Basic chemistry

Stiochiometry worksheet?

Stiochiometry worksheet?

Definition of stiochiometry:

The stoichiometry of a chemical transformation refers to the proportions with which the different chemical species (reagents and products) intervene.
These proportions are indicated by the stoichiometric coefficients present in the reaction equation.

Stoichiometric coefficients

These are the numbers noted before the chemical formulas in the reaction equations in order to balance them.
They make it possible to translate the conservation of the elements during a chemical transformation: the stoichiometric coefficients of a reaction equation are thus determined by balancing the latter.

Example of the stoichiometric coefficients of the oxidation reaction of aluminum

It is a reaction between aluminum metal Al and oxygen O 2 which produces Al 2 O 3 alumina

The unbalanced equation is written:

Al + O 2 -> Al 2 O 3

To balance the oxygen element it is necessary to find a multiple common to both oxygen oxygen and alumina three. The smallest common multiple is 6, it is obtained with a coefficient of 3 for oxygen and 2 for aluminum, which gives the following provisional equation:

Al + 3 O 2 -> 2 Al 2 O 3

It remains to balance the aluminum element present at the number of 4 in the products (2 aluminum in 2 alumina) and one in the reagents: it is enough to multiply the latter by a coefficient 4. We thus arrive at the balanced equation :

4 Al + 3 O 2 -> 2 Al 2 O 3

This equation expresses:

• that it is consumed 4 aluminum for 3 oxygen
• that 2 alumina is formed for 3 consumed dioxygen
• that half the amount of alumina is formed than aluminum consumed

Stoichiometric conditions of a chemical transformation

The proportions of the reagents of a given chemical transformation are said to be stoichiometric if they coincide with the proportions corresponding to the stoichiometric coefficients.

Example
For the oxidation reaction between aluminum and dioxygen whose equation is:

4 Al + 3 O 2 -> 2 Al 2 O 3

The reagents are in stoichiometric proportion if, for example:

• there are 4 moles of aluminum and 3 moles of oxygen
• there are 8 moles of aluminum and 6 moles of oxygen
• there are 12 moles of aluminum and 9 moles of oxygen
• there are 2 moles of aluminum and 1.5 moles of oxygen

Etc ..

If the reactants of a chemical transformation are introduced into the reaction medium in stoichiometric proportions then they are consumed entirely (provided that the reaction is complete) and they are absent from the final state of the chemical system (their amount of matter is null)

Non stoichiometric conditions

If the proportions of the reactants, in the initial state of the chemical system, do not coincide with the stoichiometric proportions then:

• one of the reagents is completely consumed, it is absent from the final state (its quantity of material and its advancement are zero). This reagent is then qualified as a faulty reagent and also corresponds to the limiting reagent
• the other reagent is not entirely consumed (its quantity of material is not zero in the final state). This reagent is said in excess

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