Stoichiometry is a branch of chemistry that studies the quantitative relationships between substances that have entered into a reaction and formed during it (from other Greek: "stoichion" - "elemental composition", "meytren" - "measure").
Stoichiometry is the most important theoretical basis for material and energy calculations, without which it is impossible to organize any chemical production. Chemical stoichiometry allows you to calculate the amount of raw materials needed for a particular production, taking into account the desired performance and possible losses. No enterprise can be opened without preliminary calculations.
A bit of history
The very word "stoichiometry" is an invention of the German chemist Jeremiah Benjamin Richter, proposed by him in his book, which first described the idea of the possibility of calculations using chemical equations. Later, Richter's ideas received theoretical justification with the discovery of the laws of Avogadro (1811), Gay-Lussac (1802), the law of constancy of composition (J.L. Proust, 1808), multiple relations (J. Dalton, 1803), and the development of atomic-molecular theory. Now these laws, as well as the law of equivalents formulated by Richter himself, are called the laws of stoichiometry.
The concept of "stoichiometry" is used in relation to both substances and chemical reactions.
Stoichiometric equations
Stoichiometric reactions are reactions in which the starting materials interact in certain proportions, and the amount of products corresponds to theoretical calculations.
Stoichiometric equations are equations that describe stoichiometric reactions.
The stoichiometric coefficients (coefficients of the equations) show the quantitative relationships between all participants in the reaction, expressed in moles.
Most inorganic reactions are stoichiometric. For example, three sequential reactions for producing sulfuric acid from sulfur are stoichiometric.
S + O 2 → SO 2
SO 2 + ½O 2 → SO 3
SO 3 + H 2 O → H 2 SO 4
By calculating these reaction equations, you can determine how much each substance needs to be taken to get a certain amount of sulfuric acid.
Most organic reactions are non-stoichiometric. For example, the ethane cracking reaction equation looks like this:
C 2 H 6 → C 2 H 4 + H 2 .
However, in fact, the reaction will always produce different amounts of by-products - acetylene, methane and others, which are theoretically impossible to calculate. Some inorganic reactions also cannot be calculated. For example, the decomposition reaction of ammonium nitrate:
NH 4 NO 3 → N 2 O + 2H 2 O.
It goes in several directions, so it is impossible to determine how much you need to take the starting material to get a certain amount of nitric oxide (I).
Stoichiometry is the theoretical basis of chemical production
All reactions that are used in chemical analysis or in production must be stoichiometric, that is, subjected to accurate calculations. Will a plant or a factory bring benefits? Stoichiometry makes it possible to find out.
Based on stoichiometric equations, a theoretical balance is made. It is necessary to determine how much starting material is required to obtain the right amount of the product of interest. Next, operational experiments are carried out that show the real consumption of the starting materials and the yield of products. The difference between theoretical calculations and practical data allows us to optimize production and evaluate the future economic efficiency of the enterprise. Stoichiometric calculations, in addition, make it possible to draw up a heat balance of the process in order to select equipment, determine the mass of the resulting by-products that will need to be removed, and so on.
Stoichiometric substances
According to the law of constancy of the composition proposed by J.L. Proust, any chemically pure substance has a constant composition, regardless of the method of preparation. This means that, for example, in a sulfuric acid molecule of H 2 SO 4, regardless of the method by which it was obtained, two hydrogen atoms will always have one sulfur atom and four oxygen atoms. Stoichiometric are all substances having a molecular structure.
However, substances are widely distributed in nature, the composition of which may differ depending on the method of preparation or source of origin. The vast majority of them are crystalline substances. You could even say that for solids, stoichiometry is the exception rather than the rule.
As an example, consider the composition of well-studied carbide and titanium oxide. In titanium oxide, TiO x X = 0.7–1.3, that is, from 0.7 to 1.3 oxygen atoms per titanium atom, in TiC x X = 0.6–1.0.
The non-stoichiometry of solids is explained by a defect of penetration at the sites of the crystal lattice or, conversely, the appearance of vacancies at the sites. Such substances include oxides, silicides, borides, carbides, phosphides, nitrides and other inorganic substances, as well as high molecular weight organic.
And although evidence of the existence of compounds with variable composition was presented only at the beginning of the 20th century by I. S. Kurnakov, such substances are often called bertollides by the name of the scientist K.L. Bertollet, who suggested that the composition of any substance changes.