Formula sales

In chemistry, a formula conversion is the specification of a complete conversion of starting materials into products according to an existing chemical reaction equation . In the case of an elementary formula conversion of a reaction N ₂ + 3 H ₂ 2 NH ₃ , one molecule of nitrogen and three molecules of hydrogen are converted into two molecules of ammonia in accordance with the stoichiometry . Often a molar formula conversion is considered, in which the corresponding molar amounts of substance of the molecules are completely converted. Such a formula conversion is used to indicate chemical data of a reaction, for example to indicate the reaction enthalpy of a reaction: N ₂ + 3 H ₂ 2 NH ₃ , Δ _R H = −92.4 kJ · mol ⁻¹ . ${\displaystyle \rightleftharpoons }$ ${\displaystyle \rightleftharpoons }$

In many reaction equations, not just molecules, as in the example, but also atoms , cations , anions , formula units and, in electrochemistry, electrons . In the case of formula conversions, we often speak of stoichiometric "object quantities" in order to have a collective term for the various particles and ratio formulas in a reaction equation. The molar formula conversion of a reaction equation corresponds to the conversion variable (reaction run number) ξ = 1 mol.

Formula notation and formula conversion

For reaction equations, it is generally true that only integer stoichiometric numbers are used, which are, however, as small as possible. Such reaction equations are called cardinal equations according to DIN 32 642. With a formula conversion according to the reaction equation

${\displaystyle \mathrm {2\ H_{2}+O_{2}\longrightarrow 2\ H_{2}O} }$

2 mol H ₂ and 1 mol O _{2 are converted} to 2 mol H ₂ O. Occasionally, fractional stoichiometric numbers are also used:

${\displaystyle \mathrm {H_{2}+{\frac {1}{2}}\ O_{2}\longrightarrow H_{2}O} }$

With a formula conversion, 1 mol of H ₂ and 0.5 mol of O _{2 are converted} to 1 mol of H ₂ O here.

The form of the reaction equation has an impact if, for example, a molar standard reaction enthalpy is specified for the reaction. While the value in the upper cardinal equation is Δ _r H ⁰ = −571.6 kJ / mol, the value in the lower equation is only half as large (Δ _r H ⁰ = −285.8 kJ / mol).

If the reaction is an equilibrium reaction, the reaction equation is read from left to right.

${\displaystyle \mathrm {3\ H_{2}+N_{2}\rightleftharpoons 2\ NH_{3}} ,}$Δ _R H = −92.4 kJ mol ⁻¹

With a formula conversion in the cardinal equation, 3 mol of hydrogen are converted with one mol of nitrogen to 2 mol of ammonia. The molar enthalpy of reaction is exothermic in this reaction direction; the value is accordingly given negative.

Mathematical definition

With a formula conversion, the conversion variable changes by 1 mol:

${\displaystyle \Delta \xi =1{\text{mol}}={\frac {\Delta n_{i}}{\nu _{i}}}}$

• ξ: sales variable
• v _i : stoichiometric number of particles i
• Δ n _i : change in the amount of substance of the particles i

The stoichiometric number and the change in the amount of substance of the i-th particles have negative signs when they are consumed and positive when they are formed. The turnover variable is therefore always positive. The amount of substance has the unit mol , the stoichiometric number is dimensionless.

Individual evidence

1. ↑ Hans-Dieter Jakubke, Ruth Karcher (Ed.): Lexicon of Chemistry , Spectrum Academic Publishing House, Heidelberg, 2001.
2. ^ Karl-Heinz Lautenschläger, Werner Schröter, Joachim Teschner, Hildegard Bibrack, Taschenbuch der Chemie , 18th edition, Harri Deutsch, Frankfurt (Main), 2001.
3. ↑ Entry on turnover variable. In: Römpp Online . Georg Thieme Verlag, accessed on June 20, 2014.