Stoichiometry (From The Greek For Element And Measure)

In chemical equations the chemical reactions of the measurable relation of products to reactants is referred to as reaction stoichiometry.

The law of conservation of mass, the law of definite proportions and the law of multiple proportions support stoichiometry in the definite ratios of chemicals combined in reactions.

The quantity of an element on one side of the equation must be the same as the other side. Chemical reactions do not allow for gain or loss of mass, nor the transmutation of one element into another. The quantity of the elements must remain constant throughout the entire reaction. The mass of the products always equals the mass of the reactants.

One of the most universal scientific laws, the Law of Conservation of Mass holds that regardless of the processes acting inside the system, the mass of a closed system will remain constant. The law is commonly stated as Matter cannot be created, nor destroyed, although it may be rearranged. Though the idea had been previously expressed and experimented with, it was attributed to Antoine Lavoisier in 1789.

Stoichiometry is often used in balancing the equations of chemical reactions, in converting from grams to moles (or grams to milliliters), and to find the correct quantity of a reactant to be used in a reaction.

In order to use certain methods, one must begin by determining the molar mass of any reagent and product and balance the reaction (equation). Calculate the number of moles of each known by using the known masses of the compounds in the reaction. Next, determine the limiting reactant (the chemical of which there are fewer moles than the proportion requires – which determines the extent of the reaction until the chemical itself gets used up, causing the reaction to cease).

The amount of an element involved in a reaction (or that reacts) with 1 mole of electrons is the Equivalent Weight. Compounds with higher equivalent weights are less likely to have weighing errors and are therefore preferred. This is most important in analytical chemistry when choosing primary standards.

Stoichiometry also refers to molar proportions of elements in stoichiometric compounds (those whose molar proportions are whole numbers – definite proportions). Water’s (H2O) stoichiometry of hydrogen and oxygen is 2:1.

Even starting with the exact same substances, there is the potential for multiple reactions. The stoichiometry of the reactions may also differ. Which reaction that occurs may be controlled partly by the concentrations relative to the reactants.

The stoichiometric coefficient is the degree that a chemical participates in the reaction. Typically, positive coefficients are assigned to products and negative ones to reactants (or reagents that are consumed in the process).

In accordance with the principles of chemical kinetics and thermodynamic equilibrium, all reactions are reversible (atleast to some extent) and may be viewed from left-to-right or vice versa. Depending on the quantities of elements involved at any point equilibrium determines how the scales are tipped and therefore the sign of the coefficients (and that of the thermodynamic free energy).

Although not yet in wide usage, the extent of reaction provides a clear and explicit representation of compositional change and can be regarded as either a real or hypothetical product, one molecule of which is produced every time the reaction event occurs.

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