Intermediate vs Transition State
Quick Answer
An intermediate is a real molecule that sits in a valley on the energy diagram. A transition state is just a moment at the top of the hump, and you can never isolate it.
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What Each One Really Means
This one has finished many students in exams, so let me settle it once and for all. An intermediate is a real molecule. It has full bonds. It sits in the valley on the energy diagram, and if you are lucky, you can trap it or detect it with spectroscopy. A transition state is not a molecule at all. It is a moment. It is the highest point on that same curve, where old bonds are half-broken and new bonds are half-formed. It lasts for about a femtosecond. You cannot put it in a bottle. So when you look at the energy diagram, count the humps. If you see two humps, that means there is one intermediate sitting in the valley between them. If you see one hump only, there is no intermediate. That single hump is the transition state. A common exam question shows a two-step reaction and asks you to label the intermediate. Students will point to the top of the humps. No. The intermediate is the valley. The humps are transition states. Learn to read the shape of the curve and this one will never confuse you again.
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Side-by-Side Comparison
| Aspect | Intermediate | Transition State |
|---|---|---|
| What it is | A real molecule with full bonds | A fleeting moment, not a molecule |
| Position on energy diagram | Local minimum (valley) | Local maximum (peak) |
| Lifespan | Short but measurable | About a femtosecond |
| Can it be isolated? | Sometimes, with special techniques | Never |
| Bonds | Fully formed | Half-broken, half-formed |
Memory Device
Intermediate sits in the valley. Transition state stands on the mountain top.