In the Large Explanatory Dictionary, the word equivalent (in Latin sounds like aequivalens) is explained as something equivalent, equivalent or equivalent to another, which can completely replace it. In chemistry, the law of equivalents (used since the end of the 18th century, it is studied at school, chemists and biologists from different countries apply in theory and practice) establishes that all chemicals enter into reactions in quantities proportional to their equivalents. The law was discovered by the German chemist I.V. Richter, whose works were unknown for a long time. In his three-volume work, published in the period from 1792 to 1794 under the title "The Beginnings of Stoichiometry, or the Method of Measuring Chemical Elements", the scientist showed that chemicals react in a strict ratio. He also introduced such a term as “stoichiometry”. Now this is a whole branch of chemistry, which describes the ratio of reagents that enter into a chemical interaction.
Richter was the first in his work to give quantitative equations of reactions. They are a conditional record containing qualitative and quantitative information about the processes that occur during the interaction of various chemicals called reagents. Back in the days of alchemical science, scientists used various symbols to designate simple elements; later, formulas for complex (consisting of several elements) chemicals were discovered. But only I.V. Richter (under the influence of his teacher and philosopher Immanuel Kant, who claimed that certain areas of the natural sciences contain as much true science as there are mathematics in it) used chemical formulas and the concept of “stoichiometry” in his dissertation, described the quantitative equations of the reaction and discovered the law of equivalents. The formula expressing it can be written: E2 • m1 = E1 • m2. Where m1 and m2 are the masses of substances "1" and "2" that have reacted, and E1 and E2 are their chemical equivalents.
To understand the Law of equivalents, it is necessary to clarify that the equivalent is a conditional or real amount of a substance that can attach a hydrogen cation in acid-base reactions or an electron as a result of redox reactions. Equivalent mass is the mass of one equivalent. It is believed that 1 equivalent of a substance reacts (or displaces) with 1 gram of hydrogen or with 8 grams of oxygen, or 35.5 grams of chlorine. In practice, the amount of the substance in the equivalent is often very small, so it is customary to express it in moles. 1 mole contains a constant number of particles (atoms, ions or molecules), it is equal to the Avagadro number: NA = 6.02214179 (30) · 1023. The mass of one mole of a substance, expressed in grams, is numerically equal to its mass in atomic units of mass.
Based on the Law of Equivalents, it can be argued that when acid-base titration proceeds according to the reaction equation: KOH + HCl → KCl + H2O, as a result of the interaction of 1 mole of potassium hydroxide with 1 mole of hydrochloric acid, 1 mole of a salt called potassium chloride is obtained , and 1 mole of water. That is, the equivalent mass of potassium hydroxide is equal to E KOH = 39 + 16 + 1 = 56 g, hydrochloric acid - E HCl = 1 + 35 = 36 g, potassium chloride - E KCl = 39 + 35 = 74 g, water - E H2O = 1 • 2 + 16 = 18 g. In order to completely neutralize 56 g of potassium hydroxide, 36 g of hydrochloric acid are needed. The result is 74 g of potassium chloride and 18 g of water. But since it is established by law that the masses of substances that reacted are proportional to their equivalents, then knowing the amount of one reagent, you can calculate how much the second reagent will enter into the reaction or calculate the yield of the product.
For example, how much potassium chloride will be obtained if it is known that 100 g of potassium hydroxide was completely neutralized with hydrochloric acid? Using the law of equivalents, we can write: 56 • mKCl = 74 • 100. Then mKCl = (74 • 100) / 56 = 132 g. And hydrochloric acid will need 64 g to neutralize 100 potassium hydroxide. If 100 g of potassium hydroxide is neutralized with sulfuric acid: 2KOH + H2SO4 → K2SO4 + 2H2O, then this will require a completely different amount of acid. As the stoichiometric coefficients of this reaction indicate, with 2 moles of potassium hydroxide, 1 mole of sulfuric acid will react, and as a result, 1 mole of potassium sulfate and 2 moles of water will be obtained. Knowing that the masses of the reacted substances are proportional to the equivalent masses, we can write: 2 • 56 • mH2SO4 = 98 • 100, then to neutralize 100 potassium hydroxide, mH2SO4 = 88 g of sulfuric acid is required. This forms 155 g of potassium sulfate. The amount of water released as a result of neutralization of 100 g of potassium hydroxide with hydrochloric or sulfuric acids will be the same and equal to 32 g.
Applies the Law of equivalents of chemistry (analytical, inorganic, organic, etc.) for the study of substances and other experiments based on the calculation of the balance of chemical reactions. In addition, it is used (for the preparation of material balances) in the design and operation of laboratory, pilot or industrial plants intended for the synthesis of chemicals. Experts from chemical, medical, biological, and sanitary-epidemiological laboratories constantly use it, since it underlies the formulas by which many of the analysis results are calculated.