# Activation energy and energy profile

> Why reactions need a start


## Objectives

- Define activation energy in the book's words.
- Explain why only collisions with enough energy lead to a reaction.
- Read an energy profile: activation energy versus net energy released.

## 1. Magnetic properties and catalytic activity




## 2. Strike the match

Predict: why must you strike it first?

- Striking supplies all the heat that the flame later gives out
- Striking gives the molecules the minimum energy they need to start reacting ✓
- Striking pushes air into the match

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.

[ar-EG] الجزيئات المتفاعلة لازم يكون معاها حد أدنى من الـ energy قبل ما تتفاعل: ودي الـ activation energy. حكّة عود الكبريت هي اللي بتدّيها، وبعدين اللهب بيطلّع حرارة أكتر بكتير من اللي الحكّة دخّلتها.
## 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.


## 4. Collide!

Here the activation energy is 60 units (made-up). Set the energy of a collision. energy of the colliding molecules Less than the activation energy: the molecules bounce apart. No reaction. Equal to or more than the activation energy: old bonds break and new bonds form.


## 5. The energy profile

Energy profile A graph of potential energy against the progress of the reaction. The hump is the barrier the molecules must climb. Activation energy is measured from the reactants **up** to the top of the hump. Activation energy is the energy the reaction gives out.


## 6. Read a new profile

Which arrow shows the activation energy of this reaction?

- Y
- Z
- X ✓

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.

[ar-EG] السهم X طالع من energy الـ reactants لحد قمة الحدبة: والطلعة دي هي الـ activation energy. أما Z فبيقارن الـ reactants بالـ products، و Y بيبدأ من الـ products.
## 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. A catalyst lowers the hump: how transition elements speed up reactions. Back to the hook: can you explain why a match must be struck, though burning gives out heat? 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.



Chapter: Transition Elements (https://yaani.ai/ar/learn/chemistry/chem-s3-en.ch1)
Source: https://yaani.ai/ar/lesson/chem-s3-en.lesson-kin.activation-energy