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I was going through my chemistry book (RT Morrison and Boyd) and found a statement:

" the collision frequency of isopropyl and n-propyl w.r.t. Chlorine atoms is the same."

Is it correct because I think the size will matter in this case while both have the same weight?

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    $\begingroup$ Welcome! A little bit more context would be nice, like which reaction is this talking about, and what environment. $\endgroup$ – Martin - マーチン Feb 17 '15 at 9:04
  • $\begingroup$ While the collision frequency may be the same, the reaction rates will differ. This is because not all collisions cause a reaction. The isopropyl radical will be more stericly hindered than the n-propyl radical. $\endgroup$ – Mr T Apr 19 '15 at 21:15
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The statement is:

This[collision frequency] must be the same for the two reactions, since both involve collisions of the same particles: a propane molecule and a chlorine atom.

It is clear that n-propyl radical and Isopropyl radical have same carbon structure. In the next step the hydrocarbon radical reacts with a chlorine ($\ce{Cl2}$) and since chlorine is symmetrical the collision frequency cannot depend upon it. It should be easy to judge from the following structures, the very minor difference:

Another reason is: Since the difficult step is the abstraction of a hydrogen and the determination of rates should be done while comparing primary, secondary or tertiary radicals, which have different stability.

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  • $\begingroup$ This answer, while correct, adds some detail that may occlude the initial point about the probability of collisions. $\endgroup$ – Lighthart Feb 19 '15 at 17:06

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