Question

Identify the most important types of interparticle forces present in the solids of each of the following substances. a. \(\mathrm{BaSO}_{4}\) e. Csl b. \(\mathrm{H}_{2} \mathrm{~S}\) f. \(P_{4}\) c. Xe g. \(\mathrm{NH}_{3}\) d. \(\mathrm{C}_{2} \mathrm{H}_{6}\)

Short answer

a. BaSO4: Ionic forces b. H2S: Dipole-dipole interactions c. Xe: London dispersion forces d. C2H6: London dispersion forces e. CsI: Ionic forces f. P4: Covalent bonds g. NH3: Hydrogen bonding

Step-by-step solution

a. BaSO4

BaSO4 is a classic ionic compound made up of Ba2+ and SO4^2- ions. Therefore, BaSO4 will have predominant ionic forces acting between its constituent ions.

b. H2S

H2S is a nonpolar molecule with covalent bonds between the hydrogen and sulfur atoms. It also possesses Van der Waals forces in the form of dipole-dipole interactions (due to differences in electronegativity between hydrogen and sulfur) among its molecules. The most important interparticle force in H2S is the dipole-dipole interaction.

c. Xe

Xe is a noble gas, which means that it has full octets and no charge. Therefore, Xe atoms are not attracted to each other with any strong force, like ionic or covalent bonds. The most important interparticle force present in solid Xe is the weakest Van der Waals force, the London dispersion forces.

d. C2H6

C2H6 or ethane is a nonpolar molecule as it has covalent bonds and no charge. Therefore, the most important interparticle forces present in C2H6 would be London dispersion forces.

e. CsI

CsI (cesium iodide) is an ionic compound that forms from the transfer of an electron from Cs to I. Cesium iodide will have predominant ionic forces acting between its constituent Cs+ and I- ions.

f. P4

P4 (tetraphosphorus) is a covalent network solid. Within P4, phosphorus atoms are covalently bonded to each other, making the covalent bond the most important interparticle force present in solid P4.

g. NH3

NH3 (ammonia) has polar covalent bonds between nitrogen and hydrogen atoms. Ammonia exhibits strong hydrogen bonding between its molecules. Therefore, the most important interparticle force in NH3 is hydrogen bonding.

Key concepts

Ionic Forces

Ionic forces are the electrostatic attractions between positively and negatively charged ions. These forces are typically found in ionic solids, where the structure is composed of alternating positive and negative ions, such as in the compounds BaSO₄ and CsI mentioned in the exercise.

In an ionic solid like barium sulfate (BaSO₄), barium ions (Ba²⁺) and sulfate ions (SO₄^2-) are held together by the strong ionic bonds that form due to their opposite charges. Similarly, in cesium iodide (CsI), cesium (Cs⁺) and iodide (I^-) ions experience a powerful attraction towards each other. These ionic forces are what give these types of solids their high melting points and make them soluble in polar solvents like water.

Dipole-Dipole Interactions

Dipole-dipole interactions are forces of attraction between polar molecules, where each molecule has a partial positive charge on one side and a partial negative charge on the other. These polar molecules attract each other by aligning their opposite charges closely.

As an example from the exercise, hydrogen sulfide (H₂S) exhibits dipole-dipole interactions. Although the bonds within the H₂S molecule are covalent, the difference in electronegativity between hydrogen and sulfur creates a dipole moment. This causes the molecules to attract one another such that the positive hydrogen end of one molecule is near the negative sulfur end of another, leading to a significant interparticle force that affects its physical properties, including boiling point and vapor pressure.

London Dispersion Forces

London dispersion forces, also known as Van der Waals forces, are weak intermolecular attractions occurring between all atoms and molecules, resulting from the temporary fluctuations in the electron distribution within atoms or molecules. These forces are the only type of interparticle force present in nonpolar molecules and noble gases, such as Xe and C₂H₆ (ethane), as discussed in the exercise.

Solid xenon (Xe), being a noble gas, relies solely on London dispersion forces to maintain its solid structure at low temperatures. In ethane (C₂H₆), although the molecule is nonpolar and there is no permanent dipole, electrons may still be unevenly distributed at any instant in time, creating instantaneous dipoles and, hence, temporary attractions between the molecules, which are the London dispersion forces.

Covalent Network Solid

A covalent network solid is a type of material where atoms are bonded together in a continuous network entirely by covalent bonds. These bonds extend throughout the material, creating a strong and rigid structure, as seen in substances like diamond, quartz, and in the context of the exercise, tetraphosphorus (P₄).

In tetraphosphorus, phosphorus atoms are covalently bonded to form a stable network structure, and these covalent bonds are the primary interparticle force holding the solid together. Covalent network solids are typically very hard, have high melting points, and do not conduct electricity in their pure state because there are no free charges available for charge transport.

Hydrogen Bonding

Hydrogen bonding is a special case of dipole-dipole interaction occurring when hydrogen is bonded to highly electronegative atoms such as nitrogen, oxygen, or fluorine. The hydrogen bond is an attraction between the hydrogen attached to one of these electronegative atoms and the lone pair of electrons on another electronegative atom in a different molecule.

Ammonia (NH₃), used in the exercise, exhibits hydrogen bonding due to the polarity generated by the nitrogen and hydrogen atoms. Nitrogen's high electronegativity creates a large dipole in the molecule and the hydrogen atoms become positive centers that can attract the lone pairs from nearby nitrogen atoms, creating a strong intermolecular force. This hydrogen bonding greatly influences ammonia's boiling point and its ability to dissolve in water.