Stoichiometry - SpeedyLook encyclopedia

Principle

At the time of a Chemical reaction one observes a modification of the substances present: certain substances are consumed, they are the Réactifs, other substances are formed, they are the produced .

On a microscopic scale, the chemical reaction is a modification of the bonds between atoms, by displacement of the electrons: some connections are broken, others are formed, but the atoms themselves are preserved. It is what is called the Conservation of the matter which results in two laws:

conservation of the number of atom of each chemical element;
conservation of the total load.

The stoechiometric relations between the quantities of consumed reagents and formed products rise directly from the laws of conservation. They are given starting from the equation-assessment of the reaction.

To write an equation of balanced reaction

When one writes the equation-assessment of a chemical reaction, it must comply with the rules of conservation of the matter.

To comply with these rules one is brought to place in front of the chemical formula of each chemical species a number, called coefficient (or number) stoechiometric , which indicates the proportions between the committed species and the formed species . In fact thus numbers without dimension should not be confused with a quantity of matter, N. The equation-assessment is indeed independent of the quantity of matter but it makes it possible to calculate the quantities of matter after reaction if one knows the real quantities concerned at the beginning.

Example

During the combustion of the Methane (CH₄), this one reacts with the Dioxygène (O₂) of the air; during this reaction it is formed Carbon dioxide (CO₂) and Eau (H₂O).

the qualitative starting point of the equation of reaction will be thus of the form:

CH₄ + O₂ → CO₂ + H₂O

but in the state this equation is not correct since it does not comply with the rules of conservation; for the hydrogen element (H) for example, one finds 4 hydrogen atoms in the reagents and 2 only in the products. One thus balances this chemical reaction by introducing in front of the chemical formulas of each species a stoechiometric coefficient .

thus, if one writes:

CH₄ + O₂ → CO₂ + 2:00 ₂O

what complies with the rule of conservation for the elements carbon (C) and hydrogen (H) but not for oxygen (O); one thus corrects:

CH₄ + 2 O₂ → CO₂ + 2:00 ₂O

which is the equation-assessment correct of the reaction of combustion of methane.

It translates that the fact that the assessment of the chemical reaction is the following: 1 mole of methane reacts with 2 moles of dioxygene to form 1 mole of carbon dioxide and 2 moles of water.

From a molecular point of view, the assessment is obviously the same one: 1 molecule of methane and 2 molecules of dioxygene disappear to form 1 molecule of carbon dioxide and 2 molecules of water, but that does not want to say that the reaction is done by direct reaction of a methane molecule with two molecules of dioxygene. Reality at the molecular level is more complex. and utilizes several elementary reactions whose assessment is well that indicated in the equation.

Stoechiometric coefficient

The stoechiometric coefficient of a chemical Espèce is the coefficient which is affected for him in the chemical equation considered. In the preceding example:

CH₄ + 2 O₂ → CO₂ + 2:00 ₂O

the stoechiometric coefficient of methane is 1, that of dioxygene is 2, that of carbon dioxide is 1 and that of water is 2.

The stoechiometric coefficient is in theory a Integer, although to reduce certain equations one uses sometimes fractions, even of the decimal numbers.

When the stoechiometric coefficient is equal to 1, he is not written, this is why in the example CH₄ and CO₂ are preceded by no coefficient.

Note:

the stoechiometric coefficients are numbers without dimension which allow the calculation of the quantities of consumed reagents or products formed during a complete reaction.
if the reaction is not complete one defines the advance of the reaction ξ which is a concept impossible to circumvent in thermodynamics and chemical kinetics. In the definition of ξ the coefficient stoechiometric of the reagents are affected sign - and those of the formed products of the sign +.

Mix/stoechiometric proportions/conditions

When the quantities of matter of all the reagents are proportional to their stoechiometric coefficients at the beginning of the reaction, it is said that

the mixture is stoechiometric;
the reagents are in the stoechiometric proportions (or " was introduced into the stoechiometric proportions ") ;
the reaction takes place under the stoechiometric conditions;

these three expressions having strictly the same significance…

Under these conditions, if the reaction is total, all the reagents will be entirely consumed.

If the reagents are not introduced initially into the stoechiometric proportions, and if the reaction is total:

one two will disappear completely at the end of the reaction; it is called reactive limiting or at fault .
it (or them) others (S) reactive (S) will not be completely consumed at the end of the reaction, and it will thus remain about it in the medium; one it (them) calls reagent (S) in excess .

Note: this is valid for a total reaction , knowing that certain reactions are limited or can be reversed. At the end of the reaction, the reagents are not entirely consumed, even if they had been introduced into the stoechiometric proportions! This is with the fact that the products of a limited reaction can themselves react together to give again the starting reagents, which is not possible in a total reaction. This inversability leads to a state of chemical balance in which the reagents and the products in a proportion fixed by a constant coexist known as " constant of équilibre" (see chemical balance).

To determine the quantities of consumed reagents/formed products

Whatever the initial conditions, the quantities of matter of consumed reagents and products formed are proportional to the stoechiometric coefficients of the equation-assessment

Exemple

See too

chemical Chemical reaction
Equation

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