O Level Notes : Chemistry - Energy from Chemicals
An exothermic reaction is one where energy (heat) is given out causing a temperature rise in the surroundings. The temperature of the reaction mixture rises. An endothermic reaction is one where energy (heat) is taken in causing a temperature drop in the surroundings.
Exothermic and Endothermic Changes:
An exothermic reaction is one where energy (heat) is given out causing a temperature rise in the surroundings. The temperature of the reaction mixture rises. The container feels hot. Examples of exothermic reactions include:
- Combustion of fuels
- Rusting of iron
- Corrosion of metals
- Respiration
- Neutralization reaction
An endothermic reaction is one where energy (heat) is taken in causing a temperature drop in the surroundings. The temperature of the reaction mixture falls. The container feels cold. Examples of endothermic reactions include:
- Photosynthesis
- Thermal decomposition
The amount of energy involved in a reaction is known as he heat change or the enthalpy change of the reaction. ΔH for an exothermic reaction is negative. This is because chemicals have lost energy to the surrounding. ΔH for an endothermic reaction is positive. This is because the chemicals gain energy from the surroundings.
Making and Breaking Bonds:
An exothermic change occurs when chemical bonds are made.
An endothermic change occurs when chemical bonds are broken.
The overall energy change of a reaction is the difference between the energy given out when bonds are made and the energy taken in when bonds are broken.
Activation Energy:
All reactions need energy in order to get started, weather it is an endothermic reaction or an exothermic one. The minimum energy that reacting particles must possess in order for a chemical reaction to occur is called the activation energy, EA.
Energy Profile Diagrams:
To show the activation energy of a reaction, energy profile diagrams are used. This diagram is a way of representing the energy changes that occur during a chemical reaction. The energy difference between the products and reactants represents the enthalpy change of the reaction. So, an energy profile diagram shows the activation energy required and the enthalpy change for a reaction.
The energy profile diagram for an exothermic reaction would be:
The energy profile diagram for an endothermic reaction would be: