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So I want maybe a simple question that I had on a test today. For a reaction to take place it needs a minimum energy called activation energy. At school I heard that this energy is coming from the kinetic energy of the particles that collide. When the reaction takes place, is the kinetic energy small because it turned into activation energy?
If you need any clarification just ask me and sorry for any misspell or grammar error.

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You got it right: when the reaction takes place, the kinetic energy of the reacting particles turns smaller because some of it is (sort of) spent on activation energy. It is just that the "when" in question is incredibly short.

Imagine two tennis balls flying at great speed and hitting each other head on. Sure enough, they will bounce off each other and fly in the opposite directions with the same speed. Some time into the collision, there must be an instant when they are not flying anywhere; their speed is 0, and so is kinetic energy. All energy they possessed has converted into the potential energy of elastic deformation, only to return into kinetic energy again really soon.

Same thing with molecules, except they are way smaller and fly way faster. Also, there might or might not be a reaction, and if it happens, then the kinetic energy of the molecules after the collision will not be the same as before. It may be greater, or it may be smaller; that's how exothermic and endothermic reactions happen.

So it goes.

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  • $\begingroup$ Could not some of the kinetic enery also be transferred into deformations and subsequent vibrations in the molecules, as well as rotational energy? $\endgroup$
    – Arthur
    Commented Nov 22, 2017 at 21:01
  • $\begingroup$ @Arthur It could all right. $\endgroup$
    – Ivan Neretin
    Commented Nov 22, 2017 at 21:08
  • $\begingroup$ @IvanNeretin Can we say that for exothermic (the reverse for endothermic) reactions because potential energy is lower than kinetic then the kinetic energy is higher (in order to be conserved) so heat is produced? $\endgroup$
    – ado sar
    Commented May 21, 2020 at 14:32
  • $\begingroup$ @ado Yes.$\mathstrut$ $\endgroup$
    – Ivan Neretin
    Commented May 21, 2020 at 14:37
  • $\begingroup$ @IvanNeretin When I said "potential energy is lower than kinetic" I wanted to say that poetntial reactants is lower than products. $\endgroup$
    – ado sar
    Commented May 21, 2020 at 15:02

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