Answer: a rection will most likely occur if the colliding partilces have the proper orientation and energy.
The reactions occur becasue the molecules collide.
But not all the collisions result in a reaction.
The collisions have to meet some requirements.
Two of the basic requirements is that the collision has enough energy to overcome the activation energy and that the molecules collide in the riight way.
When two molecules react it is necessary that one element of one of the molecules interact when a specific element or group in the other molecule. That is the orientation must be the right one.
Collisions that to not have the proper orientation or enough energy will not cause reaction (changes in the bonds of the molecules).