Enthalpy is a state and calculation variable in thermodynamics. Enthalpy (H) stands for the amount of heat given off or absorbed by exothermic or endothermic reactions. In the past, this quantity was also called the unit of heat. Its unit of measurement is kilojoules (kJ). Enthalpy also describes the measure of the energy of a thermodynamic system. It is calculated as the sum of the internal energy (U) of a system and its product of pressure ( p ) and volume (V).
Exothermic and endothermic reactions
Both reactions are basically accompanied by a turnover of energy.
If thermal energy is released in the form of heat during a reaction, we speak of an exothermic (ex = outward, therm = heat) reaction. Examples from everyday life are the burning of wood in a stove and the burning of gasoline/diesel in an engine.
If thermal energy is added to a reaction in the form of heat, we speak of an endothermic (endo = inward, therm = heat) reaction. An example from everyday life is sherbet powder in water. When the sherbet powder (citric acid + sodium hydrogen carbonate) dissolves in the water, the energy from the water is needed. This reaction also makes the water a little colder.
The entropy (S) influences in which direction a reaction proceeds. A reaction likes to go from an ordered state to a disordered state. This is also the reason why the burned ash (-> disordered state) will not form a log (-> ordered state). Also the dissolved effervescent tablet (-> disordered state) will not assemble to a solid tablet (-> ordered state).
Therefore the statement is true: The more possibilities there are, which a state can take, the more likely they meet.« Zurück zum Glossar Index