Stoichiometric number

The stoichiometric number (also stoichiometric coefficient , stoichiometric number or stoichiometric factor ) with the symbol ( ny ) is a chemical quantity . It is used to indicate the proportions of the amount of substance in which substances are converted in a chemical reaction . Stoichiometric numbers are positive for products and negative for reactants , but only their absolute value appears in reaction equations . ${\displaystyle \nu }$

${\displaystyle |\nu _{\mathrm {A} }|\,\mathrm {A} \ +\ |\nu _{\mathrm {B} }|\,\mathrm {B} \ +\ \dots \longrightarrow \ |\nu _{\mathrm {K} }|\,\mathrm {K} \ +\ |\nu _{\mathrm {L} }|\,\mathrm {L} \ +\ \dots }$

The factor 1 is usually left out.

For example is for the reaction

${\displaystyle 4\,\mathrm {Fe} +3\,\mathrm {O} _{2}\longrightarrow 2\,\mathrm {Fe} _{2}\mathrm {O} _{3}}$

the stoichiometric number of ferric oxide , that of iron and that of oxygen . ${\displaystyle \nu (\mathrm {Fe} _{2}\mathrm {O} _{3})=+2}$ ${\displaystyle \nu (\mathrm {Fe} )=-4}$ ${\displaystyle \nu (\mathrm {O} _{2})=-3}$

Dependence on the formulation of the equation

The stoichiometric numbers in a reaction equation can be multiplied by rational numbers as desired, the ratios of the stoichiometric numbers being retained. This illustrates the following alternative formulation of the above equation:

${\displaystyle \textstyle 2\,\mathrm {Fe} +{\frac {3}{2}}\,\mathrm {O} _{2}\longrightarrow \mathrm {Fe} _{2}\mathrm {O} _{3}}$.

In this reaction equation, too, there is the ratio between the amount of iron oxide formed and the amount of iron converted , but the values ​​of the stoichiometric numbers themselves differ. ${\displaystyle \textstyle {\frac {1}{2}}}$

The cardinal equation represents a special position among the formulations of a reaction with a given stoichiometry . Here, all stoichiometric coefficients are whole numbers that do not have a common divisor (smallest whole number ratio). The stoichiometric numbers in a cardinal equation are unambiguous except for their sign, whereby a reversal of the sign is equivalent to a formulation of the reverse reaction.

In stoichiometric calculations , ratios of stoichiometric numbers are always used, so that the formulation of the reaction equation does not matter. When calculating equilibrium constants , however, the stoichiometric numbers themselves are used. A doubling of the stoichiometric numbers is then accompanied by a squaring of the equilibrium constants, a reversal of their signs results in the reciprocal of the equilibrium constants. For calculations in which stoichiometric numbers are used not only in the form of their ratios, a reaction equation must therefore always be given, since otherwise the calculated quantities cannot be interpreted.

See also

• Sales variable

Individual evidence

1. ↑ Entry on stoichiometric number . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.S06025 Version: 2.3.1.
2. ↑ Peter W. Atkins, Julio de Paula: Physical chemistry . Wiley-VCH Weinheim, 4th edition 2006, page 228. ISBN 3-527-31546-2 .
3. ↑ Peter W. Atkins, Julio de Paula: Physical chemistry . Wiley-VCH Weinheim, 4th edition 2006, page 228. ISBN 3-527-31546-2 .