Explaining energy changes in reactions - Higher
Breaking and making bonds
energyThe capacity of a system to do work or the quantity required for mechanical work to take place. Measured in joules (J). For example, a man transfers 100 J of energy when moving a wheelbarrow. is transferred when bondThe chemical link that holds molecules together. are broken or are formed.
| Breaking bonds | Forming bonds | |
| Type of process | Endothermic | Exothermic |
| Heat energy transferred | Taken in | Given out |
| Type of process | |
|---|---|
| Breaking bonds | Endothermic |
| Forming bonds | Exothermic |
| Heat energy transferred | |
|---|---|
| Breaking bonds | Taken in |
| Forming bonds | Given out |
During a chemical reaction:
- bonds in the reactantA substance that reacts together with another substance to form products during a chemical reaction. are broken
- new bonds are made in the productA substance formed in a chemical reaction.
The difference between the energy needed to break bonds and the energy released when new bonds are made determines the type of reaction.
A reaction is:
- exothermicReaction in which energy is given out to the surroundings. The surroundings then have more energy than they started with so the temperature increases. if more heat energy is released in making bonds in the products than is taken in when breaking bonds in the reactants
- endothermicReaction in which energy is taken in. if less heat energy is released in making bonds in the products than is taken in when breaking bonds in the reactants