Question

Arrange the following (i) \(\mathrm{CaH}_{2}, \mathrm{BeH}_{2}\) and \(\mathrm{TIH}_{2}\) in order of increasing electrical conductance. (ii) LiH. NaH and CsH in order of increasing ionic character. (iii) \(\mathrm{H}-\mathrm{H}, \mathrm{D}-\mathrm{D}\) and \(\mathrm{F}-\mathrm{F}\) in order of increasing bond dissociation enthalpy. (iv) NaH, \(\mathrm{MgH}_{2}\) and \(\mathrm{H}_{2} \mathrm{O}\) in order of increasing reducing property.

Short answer

(i) \(\mathrm{BeH}_{2} < \mathrm{CaH}_{2} < \mathrm{TlH}_{2}\), (ii) \(\mathrm{LiH} < \mathrm{NaH} < \mathrm{CsH}\), (iii) \(\mathrm{F}-\mathrm{F} < \mathrm{H}-\mathrm{H} < \mathrm{D}-\mathrm{D}\), (iv) \(\mathrm{H}_{2}\mathrm{O} < \mathrm{MgH}_{2} < \mathrm{NaH}\).

Step-by-step solution

Introduction to Concepts

Before we start ordering the given compounds, we need to understand two different concepts: electrical conductance, ionic character, bond dissociation enthalpy, and reducing property. Electrical conductance depends on a compound’s ability to allow electrical charge flow, ionic character is the degree of charge separation in a bond or compound, bond dissociation enthalpy relates to the energy required to break a bond, and reducing property is the ability of a substance to donate electrons.

Order of Increasing Electrical Conductance

For ionic hydrides such as \(\mathrm{CaH}_{2}, \mathrm{BeH}_{2}\), and \(\mathrm{TlH}_{2}\), conductance is dependent on the metal's ability to donate electrons. \(\mathrm{BeH}_{2}\) has covalent character, \(\mathrm{CaH}_{2}\) is an ionic conductor, and \(\mathrm{TlH}_{2}\), being metallic, has the highest conductance. Therefore, the order is \(\mathrm{BeH}_{2} < \mathrm{CaH}_{2} < \mathrm{TlH}_{2}\).

Order of Increasing Ionic Character

Considering \(\mathrm{LiH}, \mathrm{NaH}\), and \(\mathrm{CsH}\), the ionic character depends on the size of the metal cation and its position in the periodic table. The ionic character increases down the group, so the order is \(\mathrm{LiH} < \mathrm{NaH} < \mathrm{CsH}\).

Order of Increasing Bond Dissociation Enthalpy

For bonds \(\mathrm{H}-\mathrm{H}, \mathrm{D}-\mathrm{D}\), and \(\mathrm{F}-\mathrm{F}\), the bond dissociation enthalpy is generally higher for heavier isotopes and stronger bonds. \(\mathrm{F}-\mathrm{F}\) is weaker due to repulsion between lone pairs of electrons, thus \(\mathrm{D}-\mathrm{D} > \mathrm{H}-\mathrm{H} > \mathrm{F}-\mathrm{F}\).

Order of Increasing Reducing Property

For hydrides like \(\mathrm{NaH}, \mathrm{MgH}_{2}\), and \(\mathrm{H}_{2}\mathrm{O}\), reducing property relates to the ease of donating electrons. \(\mathrm{H}_{2}\mathrm{O}\) is a weak reducing agent, \(\mathrm{MgH}_{2}\) is intermediate, and \(\mathrm{NaH}\) is the strongest. Hence, the order is \(\mathrm{H}_{2}\mathrm{O} < \mathrm{MgH}_{2} < \mathrm{NaH}\).

Key concepts

Electrical Conductance

When discussing chemical bonding, electrical conductance is a key concept. It's the ability of a material to conduct electricity. This occurs when charged particles, such as ions or electrons, are able to move freely within a substance.
For hydrides like \( \mathrm{BeH}_{2}, \mathrm{CaH}_{2}\), and \(\mathrm{TlH}_{2}\), electrical conductance is influenced by the type of bonding and the nature of the involved elements.

