Chemical basics

Chemical basics

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The enthalpy of reaction (ΔH) indicates how much heat is released or has to be added during a reaction (at constant pressure). If the enthalpy is negative (ΔH <0), this means that energy is released during the reaction. is ΔH > 0, energy must be supplied to the system, the reaction is endothermic.

If one considers the reaction of a mole of a substance under standard conditions (25 ° C = 298 K, 1013 mbar = 101,3 kPa), the standard enthalpy of reaction is obtained ΔH°:

H2+½O2H2OΔH°=-286 kJmol-1

The reaction of hydrogen and oxygen, i.e. the formation of water from the elements, is strongly exothermic. When an oxygen-hydrogen mixture ignites, it occurs suddenly with a loud bang, which is why it is referred to as an "oxyhydrogen reaction".

For the reverse reaction, i.e. the decomposition of water into the elements, there is an energy of +286 kJmol-1 necessary; since the forward reaction is exothermic, the reverse reaction is endothermic.