Determination of the 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:
- 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
- Nucleosynthesis (fusion) of elements heavier than nickel
- Evaporating liquids
- Melting solids
- 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)
- Most polymerization reactions
- Corrosion of metals (an oxidation reaction)
- Most crystallization processes
Enthalpy determination with a calorimetry
In Differential Scanning Calorimetry, the sample temperature is compared with an ideally non-reactive reference, which is located in the same sample chamber and thus in the same atmosphere. Both experience the same temperature environment. If the temperature of the sample to be measured rises or falls in comparison to the reference, a reaction takes place here.