Activation energy and energy profile
Why reactions need a start
1. Magnetic properties and catalytic activity
2. Strike the match
A burning match gives out a lot of heat. Yet a match does not light by itself: you must strike it.
Pick the right answer
Predict: why must you strike it first?
- A. Striking gives the molecules the minimum energy they need to start reacting
- B. Striking supplies all the heat that the flame later gives out
- C. Striking pushes air into the match
الشرح
The reacting molecules must have a minimum amount of energy before they react: the activation energy. Striking the match supplies it; the flame then gives out much more heat than the strike put in.
3. Enough energy to react
Molecules react only when they collide. Only fast molecules have enough energy to break their bonds on collision.
Activation energy
The minimum amount of energy that must be gained by a molecule to react at collision.
Slower molecules just bounce apart, unchanged.
- Molecules collide → Energy ≥ activation energy: old bonds break, new bonds form (reaction) (energy ≥ activation energy)
- Molecules collide → Energy < activation energy: bounce apart, unchanged (no reaction) (energy < activation energy)
4. Collide!
Here the activation energy is 60 units (made-up). Set the energy of a collision.
energy of the colliding molecules 30 units
5. The energy profile
Energy profile
A graph of potential energy against the progress of the reaction.
Activation energy is measured from the reactants up to the top of the hump.
6. Read a new profile
Pick the right answer
Which arrow shows the activation energy of this reaction?
- A. X
- B. Y
- C. Z
الشرح
Arrow X goes from the energy of the reactants up to the top of the hump: that climb is the activation energy. Z compares the reactants with the products, and Y starts at the products.
True or false?
A reaction that releases a lot of energy cannot have a high activation energy.
الشرح
The activation energy (the climb to the top of the hump) and the energy released (the drop to the products) are separate. A reaction can release a lot of energy and still need a high activation energy, like a match that needs a strike.
7. Summary
- Activation energy: the minimum amount of energy a molecule must gain to react at collision.
- Activation energy: from the reactants up to the top of the hump.
- Net energy released: from the reactants down to the products.
Back to the hook: can you explain why a match must be struck, though burning gives out heat?
Reveal
The molecules must first gain the activation energy before they react: striking the match supplies it. The flame then gives out much more heat than the strike put in.