# Bond breaking and bond forming

> Where the energy of a reaction comes from


## Objectives

- State that in a reaction the bonds of the reactants break and new bonds form in the products.
- Recognize that breaking bonds absorbs energy and forming bonds releases energy.
- Decide from the balance whether a reaction gives out or takes in energy.

## 1. Bond breaking and bond forming: from zero




## 2. Where does the heat come from?

Predict: where does the heat come from?

- From forming the new H–Cl bonds, which gives out more energy than breaking took ✓
- The atoms are destroyed and turn into heat
- From breaking the bonds in $\ce{H2}$ and $\ce{Cl2}$

Breaking the bonds in $\ce{H2}$ and $\ce{Cl2}$ takes energy in; forming the new H–Cl bonds gives energy out. Forming gives out more than breaking took, so heat is given out overall.

[ar-EG] كسر الـ bonds اللي في الـ $\ce{H2}$ والـ $\ce{Cl2}$ بياخد energy، وتكوين الـ bonds الجديدة H–Cl بيطلّع energy. والتكوين بيطلّع أكتر من اللي الكسر خده، فالمحصلة إن فيه حرارة بتطلع.
## 3. Break, then form

A reaction breaks the bonds of the reactants and forms new bonds in the products. Breaking a bond **absorbs** energy. Forming a bond **releases** energy. Breaking bonds releases energy.


## 4. The energy balance

Made-up numbers: breaking the reactants' bonds absorbs 600 kJ (kilojoules, a unit of energy). Change the energy released by the new bonds. energy released by forming bonds absorbed by breaking energy change (ΔH) = absorbed − released Chemists write the energy change, ΔH, as absorbed − released. A minus sign means energy is given out. More released than absorbed: ΔH is negative and the reaction **gives out** energy as heat. Released = absorbed: ΔH = 0, no net energy change. More absorbed than released: ΔH is positive and the reaction **takes in** energy.


## 5. Your turn

In a reaction, breaking bonds absorbs 500 kJ and forming bonds releases 300 kJ. What is the result?

- It releases 200 kJ
- It releases 800 kJ
- It takes in 200 kJ ✓

ΔH = absorbed − released = 500 − 300 = +200 kJ. Positive, so the reaction takes in 200 kJ.

[ar-EG] ΔH = absorbed − released = 500 − 300 = +200 kJ. الإشارة موجبة، يعني الـ reaction بيمتص 200 kJ.
## 6. Summary

A reaction = breaking the reactants' bonds + forming the products' bonds. Breaking bonds absorbs energy; forming bonds releases energy. The balance is the energy change, ΔH = absorbed − released: negative = given out, positive = taken in. Before any bond can break, colliding molecules need a minimum energy: the activation energy. Back to the hook: can you explain why hydrogen burning in chlorine gives out heat? Breaking the bonds in $\ce{H2}$ and $\ce{Cl2}$ absorbs energy. Forming the new H–Cl bonds releases more energy than breaking took, so ΔH is negative and the reaction gives out heat.



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