1. Bond breaking and bond making
Learning Objectives:
To know what happens to the energy when bonds are broken and formed.
To be able to interpret activation energies in terms of bond breaking and bond forming
Key words: Bond energy, bond forming, bond breaking, activation energy
Homework: Workbook p Q11-19 inclusive
Due: Friday 26th Feb

2. Activity

3. Activity

4. Breaking chemical bonds
Heat taken in
Energy in chemicals
Energy needed
Energy needed to overcome the bonding between the atoms
Making chemical bonds
Energy in chemicals
Energy given out
Heat given out
Energy given out as bonds form between atoms

5. Changes to chemical bonds: Endothermic Reactions
Energy in chemicals
reactants
products
Energy given out as new bonds form
Energy taken in as old bonds break H
Overall endothermic in this case

6. Changes to chemical bonds: Exothermic Reactions
Energy in chemicals
reactants
products
Energy taken in as old bonds break
Energy given out as new bonds form H
Overall exothermic – in this case

7. Ac7.8 Q5+6 ONLY – Burning Methane
Summary – Bond Changes
If the energy from bond forming is greater than that needed for bond breaking the reaction is exothermic.
If the energy for bond breaking is greater that from bond forming the reaction is endothermic.
Energy in chemicals
reactants
products H
Bonds break
Bond forming
Bonds form
Exo
Endo
Ac7.8 Q5+6 ONLY – Burning Methane

8. H Burning Methane: An exothermic reaction C H O H O C H O C Bond
Progress of reaction
Energy in chemicals O C H
Bond
Breaking
Bond
Forming O H C O C H H

9. Activity Hydrogen peroxide decomposes as shown: O
Calculate energy for bond breaking.
Calculate the energy from bond making
What is the value of H for the reaction shown O H
Bond
Energy (kJ)
H-O
464
O-O
146
O=O
498 O H O O H O H

10. Bond breaking. (endothermic) 4(O-H) + 2(O-O) =1856+292 = +2148kJ
Answer
Bond
Energy (kJ)
H-O
464
O-O
146
O=O
498
Bond breaking. (endothermic)
4(O-H) + 2(O-O)
= = +2148kJ
Bond forming: (exothermic)
4(O-H) + 1(O=O)
= = -2354kJ
H = – 2354 = -206kJ
(Exothermic) O H O H O O H O H

11. Rates of reaction

12. Activation energy
There must be enough energy (activation energy) to start breaking the old bonds before a reaction can occur.
Activation energy needed to break existing bonds
Reactants
New bonds form
Old bonds start to break
Collision theory animation

13. Reaction Activation energy
Most reactions are exothermic (give out heat) overall but there is still a need for energy to get the reaction started.
Energy in chemicals
Reaction
Activation energy
Break old bonds
Energy taken in
Form new bonds
Energy given out

14. Activity Write a definition of activation energy (Ea) with a diagram.
Read p and answer Q1+2
Read p and identify why an understanding of activation energy is essential when working with explosives.

15. How much do you know?