H2 +Br2 =2HBr so suggest twoo reasons why all the collisions between hydrogen and bromine gas molecules do bot result in reaction?

3 answers

Well one reason is that the energy of the collision might be lower than the activation energy.
If one elevates the temperature (ie, increase the KE of the molecules), read what happens...http://www.chemguide.co.uk/inorganic/group7/otherreactions.html

The second reason is the energies of the molecules might be sufficient, but the angle of collision might be such that not enough KE can be transfered to activation: ie, a glancing collision. In a car wreck, there is a big difference between a head on collision and a bump in the rear bumper ..
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Find the mean absolute deviation (MAD) of the data

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