Reactions with Energy to Spare
We can tell a chemical reaction has occoured when there is a temperature change.
When heat is given to the surroundings and the product gets colder an exothermic reaction has occoured.
Exothermic reactions always occour:
When a metal and acid react
In displacement reactions
The giving out of heat is beneficial in hand warmers and hot coffee.
Reactions that Take in Energy
Reactions that take in heat making the product hotter are endothermic reactions.
They take in energy because for the reaction to take place there must be some form of energy given, like from a Bunsen Burner.
All decomposition reactions are endothermic.
Endothermic reactions can be used for energy storage.
A main example of this is photosynthesis where glucose and oxygen are made so that they can then react and making energy.
Energy in Reversible Reactions
If a reaction is reversible, one side of the reaction will always be endothermic while the other is exothermic.
You test for water by adding it to anhydrous copper sulphate. This gives of heat (exothermic reaction) and turn the copper sulphate blue, making it hydrated copper sulphate.
If the hydrated copper sulphate is then heated, it will turn back to its orginal colour (white / grey) and since it is heated, it is an endothermic reaction.
This reversibility is used for re-usable hand warmers.
Photosynthesis and respiration are also reversible reaction and this is used by plants to get energy for growth and movement.