# Why is the enthalpy of formation negative

Page - 15 - Enthalpy H

[Greek: en darin, within + thalpein warm or thalpos warm ]

The enthalpy H is a thermodynamic state variable. It is a designation for the amount of heat given off or absorbed by a reaction. It is measured in kJ (kilojoules). You cannot measure the enthalpy of a state, but only the difference between two states. If one state changes into another through a reaction, the amount of heat given off or absorbed can be measured; it is called DH: Reactions in which heat is given off are called exothermic. The numerical values ​​are given a negative sign.
Such reactions that will need heat endothermic called. Your numerical values ​​are given a positive sign.
An example of one exothermic reaction is the oxyhydrogen reaction between oxygen and hydrogen to form water: (22)

There is an enthalpy DH of - 484 kJ / mol released. It expresses itself, among other things, in a loud bang, which gave this reaction its name.
The previously mentioned reaction of elemental sodium with water (see Gay-Lussac's gas law) is an exothermic reaction:  (23)

This releases enthalpy, which can be seen, among other things, in the rapid movement of sodium over the surface of the water.

Endothermic reactions, on the other hand, require heat to take place. If, for example, the two gases nitrogen and oxygen are allowed to react with each other to form nitrogen monoxide, an amount of heat is required for this reaction to take place. In nature, these two gases (which both occur in the air) react with each other, e.g. in the event of a lightning strike, as this provides sufficient heat: (24)

There is an enthalpy DH of + 180.6 kJ / mol needed.
Another example is the extraction of pure silicon. For this, quartz (SiO2 = Silicon oxide) reacted with coke (C = carbon) in an electric furnace:  (25)

There is an enthalpy DH of + 690 kJ / mol needed.   Enthalpy entropy free enthalpy

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