Meaning of Stoichiometry

What is Stoichiometry:

Stoichiometry is the calculation for a balanced chemical equation that will determine the ratios 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 states 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 are 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 trial and error method to calculate the stoichiometry of an equation should be followed by the following steps:

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

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

CH4 + 2O2 → CO + 2H2O

Carbon is balanced because there is 1 molecule on each side of the equation. Hydrogen also has the same amounts on each side. The oxygen, on the other hand, adds up to 4 on the left side (reactants or reactants) and only 2, therefore by trial and error a subscript 2 is added to transform CO into CO2.

In this way, the balanced chemical equation in this exercise results: CH4 + 2O2 → CO2 + 2H2O

The numbers preceding the compound, in this case 2 for O2 and 2 for H2O are 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 solve the rest of the unknowns
  4. Simplify

See also Catalyst.

Stoichiometric ratios

Stoichiometric relationships 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 the quantities obeys the principles of conservation and the atomic models that affect the chemical processes.

Conservation principles

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

Law of Conservation of Mass: There are no detectable changes in total mass during a chemical reaction. (1783, Lavoisier)

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

Dalton atomic model

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

  1. Chemical elements are made up of identical atoms for one element and it is different for any other element.
  2. Chemical compounds are formed by combining a defined amount of each type of atom to form a molecule of the compound.

Furthermore, Dalton's law of multiple proportions defines that when 2 chemical elements combine to form 1 compound, there is an integer relationship between the various masses of one element that combine with a constant mass of another element in the compound.

Therefore, in stoichiometry cross relationships between reactants and products is possible. What is not possible is the mixture 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, N2 that indicates 2 molecules of N2 and 2 atoms of Nitrogen towards the macroscopic world by the molar relationship between the amounts of reactants and products expressed in moles.

In this sense, the N2 molecule at the microscopic level has a molar ratio that is expressed as 6.022 * 1023 (one mole) of N2 molecules.

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