Question

Which of the following statements is (are) true? a. LiF will have a higher vapor pressure at \(25^{\circ} \mathrm{C}\) than \(\mathrm{H}_{2} \mathrm{S}\) . b. HF will have a lower vapor pressure at \(-50^{\circ} \mathrm{C}\) than \(\mathrm{HBr}\) . c. \(\mathrm{Cl}_{2}\) will have a higher boiling point than Ar. d. HCl is more soluble in water than in \(\mathrm{CCl}_{4}\) e. \(\mathrm{MgO}\) will have a higher vapor pressure at \(25^{\circ} \mathrm{C}\) than \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) .

Short answer

The true statements are: b. HF will have a lower vapor pressure at \(-50^{\circ} \mathrm{C}\) than HBr. c. Cl2 will have a higher boiling point than Ar. d. HCl is more soluble in water than in CCl4.

Step-by-step solution

Statement A: LiF will have a higher vapor pressure at 25°C than H2S

This statement is false. LiF is an ionic compound, while H2S is a covalent compound. Ionic compounds generally have lower vapor pressures than covalent compounds because the ionic bonds are stronger and prevent the molecules from vaporizing easily. In contrast, H2S has weaker covalent bonds and will have a higher vapor pressure than LiF.

Statement B: HF will have a lower vapor pressure at -50°C than HBr

This statement is true. HF has a higher boiling point than HBr because of the presence of hydrogen bonding. Hydrogen bonding is a stronger intermolecular force compared to the dipole-dipole interaction present in HBr. Due to the stronger intermolecular forces, HF has a lower vapor pressure compared to HBr at the same temperature.

Statement C: Cl2 will have a higher boiling point than Ar

This statement is true. Cl2 and Ar are both nonpolar, so the main intermolecular force between them is London dispersion forces. Cl2 has a higher molecular weight compared to Ar. As molecular weight increases, the strength of London dispersion forces increases, leading to a higher boiling point. Therefore, Cl2 will have a higher boiling point than Ar.

Statement D: HCl is more soluble in water than in CCl4

This statement is true. The solubility of a substance depends on the polarity of the solvent. Water is a polar solvent, while CCl4 is a nonpolar solvent. HCl is a polar covalent compound, which means it will easily dissolve in polar solvents like water. As the saying goes, "like dissolves like." Hence, HCl will be more soluble in water than in CCl4.

Statement E: MgO will have a higher vapor pressure at 25°C than CH3CH2OH

This statement is false. MgO is an ionic compound, while CH3CH2OH (ethanol) is a covalent compound. As mentioned before, ionic compounds generally have lower vapor pressures than covalent compounds due to the strong ionic bonds. Ethanol, a covalent compound, has weaker intermolecular forces (hydrogen bonding and London dispersion forces) and will have a higher vapor pressure than MgO.

Key concepts

Ionic vs Covalent Compounds

Understanding the distinction between ionic and covalent compounds is crucial in grasping vapor pressure concepts. Ionic compounds are made up of positive and negative ions held together by strong ionic bonds. This results in a solid form with a high melting and boiling point, as the energy required to break these bonds is significant.
Covalent compounds, on the other hand, form when atoms share electrons. These generally have lower melting and boiling points compared to ionic compounds because the intermolecular forces are weaker than ionic bonds.
It's these intrinsic bonding differences that result in ionic compounds having lower vapor pressures than covalent compounds. This is because the stronger bonds in ionic compounds don't break easily to form vapor, whereas the weaker bonds in covalent compounds allow for easier vaporization.

Intermolecular Forces

Intermolecular forces play a vital role in determining the physical properties of substances, such as boiling point and vapor pressure. There are several types of intermolecular forces to consider:

• London Dispersion Forces: Present in all molecules, these are the weakest forces and occur due to temporary dipoles in molecules. Larger molecules have stronger dispersion forces, leading to higher boiling points.
• Dipole-Dipole Interactions: These occur between polar molecules where positive ends attract negative ends of different molecules, resulting in moderate strength.
• Hydrogen Bonding: This is a strong type of dipole-dipole interaction found particularly in molecules where hydrogen is bonded to highly electronegative atoms like nitrogen, oxygen, or fluorine.

The boiling point of a substance increases with the strength of its intermolecular forces. Substances like HF have strong hydrogen bonds, giving them higher boiling points compared to molecules like HBr with weaker dipole-dipole interactions.

Boiling Point

The boiling point of a substance is the temperature at which its vapor pressure equals the surrounding pressure, allowing the liquid to turn into a vapor. Factors affecting boiling points include molecular weight and the type of intermolecular forces present.

• Heavy molecules, like \( ext{Cl}_2\), have higher boiling points due to stronger London dispersion forces than lighter molecules like Argon (Ar), despite both being nonpolar.
• Molecules with strong interactions, such as hydrogen bonds in HF, will also have higher boiling points compared to those with weaker interactions, such as dipole-dipole in HBr.

Understanding these concepts helps explain why different substances boil at different temperatures under the same conditions.

Solubility in Solvents

Solubility is the capacity of a substance to dissolve in a solvent, and it largely depends on the nature of the solute and solvent. One pivotal principle is the 'like dissolves like' rule, which helps predict solubility:

• Polar substances: Such substances will dissolve in polar solvents. For example, HCl dissolves well in water due to the polarity compatibility.
• Nonpolar substances: These tend to dissolve in nonpolar solvents like \( ext{CCl}_4 \). However, HCl is not soluble in \( ext{CCl}_4 \) due to its polar nature.

This principle guides us in predicting the solubility of compounds within different solvents. Understanding the polarity of both the solute and solvent is crucial for determining how and why certain substances dissolve more effectively in specific solvents.