0.19 Reaction kinetics  (Page 2/7)

O ₃ (g) + Cl(g) → O ₂ (g) + ClO(g)

Rate = k[O ₃ ][Cl]

What must happen for a reaction to occur between an O ₃ molecule and a Cl atom? Obviously, for these two particles to react, they must come into close proximity to one another so that an O atom can be transferred from one to the other. In general, two molecules cannot trade atoms to produce new product molecules unless they are close enough together for the atoms of the two molecules to interact. Although technically this is just a close encounter, chemists commonly refer to this as a collision between molecules.

Since the molecules must collide to react, the rate of the reaction cannot be any faster than the rate of collisions. This means that, for a reaction to occur quickly, the reacting molecules must collide frequently. We can also reason that the rate of collisions must depend on the concentrations of the reactants. The more molecules there are in a confined space, the more likely they are to run into each other. To write this relationship in an equation, we can think in terms of probability, and we consider the reaction above. The probability for an O ₃ molecule to be near a specific point increases with the number of O ₃ molecules, and therefore increases with the concentration of O ₃ molecules. The probability for a Cl atom to be near that specific point is also proportional to the concentration of Cl atoms. For an O ₃ molecule and a Cl atom to collide, we need both to be in the same place at the same time. Remember that the probability of two things happening is the product of the probability of the first event times the probability for the second event. Therefore, the probability for an O ₃ molecule and a Cl atom to be in close proximity to the same specific point at the same time is proportional to the [O ₃ ] × [Cl].

It is important to remember that not all collisions between O ₃ molecules and Cl atoms will result in a reaction. There are other factors to consider including how the molecules approach one another. The atoms may not be positioned properly to exchange between molecules, in which case the molecules will simply bounce off of one another without reacting. For example, if the Cl atom approaches the center O atom of the O ₃ molecule, that O atom will not transfer. Another factor is the energy associated with the reaction. As we said above, though, a collision must occur for the reaction to occur, and therefore there rate of the reaction can be no faster than the rate of collisions between the reactant molecules.

Therefore, we can say that, in a bimolecular reaction where two molecules collide and react, the rate of the reaction will be proportional to the product of the concentrations of the reactants. For the reaction of O ₃ with Cl, the rate must therefore be proportional to [O ₃ ]×[Cl], and this is exactly what we observe in the experimental rate law in Equation (2). Thus, it appears that we can understand the concentration dependence of the rate law by understanding the collisions that must occur for the reaction to take place.