A fast step and a slow step in a reaction mechanism differ like this:
- Fast step:
- Happens quickly.
- Needs little activation energy.
- Does not usually control the overall reaction speed.
- Slow step:
- Happens slowly.
- Needs more activation energy.
- Acts as the rate-determining step (it controls how fast the whole reaction goes).
Think of it like traffic: the slowest car (slow step) sets the speed for the entire line, no matter how fast the others move.