Hydrides

Hydrides are those chemical compounds formed by hydrogen with other elements, exhibiting diverse properties and often classified into ionic, covalent, or metallic hydrides.

1.0Introduction 

• Hydrides definition involves compounds formed by the reaction of hydrogen with less electronegative elements. When hydrogen reacts with various elements, excluding noble gases, the resulting product is termed a hydride. The periodic table reveals a hydride gap in VA group elements, where hydride formation is not observed. Hydrides formula can be represented by EH_x or E_mH_n, where E is the symbol of the element. Hydride symbol is H^-.
• The properties of hydrides can vary based on factors such as intermolecular forces, molecular masses, temperature, and more.
• Hydride compounds can be categorized into three main types: saline (ionic), metallic, and covalent hydrides. 
• Another type, dimeric (polymeric) hydrides, can also be identified based on structure. These hydrides may exist in various forms solid, liquid, or gas and exhibit diverse properties. Some hydrides, like those of aluminium, copper, and beryllium, are nonconductors and can be thermally unstable, with some even reacting explosively upon contact with air or moisture.

2.0General Properties of Hydrides

1. Physical State:

Hydrides can exist in various physical states, including gases, liquids, or solids, depending on the nature of the compound.

2. Melting and Boiling Points:

The melting and boiling points of hydrides vary widely. Ionic hydrides tend to have higher melting and boiling points, while covalent hydrides generally have lower ones.

3. Solubility:

Solubility in water varies among different hydrides. Ionic hydrides are often insoluble or sparingly soluble, while covalent hydrides like ammonia (NH₃) are highly soluble.

4. Conductivity:

Metallic hydrides can exhibit high thermal and electrical conductivity due to the presence of metallic elements.

5. Reactivity:

Hydrides can react with acids, bases, and water. Ionic hydrides react violently with water, producing hydrogen gas and hydroxide ions.

6. Stability:

Stability varies based on the type of hydride. Some are stable under normal conditions, while others may decompose or react under specific circumstances.

7. Hydrogen Storage:

Certain metallic hydrides have the ability to absorb and release hydrogen reversibly, making them valuable for hydrogen storage applications.

8. Density:

The density of hydrides can differ significantly based on their composition, ranging from low-density gases to high-density solids.

9. Flammability:

Many hydrides, especially covalent hydrides, can be flammable or support combustion under certain conditions.

3.0Hydrides Classification

Hydrides and their classification helps understand the different bonding and properties seen in hydrides in various chemical situations. In this section we will discuss main three hydrides types which comes under hydrides classification.

Ionic or Saline Hydrides

• Saline, or ionic hydrides are characterized by the presence of hydrogen as a negatively charged ion, H⁻. Typically associated with alkali and alkaline-earth metals, these hydrides form through direct reactions with hydrogen at elevated temperatures. While pure forms are white crystalline solids, impurities often lend a gray color. 
• Formation: These hydrides are typically formed by alkali and alkaline earth metals, as well as some transition metals.
• Nature of Bond: Ionic bonds are formed between metal cations and hydride (H⁻) ions.
• Properties: They are usually hard, high-melting solids with high thermal stability.
• Reaction with Water: Ionic hydrides react vigorously with water to produce hydrogen gas and a hydroxide ion.
• Saline Hydrides examples include sodium hydride (NaH) and calcium hydride (CaH₂). Complex saline hydrides like lithium aluminum hydride (LiAlH₄) and sodium borohydride (NaBH₄) find use as reducing agents in various chemical processes.

