Confusing BR CBT question

[golgiapparatus88]

Number 28 in the BR CBT 5 PS asks "which reaction definitely requires light energy?"

The two it compares are:
1. O3 + hv -> O2 + O
2. HCF3 -> HCF2 + F

They state that it is reaction 2 due to the homolytic cleaving requiring energy. But they say about rxn 1 "in the case of upper-atmosphere reactions, this is more likely to be light rather than heat." This confuses the hell out of me since the question asks which requires light energy

---

Members don't see this ad.

 

[chemtopper]

Reaction one can be carried out in light and in heat also.If light is not there still you can decompose O3 molecule .
However reaction 2 needs only light for the homolytic cleavage of the molecule .

 

[BerkReviewTeach]

Number 28 in the BR CBT 5 PS asks "which reaction definitely requires light energy?"

The two it compares are:
1. O3 + hv -> O2 + O
2. HCF3 -> HCF2 + F

They state that it is reaction 2 due to the homolytic cleaving requiring energy. But they say about rxn 1 "in the case of upper-atmosphere reactions, this is more likely to be light rather than heat." This confuses the hell out of me since the question asks which requires light energy

I think you need to look at the answer choices again, because that might be where you are having your problem. The reaction you listed as choice 1 in your opening post is not one of the answer choices.

The question is:

• 28. Which reaction definitely requires light energy?
  
  A. Step I of the fluorocarbon sequence
  B. Step II of the fluorocarbon sequence
  C. Step III of the fluorocarbon sequence
  D. Step I of the upper-atmosphere sequence

And you are given the following in the passage:
Fluorocarbon sequence

I. HCF3 --> HCF2 + F
II. F + O3 --> FO + O2
III. FO + O --> F + O2​

Nitrogen oxide sequence (note that NO acts as a catalyst):

I. NO + 1/2 O2 --> NO2
II. NO2 --> NO + O
III. O2 + O --> O3​

Upper-atmosphere sequence:

I. NO + O3 --> NO2 + O2
II. O + NO2 --> NO + O2​

So the question reduces to:

28. Which reaction definitely requires light energy?

A. HCF3 --> HCF2 + F
B. F + O3 --> FO + O2
C. FO + O --> F + O2
D. NO + O3 --> NO2 + O2

Only choice A shows a reaction that involves exclusively bond breaking, so it is the only reaction guaranteed to require energy. Because it is occuring in the sky, there is limited heat energy available, so it must be driven by light energy (of which there is plenty).