Chemical bonds
Breaking and making bonds
The 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 The 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 A substance that reacts together with another substance to form products during a chemical reaction. are broken
- new bonds are made in the A substance formed in a chemical reaction.
The difference between the energy needed to break bonds, and the energy released when new bonds form, determines the type of reaction. A reaction is:
- Reaction 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 forming bonds in the products than is taken in when breaking bonds in the reactants
- Reaction in which energy is taken in. - if less heat energy is released in forming bonds in the products than is taken in when breaking bonds in the reactants
An Chart showing the energy in the reactants and products, and the difference in energy between them. shows these differences more clearly.
Energy level diagrams
An energy level diagram shows the energy in the reactants and products, and the difference in energy between them.
Exothermic reaction
The energy level decreases in an exothermic reaction. This is because energy is given out to the surroundings.
Endothermic reaction
The energy level increases in an endothermic reaction. This is because energy is taken in from the surroundings.