Meaning of Stoichiometry

What is Stoichiometry:

Stoichiometry is the calculation for a balanced chemical equation that will determine the proportions between reactants and products in a chemical reaction.

The balance in the chemical equation obeys the conservation principles and Dalton’s atomic models, such as the Law of Conservation of Mass, which stipulates that:

the mass of the reactants = the mass of the products

In this sense, the equation must have equal weight on both sides of the equation.

Stoichiometric calculations

Stoichiometric calculations is the way a chemical equation is balanced. There are 2 ways: the trial and error method and the algebraic method.

Stoichiometric calculation by trial and error

The method by trial to calculate the stoichiometry of an equation the following steps must be followed:

  1. Count the number of atoms of each chemical element in the reactant position (left of the equation) and compare those numbers in the elements positioned as products (right of the equation).
  2. Balance metallic elements.
  3. Balance non-metallic elements.

For example, the stoichiometric calculation with the trial and error method in the following chemical equation:

CH4 + 2Otwo → CO + 2HtwoEITHER

Carbon is balanced because there is 1 molecule on each side of the equation. Hydrogen also has the same amounts on each side. Oxygen, on the other hand, add 4 to the left side (reactants or reagents) and only 2, therefore, by trial and error, a subscript 2 is added to transform CO into COtwo.

In this way, the balanced chemical equation in this exercise is: CH4 + 2Otwo → COtwo + 2HtwoEITHER

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The numbers that precede the compound, in this case the 2 of Otwo and 2 for HtwoOr are they called stoichiometric coefficients.

Stoichiometric calculation by algebraic method

For the stoichiometric calculation by algebraic method, the stoichiometric coefficients must be found. To do this, follow the steps:

  1. Assign unknown
  2. Multiply the unknown by the number of atoms of each element
  3. Assign a value (1 or 2 is recommended) to clear the rest of the unknowns
  4. Simplify

See also Catalyst.

Stoichiometric ratios

Stoichiometric ratios indicate the relative proportions of chemicals that are used to calculate a balanced chemical equation between the reactants and their products in a chemical solution.

Chemical solutions have different concentrations between solute and solvent. The calculation of quantities obeys conservation principles and atomic models that affect chemical processes.

Conservation principles

The postulates of the conservation principles will later help to define John Dalton’s atomic models of the nature of atoms. Models constitute the first scientifically based theory, marking the beginning of modern chemistry.

Law of conservation of mass: There is no detectable change in total mass during a chemical reaction. (1783, Lavoisier)

Law of definite proportions: pure compounds always have the same elements in the same mass ratio. (1799, J. L. Proust)

Dalton’s Atomic Model

Dalton’s atomic models form the basis of modern chemistry. In 1803, John Dalton’s (1766-1844) Basic Atomic Theory postulates the following:

  1. Chemical elements are made up of identical atoms for one element and it is different in any other element.
  2. Chemical compounds are formed by the combination of a defined amount of each type of atom that forms a molecule of the compound.
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Furthermore, Dalton’s Law of Multiple Proportions states that when 2 chemical elements combine to form 1 compound, there is a whole number ratio between the various masses of one element combining with a constant mass of another element in the compound.

Therefore, in stoichiometry the cross-reactions between reactants and products is possible. What is not possible is the mixing of macroscopic units (moles) with microscopic units (atoms, molecules).

Stoichiometry and Unit Conversion

Stoichiometry uses as a conversion factor from the microscopic world by units of molecules and atoms, for example, Ntwo which indicates 2 molecules of Ntwo and 2 Nitrogen atoms towards the macroscopic world by the molar relationship between the amounts of reactants and products expressed in moles.

In this sense, the molecule of Ntwo at the microscopic level it has a molar ratio that is expressed as 6.022 * 1023 (one mole) of N moleculestwo.

See also molar mass.