Types of enthalpy
Enthalpy of reaction:
Energy can be transferred into a reaction (endothermic) or transferred from a reaction (exothermic). Some people would say that energy is “consumed” in an endothermic reaction and it is “released” in an exothermic reaction. This is not quite right because energy cannot be created, destroyed or consumed. Energy is always changing between different forms.
The energy transferred in chemical reactions is a result of breaking and re-making chemical bonds between the atoms, so that new molecules are produced. Energy is needed in order to break a chemical bond, On the other hand, when a chemical bond is made, energy is released. By accounting these two processes it is possible to calculate the energy that is transferred by a particular reaction. This will be done on the examples below.
There are various types of chemical reaction and usually the enthalpy corresponding to each receives a particular name:
Enthalpy of combustion
Combustion is the reaction with oxygen. This type of reaction usually transfers a lot of energy, due to the formation of carbon dioxide molecules.
Molecules that can transfer a large amount of energy upon combustion are called fuels (petrol, diesel, gas, coal, etc.) or explosives (TNT).
Energetic molecules that are employed in producing energy for the maintenance of life are called food. In this case, we call oxidation the reaction of these molecules, tipically glucose, with oxygen, instead of combustion. Upon oxidation, the energy from glucose molecules is transferred into the energy currency of living things: ATP.
Examples
Enthalpy of solvation (or enthalpy of solution)
Solvation is a process that occurs when, for instance, a salt is dissolved the water and the distance between its atoms increases. The variation on the electrostatic (electric) force, because of the positive and negative ions being moved further apart, is the reason for the difference in energy found between the initial and final situation. A liquid can become cooler or hotter as a result of having a particular substance dissolved into it, as solvation can be endothermic or exothermic.
Enthalpy of formation:
All substances have an enthalpy associated to it which is called entalpy of formation, and it is the enthalpy involved in forming a substance from its constituent substances in their standard state. Standard state is the substance most "natural" state for a substance. For water it is liquid.For carbon it is graphite (rather than diamond or fullerene) and for oxygen is a gas rather than liquid, which is its most common form in planets like Jupiter.
These reactions are useful to visualise the concept of enthalpy of formation of a compound, but they are not always possible to be carried out in practice (fake reactions - this idea is also employed in electrochemistry, to define the reduction potential of elements).
The standard enthalpy of formation of an element in its standard state is zero.
Other entalpies:
Enthalpy of fusion, vaporization, sublimation, hydration, mixing, ionization, etc...
How to calculate enthalpies >>