What is the concept behind standard enthalpy changes in chemical reactions?
The easiest way to think of enthalpy is as the “chemical energy” due to the forces between particles in matter which we normally call bonds. During chemical reactions, some bonds are broken and others are formed. If the bonds formed are stronger than the bonds broken, the enthalpy change for the reaction will be negative and we say it is exothermic. According to the Law of Conservation of Energy, energy cannot just be lost so the lost enthalpy is converted into a different form of energy (usually heat, but could be light, sound, etc.). If the bonds formed are weaker than the bonds broken, the enthalpy change for the reaction will be positive and we say it is endothermic. In this case heat (or other) energy is converted into enthalpy during the reaction.
