What is stoichiometry
Stoichiometry is the information on the quantities of reactants and products in a chemical reaction. This is based on the fact that the amount of reactants is equal to the amount of products and that the compounds have a fixed composition.
The word “stoichiometry” comes from the Greek stoicheon (element) and metron (extent). It was applied by Jeremias Benjamin Richter in 1792.
A stoichiometric coefficient is the number that appears in front of the chemical formula in an equation. For example, in the decomposition reaction of water H2O, hydrogen H2 and oxygen O2 are produced in the form of a gas:
In this case, H2O and H2 have a stoichiometric coefficient of 2, while O2 has no coefficient. This means that 2 molecules of H2O are transformed into 2 molecules of H2 and 1 molecule of O2.
Stoichiometry is present in everyday life. When we make a cake, we have to use fixed amounts of ingredients (reactants) to obtain a cake (product). If we use more baking powder than flour, the result will probably not be as desired.
To prepare a chocolate cake, we must use fixed amounts of flour, milk, chocolate, eggs and other ingredients. This is applying stoichiometry in our daily lives.
Stoichiometric calculations
Stoichiometric calculations are the way stoichiometric coefficients are adjusted so that a chemical equation is balanced. There are several methods, the trial method and the algebraic method being the most used.
Stoichiometric calculation to balance an equation by trial and error
This method is based on testing step by step which are the correct stoichiometric coefficients to balance a chemical equation.
For example, in the combustion of methane CH4 in the presence of oxygen O2 (reactants) carbon dioxide CO2 and water H2O are produced, as shown in the following equation:
First, we check how many atoms of each element are on each side of the equation:
We can notice that carbon is in the same amount on each side of the equation, but oxygen and hydrogen are not. We tried placing a 2 as a coefficient in front of water in the products and a 2 in front of O2 in the reactants:
Now we do have the same number of elements on the left side (reactants) and on the right side (products) of the chemical equation. That is, the equation is balanced.
Stoichiometric calculation by algebraic method
This method uses mathematics to determine stoichiometric coefficients. For this, letters are assigned to each coefficient and a system of equations is made.
Stoichiometric ratios
Stoichiometric relationships indicate how much of each reactant is needed to obtain a given amount of product.
For example, in the reaction of bismuth nitrate Bi(NO3)3 with sodium sulfide Na2S, it produces bismuth sulfide and sodium nitrate, as shown in the equation:
In this equation, the stoichiometric relationship states that 2 Bi(NO3)3 molecules react with 3 Na2S molecules and is produced 1 Bi2S3 molecule and 6 NaNO3 molecules.
See also:
References
Atkins, P., Jones, L., Laverman, L. (2016) Chemical Principles: the quest for insight. 7ed. WH Freeman. New York.
Szabadvary, F. (1962) The birth of stoichiometry. J.Chem.educ. 39:267-270