Which reaction will occur in the shortest amount of time? So shortest amount of time really means which chemical reaction is fastest. So remember we talked earlier about speed and activation energy. If we want the fastest chemical reaction, well that would mean that we want the lowest activation energy.

If we take a look here, reaction A is 143 kJ, reaction B is 80 kJ and reaction C is 215 kJ. The answer is B. Since it has the lowest activation energy. That would mean that it moves the fastest and therefore would have the shortest amount of time necessary to go to completion.