Chemical basics

We are searching data for your request:

Forums and discussions:

Manuals and reference books:

Data from registers:

Wait the end of the search in all databases.
Upon completion, a link will appear to access the found materials.

Enthalpy

The enthalpy of reaction ( $Δ H$ ) indicates how much heat is released or has to be added during a reaction (at constant pressure). If the enthalpy is negative ( $Δ H$ <0), this means that energy is released during the reaction. is $Δ H$ > 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 $Δ H °$ :

$\begin{matrix} H_{2} + ½ O_{2} & \to & H_{2} O & Δ H ° = - 286 kJ {mol}^{-1} \end{matrix}$

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 $kJ {mol}^{-1}$ necessary; since the forward reaction is exothermic, the reverse reaction is endothermic.