Enthalpy – a measured variable in thermal analysis
What is enthalpy?
The reaction enthalpy is the enthalpy change of a sample during a chemical reaction. Reactions that release energy in the form of heat are called exothermic. Reactions in which energy must be supplied as endothermic.
Melt enthalpies are a simple example of endothermic processes, since one usually has to give heat work in a system in order to break up its solid crystal structure and convert it into a liquid phase with molecules that move freely relative to one another. An example of an exothermic reaction is a simple combustion process in which a substance reacts with oxygen to release energy.
The difference between endothermic and exothermic release:
Endothermic:
Energy is consumed (heat is absorbed)
Energy is consumed or needed for reaction to occur
Increase of inner enthalphy
Examples of endothermic processes include:
Dissolving salts in solvent
Cracking alkanes
Evaporating liquids
Melting solids
Exothermic:
Energy is released (heat is released)
Reaction occurs spontaneously or triggered
Decrease of inner enthalphy
Examples of exothermic processes include:
The thermite reaction
A neutralization (e.g., mixing an acid and a base to form a salt and water)