1Breaking a bond takes energy in
Atoms in a molecule are held together by bonds. To break a bond, energy must be put in.
bond
Lesson 17.4 notes
Breaking and making bonds
Why some reactions give out energy and others take it in: breaking bonds takes energy in, making bonds gives it out.
2Making a bond gives energy out
When atoms join to make a new bond, energy is given out.
3A reaction does both
In a reaction, the old bonds break (energy in), then new bonds form (energy out).
4More out than in
If making the new bonds gives out more energy than breaking the old ones takes in, the reaction is exothermic.
exothermic
5More in than out
If breaking the old bonds takes in more energy than making the new ones gives out, the reaction is endothermic.
endothermic
6Getting started
A candle won't light on its own. A flame gives the energy to break the first bonds. Then the energy given out keeps it burning.
7Fuels
When a fuel burns, making the new bonds gives out more energy than breaking the old ones takes in. That energy heats things up.
fuel
Explore: Energy balance
Choose a reaction. Break the bonds, then make the new ones, and compare the energy in and out.
Common mistake: Breaking bonds does not give out energy
It is easy to think that breaking bonds releases energy. Breaking bonds always takes energy in. Energy is given out when new bonds form.
Remember
- Breaking bonds takes energy in. Making bonds gives energy out.
- More energy out than in: exothermic. More energy in than out: endothermic.
- A spark or flame gives the energy to break the first bonds, so burning can start.
Going further at GCSE
GCSE: Exothermic and endothermic reactions → GCSE: Energy changes →