Problem 26 - Chemistry The Central Science In Si Units

Question

Identify the following hydrides as ionic, metallic, or molecular: \((\mathbf{a}) \mathrm{MgH}_{2},(\mathbf{b}) \mathrm{HI},(\mathbf{c}) \mathrm{LaNi}_{4.7} \mathrm{Al}_{0.3} \mathrm{H}_{6}\)

Step-by-Step Solution

Answer

(a) Ionic, (b) Molecular, (c) Metallic.

Step 1: Understanding Ionic Hydrides

Ionic hydrides typically form between hydrogen and alkali or alkaline earth metals. These hydrides are characterized by the transfer of electrons from the metal to hydrogen, forming extit{H}$^-$ ions. Let's apply this logic to the given compounds.

Step 2: Analyze \\(\mathrm{MgH}_2\\)

Magnesium (Mg) is an alkaline earth metal, and it forms ionic bonds with hydrogen. In \(\mathrm{MgH}_2\), magnesium donates electrons to hydrogen, resulting in the formation of extit{H}\(^-\), an ionic hydride.

Step 3: Understanding Molecular Hydrides

Molecular hydrides are composed of nonmetals, usually forming covalent bonds with hydrogen. These are often volatile compounds with discrete molecules, such as \(\mathrm{HI}\).

Step 4: Analyze \\(\mathrm{HI}\\)

In \(\mathrm{HI}\), hydrogen is covalently bonded to iodine, a nonmetal, through a shared pair of electrons. This forms a polar covalent bond, classifying it as a molecular hydride.

Step 5: Understanding Metallic Hydrides

Metallic hydrides form when hydrogen is absorbed into a metal or metal alloy, sharing metallic character without forming true chemical bonds. These often include transition metals and their alloys.

Step 6: Analyze \\(\mathrm{LaNi}_{4.7} \\mathrm{Al}_{0.3} \\mathrm{H}_{6}\\)

In \(\mathrm{LaNi}_{4.7} \mathrm{Al}_{0.3} \mathrm{H}_{6}\), hydrogen is absorbed in a metallic lattice composed of lanthanum, nickel, and aluminum. Consequently, this compound is a metallic hydride.

Key Concepts

Ionic Hydrides

Ionic hydrides form when hydrogen bonds with alkali or alkaline earth metals. This type of hydride is characterized by a significant transfer of electrons from the metal to the hydrogen atom. For example, consider magnesium hydride (\(\text{MgH}_2\)). Here, magnesium, an alkaline earth metal, donates its electrons to hydrogen, resulting in negatively charged hydride ions (\(\text{H}^-\)).

These compounds are typically solid and non-volatile, setting them apart from other hydride types.

• They have high melting points.
• They are good for hydrogen storage due to their ability to release hydrogen upon heating.

Furthermore, ionic hydrides often react with water, releasing hydrogen gas. This makes them useful in various industrial applications, especially where controlled hydrogen release is needed.

Uses of Ionic Hydrides

Industries use ionic hydrides for reducing agents in chemical reactions. They also find applications in producing metals from their ores through processes like the van Arkel–de Boer process.

Molecular Hydrides

Molecular hydrides primarily involve covalent bonds, where hydrogen bonds with nonmetals like carbon, nitrogen, or the halogens to form discrete molecules.

Consider the compound hydrogen iodide (\(\mathrm{HI}\)). Here, hydrogen forms a polar covalent bond with iodine.

These hydrides are generally volatile, possessing low boiling points compared to other hydrides.

• Molecular hydrides are often gases or liquids at room temperature.
• They exhibit distinct molecular properties, such as volatility and lower melting points.

Examples and Applications

Examples include water (\(\text{H}_2\text{O}\)), ammonia (\(\text{NH}_3\)), and methane (\(\text{CH}_4\)). These compounds play essential roles in life, serving various functions in biological systems and industrial applications.

Molecular hydrides are critical in organic chemistry and materials science, often acting as precursors to other compounds.

Metallic Hydrides

Metallic hydrides arise when hydrogen interacts with a metal or metal alloy.
Unlike ionic or covalent bonds, the hydrogen in metallic hydrides integrates into the metal lattice, often without forming traditional chemical bonds. This results in a material that retains the metallic properties of its constituents.For instance, \(\text{LaNi}_{4.7} \text{Al}_{0.3} \text{H}_6\) is a classic example. Here, hydrogen is absorbed into the lattice made up of lanthanum, nickel, and aluminum, forming a metallic hydride without creating separate molecular entities.

These hydrides are distinct due to their ability to store hydrogen densely.

• They can conduct electricity like metals.
• They have potential use in hydrogen-powered vehicles and energy storage systems.

Properties and Uses

Metallic hydrides are essential for their capacity to absorb and release hydrogen efficiently, making them a focus of research for clean energy solutions. They are integral in applications seeking high-density hydrogen storage such as fuel cells and energy transport systems.