Cracking the AP Chemistry Exam

Part IV

Content Review for the AP Chemistry Exam

Chapter 6

Big Idea #4: Chemical Reactions and their Rates

COLLISION THEORY

According to collision theory, chemical reactions occur because reactants are constantly moving around and colliding with one another.

When reactants collide with sufficient energy (activation energy, E_a), a reaction occurs. These collisions are referred to as effective collisions, because they lead to a chemical reaction. Ineffective collisions do not produce a chemical reaction. At any given time during a reaction, a certain fraction of the reactant molecules will collide with sufficient energy to cause a reaction between them.

Reaction rate increases with increasing concentration of reactants because if there are more reactant molecules moving around in a given volume, then more collisions will occur.

Reaction rate increases with increasing temperature because increasing temperature means that the molecules are moving faster, which means that the molecules have greater average kinetic energy. The higher the temperature, the greater the number of reactant molecules colliding with each other with enough energy (E_a) to cause a reaction.

In addition, reactions only occur if the reactants collide with the correct orientation. For example, in the reaction 2NO₂F → 2NO₂ + F₂, there are many possible collision orientations. Two of them are drawn below:

The reaction will only occur if the two NO₂F molecules collide in such a fashion that the N-F bonds can break and the F-F bonds can form. While there is no way to quantify collision orientation, it is an important part of collision theory, and you should be familiar with the basic concept underlying it.