Problem 6 - Chemistry: Introducing Inorganic, Organic and Physical Chemistry

Question

Classify the following hydrides as ionic, covalent, or metallic: (a) \(\mathrm{BeH}_{2} ;(\mathrm{b}) \mathrm{PH}_{3} ;(\mathrm{c}) \mathrm{KH} ;(\mathrm{d}) \mathrm{HCl} ;(\mathrm{e}) \mathrm{FeTiH}_{1.8} .\) For the covalent hydrides, state whether they exist as discrete molecules or are linked by bridging hydrogen atoms. (Section 25.2 )

Step-by-Step Solution

Answer

(a) Covalent (Polymer), (b) Covalent (Discrete Molecule), (c) Ionic, (d) Covalent (Discrete Molecule), (e) Metallic.

Step 1: Understanding Hydride Types

Hydrides are compounds formed between hydrogen and other elements. They can be classified into three primary types: 1. **Ionic Hydrides**: These consist of hydrogen combined with electropositive metals (typically from Groups 1 and 2). 2. **Covalent Hydrides**: These form when hydrogen is bonded to non-metals or metalloids, creating discrete molecules or polymers with bridging hydrogen. 3. **Metallic Hydrides**: These include hydrogen absorbed into the lattice of transition metals.

Step 2: Analyzing Each Compound

We must analyze each given compound to determine its classification: - (a) ** BeH₄**: Beryllium is in Group 2, potentially forming ionic hydrides with bridging hydrogen as polymers. - (b) **PH₃**: Hydrogen with phosphorus forms a covalent bond, creating discrete molecules. - (c) **KH**: Potassium, a Group 1 element, forms ionic hydrides. - (d) **HCl**: A covalent hydride where hydrogen and chlorine form discrete molecules. - (e) **FeTiH₁.₈**: This compound involves transition metals, indicating a metallic hydride.

Step 3: Identifying Each Type

Using the types identified, classify each compound: - (a) BeH₂: **Covalent (Polymer with bridging hydrogen)** - (b) PH₃: **Covalent (Discrete Molecule)** - (c) KH: **Ionic Hydride** - (d) HCl: **Covalent (Discrete Molecule)** - (e) FeTiH₁.₈: **Metallic Hydride**

Key Concepts

Ionic Hydrides

Ionic hydrides are interesting compounds formed when hydrogen combines with highly electropositive metals, typically from Groups 1 and 2 of the periodic table. These metals include elements like sodium (Na), potassium (K), and calcium (Ca). Ionic hydrides are mainly characterized by the presence of hydrogen as a negatively charged ion (hydride ion, H⁻). This occurs because the metal donates its electron to the hydrogen, resulting in a strong electrostatic attraction between the positive metal ion and the negative hydride ion.

• Properties: These hydrides are solid at room temperature.
• Reactivity: They are highly reactive with water, producing hydrogen gas and a hydroxide.
• Example: Potassium hydride (KH) is a classic example, reacting vigorously when exposed to water.

Ionic hydrides are used in various applications, including as strong reducing agents in chemical synthesis. They serve important roles in industry, yet they must be handled carefully due to their reactivity.

Covalent Hydrides

Covalent hydrides are diverse and fascinating. They form when hydrogen bonds covalently with non-metals or metalloids, involving the sharing of electron pairs between atoms. These hydrides exhibit a variety of structures, either as discrete molecules or polymeric chains with bridging hydrogen atoms.

• Discrete Molecules: Simple hydrides like PH₃ (phosphine) and HCl (hydrogen chloride) are examples of discrete covalent molecules. They tend to have low melting and boiling points.
• Polymeric Forms: More complex structures, like beryllium hydride (BeH₂), form polymers with hydrogen atoms bridging between them, leading to a wide range of structural possibilities.

Given their wide array of forms and properties, covalent hydrides play crucial roles in both biological systems and industrial applications. They are central in studies of molecular bonding and chemical reactivity.

Metallic Hydrides

Metallic hydrides represent a unique class of hydrogen-containing compounds where hydrogen atoms are absorbed into the lattice of transition metals. These hydrides behave quite differently from ionic and covalent hydrides due to their special metal-hydrogen interactions. Metallic hydrides generally form with transition metals, such as titanium and iron.

• Structure: The hydrogen atoms occupy interstitial sites within the metal lattice, creating a compound that often retains metallic properties.
• Properties: They exhibit variable stoichiometry and retain the electrical conductivity of metals.
• Example: A compound like FeTiH₁.₈ illustrates this, where hydrogen atoms are intercalated within an iron-titanium alloy.

Metallic hydrides are important in numerous applications, including special roles in hydrogen storage systems. This is crucial for the development of clean energy technologies and further advancements in materials science.