All chemical reactions start with the reactants having some sort of energy activation that results in the bonding of the two material's molecules and forms a new product.
The most common form of energy is heat; either the chemical reaction will cause the blended mixture (product) to give off heat, or absorb it.
The primary differences between exothermic and endothermic
The easiest way to start to understand the differences between endothermic and exothermic reactions can be derived from their names. "Endo" means "to draw into," and "Exo" means "to give off."
* Endothermic reactions draw heat in from their surroundings.
* Exothermic reactions give heat off to their surroundings.
Endthermic properties explained
* Energy is drawn in from the external environment, causing its surroundings to loose heat, or "cool down."
* The endothermic chemical reaction creates a product that has a higher energy level than the original materials, causing the reactant's stored energy to decrease. (In scientific terms, the reactants have "less total enthalpy" than the product.)
* The resulting product of the reaction is less stable because, the higher the energy bond, the less strength its molecules possess.
* Most endothermic reactions are not spontaneous.
Exothermic properties explained
* Exothermic chemical reactions cause their surroundings to warm up by giving off heat.
* The reactants contain more stored energy than the product because energy from external sources is not required, but given off. This gives the product more stability due to the lower amount of energy needed. (In this case, the reactants have a "greater total enthalpy" than the product.)
* Exothermic reactions are usually spontaneous.
Simple examples of entothermic and exothermic reactions
* Endothermic = Melting ice. In order for ice to melt, it needs to draw in the heat (energy) from its surroundings. The ice becomes less stable as it responds to the increased heat. The ice's stored energy decreases. The end product is water, which has a higher energy level than the ice.
* Exothermic = Lighting a match. When the head of the match is struck, it results in the spontaneous release of stored energy (heat) from the reactants into the surroundings. The flame that is produced has a lower level of energy than the match and the striking surface because the reaction is giving off stored energy and not required to draw energy in from its surroundings.
Chemical reactions are all about energy and randomness. Some reactions occur as the result of entropy, which is a facet of random, spontaneous bonding, but even if a spontaneous reaction between chemicals would not normally occur, one can be created, if enough energy is applied.