2.1 The Collision Theory
We now need a theory that will help explain why different reactions occur at different rates, as well as help us understand ways in which to change the rate of a reaction. The Collision Theory does just that.
The Collision Theory
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The theory also tells us that reacting particles often collide without reacting. Certain requirements must be met if the collisions are effective enough to cause a reaction: |
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animation by Gary Frederickson & Terry Biss
Here are some simple little "experiments" you can do to get an idea of what this all means. I suggest you do these as thought experiments, rather than actually do them.
In an empty classroom blindfold a group of 10 or so students and walk slowly around the room. Occasionally, a pair of students will bump into one another. If they are moving slowly enough, nothing much will happen (that is, no pain upon contact). But if these blindfolded students begin running around the room (more speed = more energy), then a collision is likely to be successful (painfully so). If some students move about fast, while others stroll about at a more leisurely pace, successful reactions will still occur, but not as often as if all students are running.
Next, have the students move at a brisk pace, but without running. This time, however, consider that collisions that occur shoulder-to-shoulder are not successful - your shoulders act as sufficient bumpers and collisions are not painful. But if one student steps on another's toes, then we have a successful collision! Let the movement begin. We find that of the frequent collisions that result, only a few will involve stepping on another's toes. This experiment helps us realized that collisions must occur with the proper orientation. Again, collisions between students will occur, but only some will be successful.
When we examine factors that increase the rate of a reaction, we will therefore look at factors that can influence at least one of the following:
- how often collisions occur. More frequent collisions will mean a faster rate.
- more effective collisions in terms of collisions occurring with sufficient energy
- more effective collisions in terms of collisions occurring with the proper orientation.
Before we look at these factors in depth, there are two more topics that we need to first examine - reaction mechanisms and the concept of threshold energy.