Determination of the enthalpy
Reaction enthalpy refers to the enthalpy change observed in a sample during a chemical reaction. Reactions that release energy in the form of heat are classified as exothermic, while reactions requiring energy input are termed endothermic.
A straightforward illustration of an endothermic process is melt enthalpy, where heat must be supplied to transition a solid crystal structure to a liquid phase with molecules moving freely. Conversely, an example of an exothermic reaction is combustion, where a substance reacts with oxygen, resulting in the release of 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
- Nucleosynthesis (fusion) of elements heavier than nickel
- 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)
- Most polymerization reactions
- Combustion
- Respiration
- Corrosion of metals (an oxidation reaction)
- Most crystallization processes
Enthalpy determination with a calorimetry
Enthalpy determination using calorimetry involves Differential Scanning Calorimetry (DSC), where the sample’s temperature is compared with an ideally non-reactive reference in the same sample chamber and atmosphere. Both experience the same temperature environment. If the sample’s temperature deviates from the reference, a reaction is occurring. This method provides a quantitative measure of the heat changes associated with chemical reactions.
