Stoichiometric Defects

Stoichiometry is based on the law of conservation of mass, which states that the total mass of the reactants equals the total mass of the products, implying that the relationships between reactant and product quantities are often positive integer ratios. This means that if the quantities of the individual reactants are known, the quantity of the product can be determined. In contrast, if the quantity of one reactant is known and the quantity of the products can be estimated empirically, the quantity of the remaining reactants may also be predicted.

A stoichiometric amount or stoichiometric ratio of a reagent is the amount or ratio that is optimal for the reaction to complete:

• The entire amount of reagent is consumed.
• There is no reagent deficiency.
• There is no reagent excess.

Stoichiometry is founded on a number of fundamental rules that aid in its comprehension, including the law of conservation of mass, the law of definite proportions (also known as the law of constant composition), the law of multiple proportions, and the law of reciprocal proportions. Chemical reactions, in general, mix chemicals in predetermined ratios. Due to the fact that chemical reactions cannot produce or destroy matter, nor can they transform one element into another, the concentration of each element must remain constant during the reaction. For instance, the number of atoms of a certain element X on the reactant side must equal the number of atoms of that element on the product side, regardless of whether all of those atoms are engaged in the reaction.

Chemical reactions as macroscopic unit operations are composed of an infinite number of elementary reactions in which a single molecule combines with another. Because the reacting molecules (or moieties) are composed of an integer number of atoms, the ratio of reactants in a complete reaction is also an integer number. A reaction may consume several molecules, and the stoichiometric number indicates this quantity, which is defined as positive for products (added) and negative for reactants (removed). In general, the unsigned coefficients are referred to as stoichiometric coefficients.

Stoichiometry defect :-

If the crystal is defective in such a way that the ratio of cation to onions remains constant. When the stoichiometry of a substance is not disrupted, this is referred to as a stoichiometric defect. There are several forms of stoichiometric defects.

1. Vacancy defect:- When some of the lattice sites in a crystalline substance are vacant, the crystal is said to have a vacancy defect. This results in a drop in the density of the substance.
2. Interstitial defect:- During the creation of the lattice structure, certain atoms occupy interstitial sites; the defect that results is referred to as Interstitial Defects. In an interstitial defect, certain atoms occupy locations in the crystal structure that are normally devoid of atoms. Due to the presence of interstitial defects, the number of atoms exceeds the number of lattice sites. The density of a substance increases as the number of atoms increases, i.e. interstitial flaws increase the density of the substance.
3. Schottky defect- Schottky Defect is a form of simple vacancy defect that is manifested in ionic solids with cations and anions that are nearly identical in size, such as NaCl, KCl, and CsCl. AgBr exhibits both Schottky and Frenkel Defects.

When both cations and anions are absent from typical locations, this is referred to as the Schottky Defect. To maintain the ionic compound’s electrical neutrality, the number of missing cations must equal the number of missing anions.

Due to the fact that Schottky Defects occur as a result of constituent particle mission, they reduce the density of an ionic compound.

4. Frenkel defect:- It is a particular kind of vacancy fault. In ionic compounds, some of the ions (which are typically smaller in size) get displaced from their original locations, resulting in a defect. This type of flaw is referred to as Frenkel Defects. Due to the fact that this defect occurs as a result of ion dislocation, it is also referred to as Dislocation Defects. Due to the presence of a large number of cations and anions (which stay equal despite the defect), the density of the substance does not change. Ionic compounds, such as ZnS, AgCl, AgBr, and AgI, exhibit Frenkel Defects because to the huge size difference between their cations and anions. These compounds have cations that are smaller in size than anions.

Frenkel Defect Characteristics

• This is a fault that happens only when the cations are larger than the anions. Chemical properties remain the same as well.
• Additionally, because the Frenkel defect has no influence on the density of the solid, both its volume and mass are conserved.
• In such cases, substances keep their electrical neutrality.
• The dielectric constant increases as opposite charge ions become closer together.
• Materials having Frenkel defects exhibit conductivity and diffusion in the solid state due to the existence of vacant lattice sites.
• The Frenkel defect affects crystalline solids’ lattice energy and stability. This shortcoming has an effect on the chemical properties of ionic molecules.
• The entropy of the solid increases.

Conclusion-

Stoichiometric defects are so named because they do not affect the crystal’s stoichiometry ( Schottky defect and Frenkel defect). They are called intrinsic defects because they occur as a result of a variation from the regular arrangement of atoms or ions within the crystal without the addition of any external component.