Problem 9 - Chemistry

Question

Give the name of (a) an ionic hydride and (b) a covalent hydride. In each case describe the preparation and give the structure of the compound.

Step-by-Step Solution

Answer

The ionic hydride is Sodium Hydride (NaH), prepared by combining sodium and hydrogen, and has an ionic structure with Na+ cations and H- anions. The covalent hydride is Hydrogen Chloride (HCl), prepared by combining hydrogen and chlorine gases, and has a covalent structure where the H and Cl atoms share a pair of electrons.

Step 1: Name of Ionic Hydride and its Preparation

A common example of an ionic hydride is Sodium Hydride (NaH). It is typically prepared by direct combination of its two constituents, sodium (Na) and hydrogen (H2), in a reaction: \(2Na + H2 \rightarrow 2NaH\). The structure is ionic, with Na+ cations and H- anions.

Step 2: Name of Covalent Hydride and its Preparation

A common example of a covalent hydride is hydrogen chloride (HCl). It is typically prepared by the direct combination of hydrogen (H2) and chlorine (Cl2) gases, in a reaction: \(H2 + Cl2 \rightarrow 2HCl\). The structure of HCl is covalent, with the H and Cl atoms sharing a pair of electrons.

Key Concepts

Ionic Hydrides

Ionic hydrides are a fascinating class of chemical compounds formed between hydrogen and highly electropositive elements, typically alkali metals. In the specific example of sodium hydride (NaH), sodium (Na) donates an electron to hydrogen (H), leading to the formation of Na⁺ and H^- ions. This transfer creates a strong ionic bond. The creation of NaH is straightforward: it results from a direct reaction between sodium metal and hydrogen gas, as represented by the equation:

• 2Na + H₂ → 2NaH

In this solid compound, each sodium ion is surrounded by hydride ions, forming a crystalline lattice. This structure is typical of ionic compounds, reflecting their stable and strong ionic bonds.
When discussing ionic hydrides, remember that they generally form under specific conditions such as elevated temperatures, and they have applications ranging from acting as strong bases to being useful chemical reducing agents.

Covalent Hydrides

Covalent hydrides consist of hydrogen covalently bonded to one or more nonmetals. These bonds occur through the sharing of electron pairs between the atoms. Hydrogen chloride (HCl) serves as a classic example of a covalent hydride. When hydrogen gas reacts with chlorine gas, it forms HCl:

• H₂ + Cl₂ → 2HCl

Here, hydrogen and chlorine share a pair of electrons, resulting in a single covalent bond. This bond is represented as H—Cl within the molecule.
HCl is known as a polar covalent compound because of the significant difference in electronegativity between hydrogen and chlorine. Chlorine is more electronegative, which means it attracts the shared electrons more strongly, creating a dipole moment. This attribute is important in understanding the solubility and reactivity of HCl in various environments.

Compound Structure

Understanding the structure of a compound is crucial to predicting its physical and chemical properties. In ionic hydrides like NaH, the structure is defined by its ionic bonds. These bonds create a crystalline lattice structure, organizing the ions in a repeating pattern. This gives ionic compounds high melting and boiling points, as well as distinctive electrical conductivities when they are dissolved in water or melted.
In contrast, the structure of covalent hydrides like HCl is based on covalent bonds, which result in discrete molecules rather than large lattice structures. These molecules are held together by weaker intermolecular forces compared to the ionic bonds in hydrides like NaH. This difference influences many properties, such as lower melting and boiling points and differing solubilities.
The nature of these structures provides valuable insights into how substances behave in different conditions and forms the basis for further chemical analysis.

Chemical Reactions

Chemical reactions describe how compounds transform into new substances, often with distinct properties and compositions. Ionic hydrides like NaH participate in reactions as strong bases. They react vigorously with water to produce a sodium hydroxide solution and hydrogen gas:

• NaH + H₂O → NaOH + H₂

This type of reaction is a classic example of acid-base chemistry.
Covalent hydrides, such as HCl, also participate in diverse chemical reactions. When dissolved in water, HCl dissociates into H⁺ and Cl^- ions, forming hydrochloric acid, a strong acid in aqueous solutions. This property makes HCl an effective reactant in acid-base reactions and other chemical processes across various industries.
Understanding these chemical reactions helps explain the practical applications of these compounds, from laboratory settings to industrial uses.