• \(\mathrm{BeH}_{2}\) displays more covalent character, thus its ability to conduct electricity is quite limited.
• \(\mathrm{CaH}_{2}\) is an ionic compound, allowing for better electrical conductivity than \(\mathrm{BeH}_{2}\).
• \(\mathrm{TlH}_{2}\), being metallic in nature, offers the highest electrical conductance due to the presence of delocalized electrons.

Understanding these differences helps in predicting the order of increasing electrical conductance: \(\mathrm{BeH}_{2} < \mathrm{CaH}_{2} < \mathrm{TlH}_{2}\). This concept is crucial for determining the potential applications of materials in electrical and electronic devices.

Ionic Character

Ionic character refers to the extent to which a chemical bond between two atoms has ionic versus covalent characteristics. It depends primarily on the relative electronegativities of the atoms involved.
When examining \(\mathrm{LiH}, \mathrm{NaH},\) and \(\mathrm{CsH}\), consider the position of the metal in the periodic table.

• Ionic character increases as you move down a group in the periodic table.
• \(\mathrm{LiH}\) shows the least ionic character because lithium, being small and highly electronegative, holds its electron tightly.
• \(\mathrm{NaH}\) has more ionic character than \(\mathrm{LiH}\) due to a larger atomic radius and lower electronegativity of sodium.
• \(\mathrm{CsH}\) has the highest ionic character in this group because of the significantly larger size of cesium, which makes it easier for its valence electrons to be transferred.

Thus, the order of increasing ionic character for these hydrides is \(\mathrm{LiH} < \mathrm{NaH} < \mathrm{CsH}\). Grasping this concept helps in predicting the solubility and reactivity of compounds.

Bond Dissociation Enthalpy

Bond dissociation enthalpy is the energy required to break a chemical bond and separate atoms in a molecule. It is indicative of bond strength. Higher values mean stronger bonds.
In the case of \(\mathrm{H}-\mathrm{H}, \mathrm{D}-\mathrm{D}, \) and \(\mathrm{F}-\mathrm{F}\), consider different factors:

• Isotopic effects play a role. \(\mathrm{D}-\mathrm{D}\), being a heavier isotope than \(\mathrm{H}-\mathrm{H}\), has a slightly higher bond dissociation enthalpy due to the stronger bond formed.
• \(\mathrm{H}-\mathrm{H}\) bond is strong but slightly less so than the \(\mathrm{D}-\mathrm{D}\) bond.
• \(\mathrm{F}-\mathrm{F}\) bond, despite fluorine atoms being highly electronegative, is weak due to repulsion between lone electron pairs on the fluorine atoms.

Hence, the order is \(\mathrm{F}-\mathrm{F} < \mathrm{H}-\mathrm{H} < \mathrm{D}-\mathrm{D}\). Understanding bond dissociation enthalpy provides insights into reaction mechanisms and the stability of molecules.

Reducing Property

The reducing property of a substance measures its ability to donate electrons to other substances during a chemical reaction. This property is crucial for understanding redox reactions, which are key to many processes in chemistry.
For hydrides like \(\mathrm{NaH}, \mathrm{MgH}_{2}\), and \(\mathrm{H}_{2} \mathrm{O}\), the ability to donate electrons varies:

• \(\mathrm{NaH}\) has a strong reducing ability, as sodium readily donates its electron due to its low electronegativity and large atomic size.
• \(\mathrm{MgH}_{2}\) shows moderate reducing properties.
• \(\mathrm{H}_{2}\mathrm{O}\) is the weakest reducing agent among these due to the strong bonds between hydrogen and oxygen, making its electrons less available for donation.

Thus, the order of increasing reducing property is \(\mathrm{H}_{2} \mathrm{O} < \mathrm{MgH}_{2} < \mathrm{NaH}\). Mastering this concept is essential for predicting reactions in various chemical processes, including energy production and material synthesis.