Collision Theory: Hitting the right spot

The molecules must collide in the right orientation, or hit at the right spot, in order for the reaction to occur. The place on the molecule where the collision must take place is called the reactive site. For example, suppose you have an equation showing molecule A-B reacting with C to form C-A and B, like this:

A-B + C → C-A + B

The way this equation is written, the reaction requires that reactant C collide with A-B on the A end of the molecule. (You know this because the product side shows C hooked up with A — C-A.) If it hits the B end, nothing will happen. The A end of this hypothetical molecule is the reactive site. If C collides at the A end of the molecule, then there’s a chance that enough energy can be transferred to break the A-B bond. After the A-B bond is broken, the C-A bond can form. You can show the equation for this reaction process in this way (I show the breaking of the A-B bond and the forming of the C-A bond as “squiggly” bonds):

C~A~B → C-A + B

So for this reaction to occur, there must be a collision between C and A-B at the reactive site. Note that this example is a simple one. I’ve assumed that only one collision is needed, making this a one-step reaction. Many reactions are one-step, but many others require several steps in going from reactants to final products. In the process, several compounds may be formed that react with each other to give the final products. These compounds are called intermediates.  You show them in the reaction mechanism, the series of steps that the reaction goes through in going from reactants to products. But in this chapter, I keep it simple and pretty much limit my discussion to one-step reactions.