how does molecular motion help to explain how molecules combine in chemical reactions?

Answer 1

The movement of molecules during reaction of two or more chemical species helps to explain how they combine because when atoms or molecules collide eachother the involved energy (called Energy of activation) is used for formation of new compounds.

During collision of molecules also there is a interchange of electrons at orbital level that allows the breaking of old bonds and formation of new bonds between atoms.

Not all the collisions are effective and they depend on the Energy of activation of the substance and the spatial orientation of these particles at the moment of collision.

Good luck!