C7 - Energy Changes
7.1 Endo/exothermic reactions
Endothermic
Exothermic
This is when reactions transfer energy from the the surroundings to the reactants. Energy enters the reaction
This is where energy is released into the surroundings by the reactants. Energy exits the reaction.
7.3 Reaction Profiles
Exothermic reactions
Endothermic reactions
The products are at a lower energy level than the reactants. When the reactants form the products, energy is transferred to the surroundings. The surroundings get hotter.
The products are at a higher energy level than the reactants. This means that when the reactants react to form the products, energy is transferred from the surroundings into the reaction. The surroundings gets colder.
Activation energy
This is the amount of energy needed before the reactants have enough energy to start a reaction.
Overall energy change
This is the increase/decrease in energy between the reactants and the products.
7.2 Using energy transfers from reactions
Endothermic
Exothermic
Chemical cold packs - used as emergency treatment for sports injuries
Chemical hand and body warmers - used to ease aches and pains
Bond breaking and bond making
When a chemical reaction takes place, the chemical bonds between atoms/ions in the reactants are broken. The new chemical bonds are formed to make the products.
7.4 Bond Energy Calculations
Making and breaking bonds
Bond energy
In some reactions, the energy released when new bonds are formed is more than the energy needed to break the bonds in the reactants. These reactions transfer energy to the surroundings and are exothermic.
In other reactions, the energy needed to break the bonds in the reaction is more than the energy release when new bonds are formed in the products. These reactions transfer energy from the surroundings into the reaction and are endothermic.
The energy needed to break the bond between the two atoms is called the bond energy for that bond.