I found this question in the AP Chem book, and I understand why B is the answer (as the answer key says). However, I don't understand why D is not also correct.

The question is:

A multistep reaction takes place with the following elementary steps.

  • Step I. A + B <-> C
  • Step II. C + A -> D
  • Step III. C + D -> B + E

Why would increasing the temperature make the reaction rate go up?

  • (A) it is an endothermic reaction that needs an outside energy source to function
  • (B) The various molecules in the reactions will move faster and collide more often
  • (C) The overall activation energy of the rxn will be lowered
  • (D) A higher fraction of molecules will have the same activation energy

I understand that B is correct based on collision theory, but why isn't D correct as well? Don't a greater % of molecules have the same activation energy at higher temperatures? (Or am I misinterpreting answer choice D?)

Thank you!

  • $\begingroup$ “A higher fraction of collisions will have an energy higher than the activation energy” is a correct statement. This is the main reason reaction rates increase at higher temperature. They also increase due to higher frequency of collisions. I don’t understand the statement in choice D. $\endgroup$ Mar 11 '19 at 22:03
  • 3
    $\begingroup$ If you strike out the word same in (D) then the statement would be true. With same however the statement is weird. I think same was thrown in to trap a careless reading of the statement. $\endgroup$
    – MaxW
    Mar 11 '19 at 22:18
  • 3
    $\begingroup$ Tangential to MaxW's point, at any temperature, all the molecules have the same activation energy, but at higher temperature more molecules will have energy in excess of that activation energy. $\endgroup$
    – A.K.
    Mar 11 '19 at 23:07

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