Chemistry 30

2.2 Reaction Mechanism & the Rate-Determining Step

Here is another quick little experiment you may want to try (and this is one you can safely do):

Find a friend, and each of you crumple up a piece of paper. Sitting facing one another, with knees almost touching, toss your pieces of paper together, trying to get them to hit while still in the air. Do this 10 times - how many successful "collisions" did you record?

Now, grab another friend with their own wad of paper. This time, a successful collision will only occur when all three pieces of paper hit together simultaneously - just two out of three won't do. Out of 10 tosses, how many are successful?

Add one more friend and repeat the exercise. Any successful collisions, where all four pieces of paper collide at the same instant? Likely not.

Chemical reactions follow the same principles. When we write a chemical equation such as:

2 C₂H₂ + 5 O₂ → 4 CO₂ + 2 H₂O

we see that two molecules of acetylene (C₂H₂) react with 5 molecules of oxygen. According to the collision theory, molecules must collide in order to react. But it is highly unlikely that 7 molecules would collide together all at once.

Instead, the reaction most likely occurs in a series of simple steps which only required two or three molecules colliding at any one instant. Although these steps cannot always actually be observed, chemists can often make predictions about the sequence of events.

For example, nitrogen monoxide reacts with oxygen according to the equation

2 NO(g) + O₂ → 2 NO₂

This reaction does not occur in a single step, however, but rather through these two steps:

Step 1:	2 NO → N2O2
Step 2:	N2O2 + O2 → 2 NO2

Notice that if you add these two reactions together, you end up with the overall reaction:

Step 1:	2 NO → N2O2	The series of steps a reaction undergoes is called the reaction mechanism.
Step 2:	N2O2 + O2 → 2 NO2
Overall:	2 NO(g) + O2 → 2 NO2

Notice that in our example dinitrogen dioxide (N₂O₂) cancels out and does not appear in our overall equation. Substances such as this are called reaction intermediates and are typically short-lived.

Given an overall reaction, it will not be possible for you to predict what the reaction mechanism would be. However, if you are given the steps of a reaction mechanism you will need to be able to add together individual steps to end up with the overall reaction.

Rate Determining Step

Here is another reaction mechanism with some additional information concerning the relative rates of each of the individual steps:

Step 1:	HBr + O2 → HOOBr	Slow
Step 2:	HOOBr + HBr → 2 HOBr	Fast
Step 3:	HOBr + HBr → H2O + Br2	Fast
Step 4:	HOBr + HBr → H2O + Br2	Fast
Overall:	4 HBr + O2 → 2 H2O + 2 Br2

We don't have values for the actual rates of these individual steps, but here's a question for you to consider - would you consider the overall reaction to be fast or slow?

With two fast steps and only one slow step, many of you will predict that the reaction will be fast.

But let's make up some extreme numbers and ask the question again:

Step 1:	HBr + O2 → HOOBr	1 year
Step 2:	HOOBr + HBr → 2 HOBr	0.1 s
Step 3:	HOBr + HBr → H2O + Br2	0.1 min
Step 4:	HOBr + HBr → H2O + Br2	0.1 min
Overall:	4 HBr + O2 → 2 H2O + 2 Br2

Now would you consider the overall reaction to be fast or slow? Clearly it is a slow reaction, taking over a year to complete despite some fast steps.

The overall rate of any reaction depends on the rate of the slowest step. This slowest step is called the rate determining step. If you want to speed up a reaction, this is where you should focus your attention.

The slowest step of a reaction mechanism is the rate determining step

 

Try these practice questions before continuing on.