Hydrogen Compounds: Types and Properties

Hydrogen Compounds

Overview of Hydrides

Hydrogen forms compounds with many elements, known as hydrides. Hydrides are binary compounds containing hydrogen and another element. They are classified into three main types based on their bonding and properties: ionic hydrides, covalent (molecular) hydrides, and metallic hydrides.

• Ionic Hydrides: Formed with alkali and alkaline earth metals (except Be).

• Covalent Hydrides: Formed with nonmetals and metalloids.

• Metallic Hydrides: Formed with transition metals.

Ionic Hydrides

Formation and Properties

Ionic hydrides are typically white, crystalline solids formed when hydrogen reacts with Group 1A or 2A metals (except beryllium). In these compounds, hydrogen acts as a hydride ion (H-), possessing an oxidation state of -1.

• Group 1A metals: React with hydrogen to form hydrides of the type MH (e.g., NaH, KH).

• Group 2A metals: React with hydrogen to form hydrides of the type MH2 (e.g., CaH2, SrH2).

Ionic Hydrides	Reaction	Product
1A metals	M (s) + ½ H2 (g)	MH (s)
2A metals	M (s) + H2 (g)	MH2 (s)

Example: Complete and balance the following reaction:

Sr (s) + H2 (g) → SrH2 (s)

Practice: Write and balance a reaction for formation of an ionic hydride with sodium.

Na (s) + ½ H2 (g) → NaH (s)

Covalent (Molecular) Hydrides

Formation and Properties

Covalent hydrides are formed when hydrogen reacts with nonmetals and metalloids. In these compounds, hydrogen typically has an oxidation state of +1.

Common Covalent Hydrides	Formula
Methane	CH4
Ammonia	NH3
Water	H2O
Hydrogen fluoride	HF

An important example is the formation of ammonia:

$N_2 (g) + 3 H_2 (g) \rightarrow 2 NH_3 (g)$

Example: Complete and balance the following reaction:

Se (s) + H2 (g) → H2Se (g)

Practice: Provide a balanced equation for the reaction of hydrogen gas with bromine gas.

$H_2 (g) + Br_2 (g) \rightarrow 2 HBr (g)$

Metallic Hydrides

Formation and Properties

Metallic hydrides are formed when hydrogen reacts with transition metals. These hydrides are often non-stoichiometric, meaning the ratio of metal to hydrogen can vary. Hydrogen atoms occupy interstitial sites within the metal lattice structure.

• Many metallic hydrides follow a stoichiometric ratio of metal to hydrogen atoms.

• Hydrogen is located in the spaces (interstices) of the metal lattice.

Example: Identify a metallic hydride out of the following:

• HTa

• TaHx (Metallic hydride)

• RbH

• AlH3

Practice: Classify each hydride according to its type:

• SiH4: Covalent

• RbH: Ionic

• FeHx: Metallic

• CaH2: Ionic

Additional info: Hydrides are important in various chemical processes, including synthesis of ammonia (Haber process), hydrogen storage, and as reducing agents in organic and inorganic chemistry.