Stoichiometry; Stoichiometric

Stoichiometry describes the quantitative ratios of substances in chemical reactions and is based on the law of conservation of mass and the constancy of the composition of chemical compounds. A stoichiometric ratio exists when all reactants are present in exactly the same ratio as specified by the reaction equation. In this case, there is neither an excess of reactants nor any unnecessary by-products. The basis for this is a balanced reaction equation in which the same number of atoms of each element are present on the reactant and product sides. The quantity of substance n in moles is central, as it links the number of particles to measurable quantities such as mass and volume.

The following applies to a substance: m = n · M, where m is the mass in grams, n is the amount of substance in moles and M is the molar mass in grams per mole. For gases, the molar volume V_m is often used. Under standard conditions, the following approximation applies: 

V = n · V_m

Stoichiometric calculations can be used to determine the required mass of a second reactant and the theoretical yield of products from the given mass of a reactant. In process engineering, stoichiometric ratios are important for the design, safety and energy efficiency of processes. On an industrial scale, they influence raw material utilisation, emissions and the need for exhaust air and wastewater treatment. Let us consider the complete combustion of methane as an example:

CH₄ + 2 O₂ → CO₂ + 2 H₂O

CH₄ (methane) contains one carbon atom and four hydrogen atoms, O₂ (oxygen molecule) contains two oxygen atoms, CO₂ (carbon dioxide) contains one carbon atom and two oxygen atoms, and H₂O (water) contains two hydrogen atoms and one oxygen atom. The coefficients 1 : 2 : 1 : 2 indicate the stoichiometric ratio of the substances. On the reactant and product sides, there is one carbon atom, four hydrogen atoms and four oxygen atoms. If exactly n(CH₄) moles of methane and 2 · n(CH₄) moles of oxygen are present, the mixture is stoichiometrically mixed. The molar masses yield the following:

M(CH₄) ≈ 16 g/mol; M(O₂) ≈ 32 g/mol

This results in the stoichiometric mass ratio: 16 g of methane react with 64 g of oxygen, i.e. the mass ratio of methane to oxygen is 1:4. 

Stoichiometrically dosed, optimally mixed: In many processes, precise material ratios determine the yield, quality and safety of chemical reactions. amixon® mixing systems help to prepare the precisely weighed reactants in the desired stoichiometric ratio.