Formation of Saline hydride while reacting with H₂ will be -

• If We consider A as any group 1 metal Hydride will be- 

2A (s)   +    H₂(g)    →    2AH (s)

• If We consider A as any group 2 metal Hydride will be-

A (s)    +    H₂(g)     →     AH₂ (s)

Covalent or Molecular Hydrides

• Covalent hydrides definition involves compounds of hydrogen and nonmetals, characterized by electron pairs shared between atoms of comparable electronegativities. These hydrides, often volatile and exhibiting low melting and boiling points, form liquids or gases. Notably, NH₃, H₂O, and HF, representative covalent hydrides, display unique properties influenced by hydrogen bonding in the liquid state.
• In group 13 of the periodic table, boron, aluminum, and gallium form covalent hydrides. Boron forms an extensive series of hydrides, while aluminum and gallium produce elusive neutral hydrogen compounds such as AlH₃ and Ga₂H₆. Ionic hydrogen species like BH⁴⁻ and AlH⁴⁻ are widely used hydride sources.
• Formation: Typically formed by nonmetals, especially from groups 13 to 17 in the periodic table.
• Nature of Bond: Covalent bonds are formed by the sharing of electrons between hydrogen and nonmetal.
• Properties: These hydrides can exist in various states (gases, liquids, or solids) depending on the compound and conditions.
• Reactivity: The reactivity with water varies, and some molecular hydrides can act as reducing agents.
• Molecular Hydrides examples: H₂O (water), CH₄ (methane), NH₃ (ammonia).

Metallic Hydrides

• Metallic hydrides are typically formed by heating hydrogen gas with metals or their alloys. Extensively studied examples include those of the most electropositive transition metals like titanium (Ti), zirconium (Zr), and hafnium (Hf), which form nonstoichiometric hydrides exhibiting reactivity similar to the respective metals. These hydrides, with a predominantly ionic bonding character, are used as reducing agents in various applications. Inner transition metals, such as lanthanoids and actinoids, also form nonstoichiometric hydrides. 
• Formation: Generally formed by transition metals and some main group metals.
• Nature of Bond: Metallic bonding involves a delocalized electron sea surrounding positively charged metal ions and hydroge  n atoms.
• Properties: Metallic hydrides often exhibit unique properties such as high thermal conductivity and electrical conductivity.
• Hydrogen Absorption: Some metallic hydrides, like those of palladium, can absorb and release hydrogen reversibly, making them useful for hydrogen storage.
• Metallic Hydrides examples: PdHx (palladium hydride), TiH₂ (titanium hydride).

4.0Hydride Elimination

Hydride elimination refers to a chemical reaction or process in which a hydride ion (H^-) is removed from a molecule. This type of reaction is commonly observed in organic chemistry, particularly in the context of elimination reactions involving hydrides.

One common example is in the elimination reactions of alcohols, where a hydride ion is eliminated along with another leaving group to form an alkene. The general reaction scheme for a hydride elimination from an alcohol is as follows:

Here, R represents an organic group, and a hydride ion (H⁻) is abstracted by a base, resulting in the elimination of water and the formation of an alkene.

5.0Uses of Hydrides

Hydrides have diverse applications across various fields. Here are some common uses:

Energy Storage:

Certain metal hydrides, such as those involving alkali and alkaline earth metals, are investigated for their potential use in hydrogen storage, contributing to clean energy solutions.

Rocket Propellants:

Liquid hydrogen and liquid hydrazine are examples of hydrides used as rocket propellants due to their high energy content.

Catalysis:

Metal hydrides are employed as catalysts in various chemical reactions, including hydrogenation processes in the production of chemicals and pharmaceuticals.

Semiconductor Industry:

Hydrides play a role in the semiconductor industry, particularly in the production of semiconductors and as dopants in the fabrication of electronic devices.

Hydrogen Fuel Cells:

Certain hydrides are investigated for their potential use in hydrogen fuel cells, which are a clean and efficient energy conversion technology.

Reducing Agents:

Some metal hydrides, such as sodium borohydride, are used as reducing agents in chemical reactions.

Hydrogenation Reactions:

Covalent hydrides, like ammonia (NH₃), are essential in organic chemistry for hydrogenation reactions, such as in the synthesis of fertilizers and pharmaceuticals.

Hydride Batteries:

Research is ongoing on the development of hydride batteries as a potential alternative to traditional batteries for energy storage applications.

Metal Hydride Compressors:

Metal hydride compressors are used for gas compression in various applications, offering a more environmentally friendly and energy-efficient solution.

Hydride Heat Pumps:

Metal hydrides are explored in heat pump applications for heating and cooling purposes, providing an alternative to conventional refrigeration systems.