3CCl --> F3C· + Cl·
Reaction 4: Cl· + O3 --> ClO· + O2
Reaction 5: ClO· + O --> Cl· + O2
From reactions 3-5, what can be determined about the relative concentrations of CFCs and O2?
A. One equivalent of CFC is required to produce one equivalent of O2.
B. One equivalent of CFC is required to produce two equivalents of O2.
C. A catalytic amount of CFC can produce much O2.
D.The CFC is produced by a catalytic amount of O2.

The answer is C. I thought it was B because if you find the overall reaction one F3CCl produces two O2 's.

Thank you in advance!

 

[SuperSneaky24]

I also answered this question incorrectly, and wasn't quite sure why B wasn't correct too. But after looking at it again, I realized the reaction was actually:

O3 --> O + O2, and F3CCl only acts as a catalyst.

If you look at the reactions slowly, you notice that Cl· is regenerated at the end of reaction 5. If both F3C· and Cl· were required for reaction 4 to proceed, then you would have been correct. However, notice how Cl· is regenerated and then can be used (by itself and not with F3C·) to react with O3 again. So one equivalent of F3CCl can theoretically produce an infinite amount of O2 if given enough O3.

Basically, what I told myself after this problem, is that whenever you see a molecule on both the reactant and product side, 90% of the time it is a catalytic substance.

 

[30somethingyearold]

I'm being a bit slow here...how do we know that we're given enough O3?

 

[SuperSneaky24]

Nah, I'm just theoretically saying that. Basically the choice One equivalent of CFC is required to produce two equivalents of O2 is wrong because if we are given an infinite amount of O3 and only one molecule of CFC, an infinite amount of O2 can be produced. However, the opposite is not the same. One molecule of O3 can only make one molecule of O2, no matter how much CFC you put in.

Notice how only the Cl· is required for reaction 4 and 5. If both Cl· and F3C· were required for reaction 4 but not reaction 5, then one equivalent of CFC would be required to produce two equiv. of O2. Read the reactions carefully and make sure you're not assuming something wrong.

 

[30somethingyearold]

Thank you, that makes sense now!!!