Question

Gemstones Rubies are gemstones made up mainly of aluminum oxide. Their red color comes from a small amount of chromium(III) ions replacing some of the aluminum ions. Draw the structure of aluminum oxide, and show the reaction in which an aluminum ion is replaced with a chromium ion. Is this a redox reaction?

Short answer

The structure of aluminum oxide (Al2O3) consists of a lattice arrangement with two aluminum ions (Al^(3+)) and three oxygen ions (O^(2-)). The reaction replacing an aluminum ion (Al^(3+)) with a chromium(III) ion (Cr^(3+)) can be represented as Al2O3 + Cr^(3+) -> AlCrO3. This process does not involve the transfer of electrons between atoms or ions, so it is NOT a redox reaction.

Step-by-step solution

Draw the structure of aluminum oxide (Al2O3)

To draw the structure of aluminum oxide, we first need to identify the ions involved. Aluminum forms Al^(3+) ions and oxygen forms O^(2-) ions. The chemical formula Al2O3 indicates that there are two aluminum ions (Al^(3+)) and three oxygen ions (O^(2-)) in the lattice structure. The aluminum and oxygen ions are arranged in an alternating fashion to form a lattice structure, where each aluminum ion is bonded to six oxygen ions and each oxygen ion is bonded to four aluminum ions.

Show the reaction replacing an aluminum ion with a chromium ion.

The reaction in which an aluminum ion (Al^(3+)) is replaced with a chromium(III) ion (Cr^(3+)) can be represented as follows: In the initial aluminum oxide structure: Al2O3 (lattice structure) The reaction occurs, replacing one aluminum ion with a chromium(III) ion: Al2O3 + Cr^(3+) -> AlCrO3 (Ruby lattice structure) In this process, one of the Al^(3+) ions in the lattice structure is replaced with a Cr^(3+) ion, resulting in the formation of a ruby compound.

Determine if this is a redox reaction.

A redox reaction involves the transfer of electrons between atoms or ions. In this reaction, the aluminum ion (Al^(3+)) is replaced by a chromium(III) ion (Cr^(3+)). Both ions have a +3 charge, and no transfer of electrons occurs during this replacement process. Thus, this is NOT a redox reaction.

Key concepts

Aluminum Oxide

Aluminum oxide, with the chemical formula Al extsubscript{2}O extsubscript{3}, is a compound composed of aluminum and oxygen ions. It is a significant material due to its stability and high melting point, making it useful for a variety of applications, including as a refractory material that can withstand high temperatures. In its crystalline form, aluminum oxide is hard and forms a lattice structure that contributes to its strength.

In Al extsubscript{2}O extsubscript{3}, aluminum ions ( ext{Al}^{3+}) and oxygen ions ( ext{O}^{2-}) interact to maintain charge neutrality. The structure consists of aluminum ions bonding to six oxygen ions, while each oxygen ion links to four aluminum ions, creating a robust ionic lattice. This arrangement contributes to aluminum oxide's resistance to corrosion and wear.

Understanding this structure is crucial as it plays a foundational role in the evolution of various gemstones, including rubies. Alterations in this lattice due to the introduction of different ions can significantly affect its characteristics, as seen when chromium ions are introduced to form rubies.

Chromium Ions

Chromium ions, specifically in their trivalent form ( ext{Cr}^{3+}), play a pivotal role in the color and characteristics of rubies. These ions, when they replace some of the aluminum ions ( ext{Al}^{3+}) in aluminum oxide's lattice, give rubies their striking red color.

The substitution of aluminum ions with chromium ions does not alter the overall charge of the aluminum oxide structure, as both ions possess a +3 charge. Instead, the ext{Cr}^{3+} ions simply take the place of ext{Al}^{3+} ions in the lattice, fitting into the same spatial positions.

• This substitution is essential for transforming Al extsubscript{2}O extsubscript{3}, otherwise colorless, into a vibrant red hue typical of rubies.
• Despite this modification, the process is not considered a redox reaction because there is no change in the charge state of the ions involved.

By influencing how light interacts with the crystal, ext{Cr}^{3+} ions are responsible for both the color and the refractive properties that make rubies so desirable.

Ionic Lattice

An ionic lattice is a repeating three-dimensional array of ions bonded together by ionic bonds. In the context of aluminum oxide, the lattice involves a network of aluminum and oxygen ions.

It is crucial to recognize that the strength and stability of aluminum oxide come from this tightly packed ionic structure. Each ion experiences strong electrostatic forces due to the opposite charges, resulting in a solid that is both durable and heat-resistant.

• The lattice structure ensures the optimal balance of attractions amongst the ions, sustaining the material's toughness.
• Ionic lattices like that in Al extsubscript{2}O extsubscript{3} create substances that are usually hard and, in terms of gemstones, able to maintain a polished finish.

The presence of an ionic lattice in gemstones maintains critical attributes such as durability and resistance to cleavage and fracture.

Gemstone Chemistry

The chemistry of gemstones involves understanding how different elements contribute to their appearance and properties. With rubies, the presence of chromium ions in aluminum oxide defines their essence.

In gemstone chemistry, small tweaks in a crystal's elemental makeup can drastically change its characteristics. Rubies are an excellent example of how minor alterations in composition translate into notable effects. The inclusion of ext{Cr}^{3+} ions in an aluminum oxide base leads to the creation of a ruby gemstone, highlighting the following effects:

• Color: The distinctive red hue of rubies results from chromium ions substituting into the ionic lattice structure of Al extsubscript{2}O extsubscript{3}.
• Light Interaction: Chromium ions change how the crystal absorbs and emits light, contributing to ruby's specific refractive qualities.
• Value: The process of formation and the resulting gemstone quality increase the rarity and hence the market value of rubies.

Understanding gemstone chemistry allows scientists and gemologists to comprehend the processes affecting gemstone visibility and durability and guide the creation of synthetic counterparts or enhancements.