Problem 39 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following statements is not correct? (a) Heating of zinc oxide leads to the formation of a yellow coloured non- stoichiometric compound (b) The creation of cationic holes in ionic crystals imparts electrical conductivity to such crystals (c) The compound ferrous oxide can be prepared in its stoichiometric composition (d) Silver bromide shows both types of Schottky and Frenkel defects

Step-by-Step Solution

Answer

Statement (c) is not correct; pure stoichiometric FeO is rare.

Step 1: Understanding Statement (a)

Zinc oxide (ZnO) is known to change color from white to yellow upon heating. This occurs because oxygen vacancies lead to non-stoichiometric ZnO, providing a yellow color. Hence, statement (a) is correct as it describes a well-known behavior of ZnO.

Step 2: Analyzing Statement (b)

Cationic holes refer to the absence of cations, which can indeed enhance electrical conductivity in ionic crystals by allowing positive charge carriers to move more freely. Therefore, statement (b) is correct because the creation of cationic holes contributes to electrical conductivity.

Step 3: Evaluating Statement (c)

Ferrous oxide (FeO) is known to have non-stoichiometric composition due to the presence of Fe²⁺ and Fe³⁺ ions, leading to variations in composition like Fe₀.₉₅O or Fe₀.₉O. Pure stoichiometric FeO with exactly equal amounts of iron and oxygen is extremely rare. Thus, statement (c) is not correct.

Step 4: Assessing Statement (d)

Silver bromide (AgBr) is known to exhibit both Schottky and Frenkel defects. Schottky defects involve vacancy of atoms from the lattice, while Frenkel defects involve displacement of atoms within the lattice. Both types of defects can be seen in AgBr, making statement (d) correct.

Key Concepts

Zinc oxide properties

Zinc oxide (ZnO) is a key ingredient in many products due to its unique properties. When heated, ZnO transitions from white to yellow. This interesting behavior occurs because of the formation of oxygen vacancies, which results in a non-stoichiometric compound. Non-stoichiometric compounds have a variable elemental composition due to these vacancies. This color change is an indicator of defects in the crystal structure, specifically the presence of excess zinc atoms or oxygen deficiencies.
In its unheated form, zinc oxide is widely used in products like sunscreens, rubber manufacturing, and as a pigment. Its effective UV-blocking properties make it ideal for skin protection. Its antibacterial properties also make it useful in medical and skincare applications. In electronics, heated ZnO is utilized for its conductive properties, due to the movable charge carriers created by the altered stoichiometry.

Defects in ionic crystals

Ionic crystals can possess various types of defects that impact their physical properties. These defects are essential in determining the conductivity, strength, and durability of the crystal. One notable defect is the creation of cationic holes, which occur when positively charged ions are absent in the crystal lattice. This absence enables the movement of positive charge carriers, thus enhancing electrical conductivity.
Ionic defects include Schottky and Frenkel types. Schottky defects involve missing ions, while Frenkel defects involve ions displaced from their usual position. These variations disrupt the stoichiometry but can improve specific crystal properties. Understanding defects is crucial for materials scientists who aim to manipulate these properties for industrial applications like semiconductors or superconductors.

Stoichiometry of ferrous oxide

Ferrous oxide (FeO) exhibits non-stoichiometric behavior, meaning its composition does not adhere strictly to a simple whole-number ratio of iron to oxygen. FeO commonly exists in forms like Fe₀.₉₅O or Fe₀.₉O due to the presence of both Fe²⁺ and Fe³⁺ ions. This variation leads to a deficiency of iron atoms relative to oxygen, which makes exact stoichiometry rare.
Despite its non-stoichiometric nature, FeO is still utilized in various applications. It is a key component in the production of steel and other alloys. Its ability to exist in varying compositions allows it to fulfill different roles depending on the required material properties, like hardness or conductivity. As such, understanding and manipulating its stoichiometry is valuable for metallurgy and materials engineering.

Silver bromide defects

Silver bromide (AgBr), a silver salt widely used in photographic materials, exhibits two types of ionic defects: Schottky and Frenkel defects. Schottky defects occur when ions are completely missing from their lattice positions, creating vacancies. This can disrupt the lattice structure but provides pathways for charge movement, which affects conductivity.
Frenkel defects, on the other hand, happen when an ion relocates from its regular position to an interstitial site within the crystal. This does not result in a vacancy but creates a shift within the lattice that can affect the material's electrical properties. The presence of both defects in AgBr significantly influences its photographic properties, as these defects play a vital role in the sensitivity and grain quality of photographic film.