Endothermic and Exothermic reactions

A chemical reaction is a process in which one or more substances are chemically changed into one or more new substances. A chemical reaction may involve the motion of electrons in the forming and breaking of chemical bonds. Therefore when a chemical reaction takes place, energy is transferred to, or from, the environment. A Chemical reaction can either be exothermic or endothermic. Exothermic Reactions

During an Exothermic Reaction a transfer of energy to the surrounding, usually in the form of heat, takes place. This causes an increase in the temperature of the surrounding environment. The process can be easily explained using the change of phases of Water (H2O): Liquid water had to have energy put into it to become steam, and that energy is not lost. Instead, it is retained by the gaseous water molecules. When these molecules condense to form liquid water again, the energy put into the system must be released.

And this stored energy is let out as exothermic heat. The same explanation can be used for the process of freezing: energy is put into a liquid during melting, so freezing the liquid into a solid again returns that energy to the surroundings. The most stable state is where all energy has been released. When going to a more stable state, energy will be released. On an enthalpy level diagram, higher positions will be less stable therefore, if the product is lower, heat is released. Therefore we can say that the formation of bonds causes an energy release (exothermic). Examples of exothermic reactions:

-making ice cubes
-formation of snow in clouds
-condensation of rain from water vapour
-a candle flame
-rusting iron
-burning sugar
-forming ion pairs
-Combining atoms to make a molecule in the gas phase
-mixing water and strong acids

Endothermic Reactions
An Endothermic reaction is a reaction that takes place when energy is taken in from the surrounding environment. This energy is usually transferred as heat energy, causing the reaction mixture and its surrounding to decrease in temperature. The main reason a reaction is endothermic is because it requires the absorption of heat to take place. Because we must add heat, boiling water is a process that chemists call endothermic. When going to a less stable state, energy will be gained (from the surroundings). On an enthalpy level diagram lower levels will be more stable therefore if the product is higher, heat is gained. We can therefore say that breaking of bonds requires energy (endothermic).

Other examples of Endothermic reactions include:
-melting ice cubes
-baking bread
-cooking an egg
-producing sugar by photosynthesis
-melting solid salts
-mixing water and ammonium nitrate