As an analogy, imagine a relay team. One of the runners must be the slowest, just as one of them must be the fastest. On a track team, the chances are that all of the runners are pretty fast, so that the difference between the fastest and the slowest runners may be only fractions of a second.
In the chemistry world a fast reaction may occur in trillionths of a second, while a slow reaction may occur in billionths of a second. Although that is really fast, it may be 1000 times slower than a fast reaction. This is like having one member of the relay team replaced by a tortoise.
A step that is notably slower than the rest of the mechanism is called the rate determining step, because the entire reaction cannot go faster than this step. That also means that nothing can change the rate of the reaction, unless it changes the slow step. In other words, the relay team will never get faster unless you can deal with the tortoise.
An more involved analogy may help.
A “get-involved” catalyst provides an alternative way for a reaction to occur. This occurs by changing the mechanism in some way so that the result is the same, but the process occurs more quickly. To see how this might work in the analogy, go here.
An example of a get-involved catalyst in the real world would be the chlorine catalyzed destruction of ozone. This page discusses how ozone is produced and destroyed naturally and how humans have changed that process.
This mechanism is important because the activation energy of the first step in this process is significantly less than it is for the first step in the other process that destroys ozone.
Lower activation energy means that more of the collisions already have enough energy to react and the rate is higher.
The difference between this “get-involved” catalyst and a surface catalyst is that the surface catalyst is never changed in the process, the “get-involved” catalyst is consumed in the middle of the process and then reproduced.
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