Stoichiometry

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In chemistry, stoichiometry (sometimes called reaction stoichiometry distinguishing itself from composition stoichiometry) is the study and calculation of quantitative (measurable) relationships of the reactants and products in chemical reactions (chemical equations). The word is derived from the Greek word stoikheion ("element") and metriā ("measure," from metron). The Stoichiometria of Nicephorus gave line counts of the canonical books of the New Testament and some of the Apocrypha. The related term stoichiometric is often used in thermodynamics to refer to the "perfect mixture" of fuel and air.

Stoichiometry rests upon the conservation of mass and the law of definite proportions (i.e., the law of constant composition) and the law of multiple proportions. In general, chemical reactions will combine definite ratios of chemicals. Since matter cannot be created or destroyed, the amount of each element must be the same throughout the overall reaction. For example, the amount of element A on the reactant side must equal the amount of element A on the product side.

Stoichiometry is often used to balance chemical equations. For example, the two diatomic gases hydrogen and oxygen can combine to form a liquid, water, in an exothermic reaction, as described by Equation 1.

(1) H₂ + O₂ → H₂O

Equation 1 does not depict the proper stoichiometry of the reaction—that is, it does not reflect the relative proportions of the reactants and products.

(2) 2H₂ + O₂ → 2H₂O

Equation 2 does have proper stoichiometry and is therefore said to be a "balanced" equation, depicting the same number of atoms of each type on each side of the equation. There are four H on the reactant side and 4 H on the product side and there are 2 O on both sides as well; mass has been conserved.

The term stoichiometry is also often used for the molar proportions of elements in stoichiometric compounds. For example, the stoichiometry of hydrogen and oxygen in H₂O is 2:1. In stoichiometric compounds, the molar proportions are whole numbers (that is what the law of multiple proportions is about).

Compounds for which the molar proportions are not whole numbers are called non-stoichiometric compounds.

Stoichiometry is used not only to balance chemical equations but also is used in conversions—i.e. converting from grams to moles, or from grams to milliliters. For example, if there were 2.00 g of NaCl, to find the number of moles, one would do the following:

<math>\rm{} \frac{2.00 \ g \ NaCl}{58.44 \ g \ NaCl \ mol^{-1}} = 0.034 \ mol</math>

In the above, example, when written out in fraction form, the units of grams cancels out, leaving one with the amount of moles (the unit that was needed), as shown in the following equation:

<math>\rm{}\left(\frac{2.00 \ g \ NaCl}{1}\right)\left(\frac{1 \ mol \ NaCl}{58.44 \ g \ NaCl}\right) = 0.034 \ mol</math>

Another use of stoichiometry is also used to find the right amount of reactants to use in a chemical reaction. An example is shown below using the Thermite reaction: Fe₂O₃ + 2Al → Al₂O₃ + 2Fe

How many grams of Aluminum are needed to completely react with 85 grams of Iron (III) Oxide?

<math>\rm{} \left(\frac{85 \ g \ Fe_2 O_3}{1}\right)\left(\frac{1 \ mol \ Fe_2 O_3}{160 \ g \ Fe_2 O_3}\right)\left(\frac{2 \ mol \ Al}{1 \ mol \ Fe_2 O_3}\right)\left(\frac{27 \ g \ Al}{1 \ mol \ Al}\right) = 28.6875 \ g \ Al</math>

So, to completely react with 85 grams of Iron (III) oxide, 28.6875 grams of Aluminum are needed.