What is a simple way to classify the enthalpy of a reaction?
The evolution of heat from a reaction is often classified as being exothermic or endothermic, but how do you know which one it is in order to assign the appropriate sign?
A simple way to be sure of which type you are dealing with in a reaction is to consider what hapens to the bonds.
The rule is that bond making reactions are exothermic and bond breaking reactions are endothermic.
The simple explanation is that, when two atoms are joined to form a molecule, energy is released as heat when the bond is made and the new molecule stabilised. Therefore this energy is released into the surroundings and felt as heat. This is what is known as an exothermic reaction and is assigned a negative sign to indicate the flow of heat to the surroundings. An example is the heat felt when you dissolve detergent in water.
For the endothermic reaction, a bond must be broken. That is, energy is required to break the bond holding a molecule. Therefore this lose of energy is felt as a lose of heat in the surrounding environment and is assigned a positive sign to indicate the absorption of heat into the molecule. An example would be the melting of ice cubes.