with enough energy

The atoms in both H₂ and Cl₂ are bonded to each other.

So, in order to form the product molecules, I need to separate those atoms (I need to break those bonds). Since I am NOT reaching in and pulling apart the atoms, I rely on the force of the collision to break those bonds.

The consequence of this is that anything I can do to increase the energy of collisions will increase the rate of the reaction. In addition, anything I can do that weakens the bonds will increase the rate of the reaction as well

The energy required to make the reaction happen is called activation energy.

This part of collision theory is affected by temperature, catalysts and the nature of the reactants