Temperature and activation energy
Temperature and activation energy
- The activation energy ($E_A$) is the minimum energy a collision needs to be effective.
- The Boltzmann distribution shows how molecular energies are spread out.
- Raising the temperature gives a big rise in rate.
Practice
The activation energy is:
Only collisions with at least the activation energy can react.
The Boltzmann distribution
- The curve starts at the origin, peaks, then falls in a long tail. The area under it is the total number of molecules.
- Only molecules to the right of $E_A$ have enough energy to react.
Practice
On the Boltzmann distribution, the molecules that can react are those:
Molecules with energy greater than Eₐ (right of the line) have enough energy to react.
Why temperature matters
- Raising the temperature spreads the curve to the right, so a much larger fraction of molecules now exceed $E_A$.
- Molecules also collide more often — but the first effect is the bigger one.
- That's why a small rise in temperature gives a large rise in rate.
Practice
The main reason a higher temperature speeds up a reaction is that:
The curve spreads right, so far more molecules exceed Eₐ; this matters more than the increased collision rate.
Practice
A small rise in temperature can give a large rise in reaction rate.
Because the reacting fraction grows sharply with temperature, even a small increase greatly speeds the reaction.
You've got it
Key idea
- activation energy $E_A$ = the minimum energy for an effective collision
- on the Boltzmann distribution, only molecules with energy > $E_A$ react
- higher temperature → curve spreads right → far more molecules exceed $E_A$ (the main effect)
- so a small temperature rise → a large rate rise