Question

In each of the following groups of substances, pick the one that has the given property. Justify each answer. a. highest boiling point: \(\mathrm{CCl}_{4}, \mathrm{CF}_{4}, \mathrm{CBr}_{4}\) b. lowest freezing point: LiF, \(\mathrm{F}_{2}, \mathrm{HCl}\) c. smallest vapor pressure at \(25^{\circ} \mathrm{C} : \mathrm{CH}_{3} \mathrm{OCH}_{3}, \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) , \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3}\) d. greatest viscosity: \(\mathrm{H}_{2} \mathrm{S}, \mathrm{HF}, \mathrm{H}_{2} \mathrm{O}_{2}\) e. greatest heat of vaporization: \(\mathrm{H}_{2} \mathrm{CO}, \mathrm{CH}_{3} \mathrm{CH}_{3}, \mathrm{CH}_{4}\) f. smallest enthalpy of fusion: \(\mathrm{I}_{2}, \mathrm{CsBr}, \mathrm{CaO}\)

Short answer

a. Highest boiling point: \(\mathrm{CBr}_{4}\) due to the strongest London dispersion forces among the given substances. b. Lowest freezing point: \(\mathrm{F}_{2}\) due to the weakest London dispersion forces among the given substances. c. Smallest vapor pressure at \(25^{\circ} \mathrm{C}\): \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{OH}\) due to the strongest hydrogen bonding forces among the given substances. d. Greatest viscosity: \(\mathrm{H}_{2}\mathrm{O}_{2}\) due to the strongest hydrogen bonding forces among the given substances. e. Greatest heat of vaporization: \(\mathrm{H}_{2}\mathrm{CO}\) due to the strongest London dispersion forces among the given substances. f. Smallest enthalpy of fusion: \(\mathrm{I}_{2}\) due to the weakest London dispersion forces among the given substances.

Step-by-step solution

Analyze the substances and their molecular forces

The substances are \(\mathrm{CCl}_{4}, \mathrm{CF}_{4},\) and \(\mathrm{CBr}_{4}.\) These are all nonpolar molecules because the difference in electronegativity between carbon and the halogens is relatively small, resulting in nonpolar covalent bonds. The type of molecular force these substances experience is London dispersion forces.

Determine the strength of the London dispersion forces

The strength of London dispersion forces depends on the size and shape of the molecules. Larger molecules with more electrons will have stronger London dispersion forces. Among the given substances, \(\mathrm{CBr}_{4}\) has the largest halogen atoms and most electrons, leading to stronger London dispersion forces.

Choose substance with highest boiling point

Higher boiling points correspond to stronger molecular forces. Since \(\mathrm{CBr}_{4}\) has the strongest London dispersion forces, it will have the highest boiling point among the three substances. #b. Lowest freezing point#

Analyze the substances and their molecular forces

The substances are LiF, \(\mathrm{F}_{2},\) and \(\mathrm{HCl}.\) LiF is an ionic compound with strong ionic bonds. \(\mathrm{F}_{2}\) is a diatomic molecule with London dispersion forces, while \(\mathrm{HCl}\) has polar covalent bonds and experiences dipole-dipole and London dispersion forces.

Determine the strength of the molecular forces

The ionic bonds in LiF are the strongest, followed by the dipole-dipole and London dispersion forces in \(\mathrm{HCl}\) and finally the London dispersion forces in \(\mathrm{F}_{2}.\)

Choose substance with lowest freezing point

Lower freezing points correspond to weaker molecular forces. Among these substances, \(\mathrm{F}_{2}\) has the weakest London dispersion forces, which results in the lowest freezing point. #c. Smallest vapor pressure at \(25^{\circ} \mathrm{C}\)

Analyze the substances and their molecular forces

The substances are \(\mathrm{CH}_{3}\mathrm{OCH}_{3}, \mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{OH},\) and \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{CH}_{3}.\) The first substance is a nonpolar molecule with London dispersion forces. The second substance has a polar hydroxyl group and experiences hydrogen bonding and London dispersion forces. The third substance is a nonpolar molecule with London dispersion forces.

Determine the strength of the molecular forces

Hydrogen bonding in the second substance, \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{OH},\) is the strongest force, followed by the London dispersion forces in the first and third substances, which are of similar strength.

Choose substance with smallest vapor pressure

Lower vapor pressure corresponds to stronger molecular forces at a given temperature. Among these substances, \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{OH}\) has the strongest hydrogen bonding forces, resulting in the smallest vapor pressure at \(25^{\circ} \mathrm{C}.\) #d. Greatest viscosity#

Analyze the substances and their molecular forces

The substances are \(\mathrm{H}_{2}\mathrm{S}, \mathrm{HF},\) and \(\mathrm{H}_{2}\mathrm{O}_{2}.\) The first and second substances have polar covalent bonds and experience dipole-dipole and London dispersion forces. The third substance has a polar structure and experiences hydrogen bonding and London dispersion forces.

Determine the strength of the molecular forces

The hydrogen bonding forces in \(\mathrm{H}_{2}\mathrm{O}_{2}\) are the strongest, followed by the dipole-dipole forces in the other two substances.

Choose substance with greatest viscosity

Higher viscosity corresponds to stronger molecular forces. Among these substances, \(\mathrm{H}_{2}\mathrm{O}_{2}\), with hydrogen bonding forces, will have the greatest viscosity. #e. Greatest heat of vaporization#

Analyze the substances and their molecular forces

The substances are \(\mathrm{H}_{2}\mathrm{CO}, \mathrm{CH}_{3}\mathrm{CH}_{3},\) and \(\mathrm{CH}_{4}.\) They all have nonpolar covalent bonds and experience London dispersion forces.

Determine the strength of the London dispersion forces

The greater the size and number of electrons in a molecule, the stronger the London dispersion forces. \(\mathrm{H}_{2}\mathrm{CO}\) is the largest molecule with the highest number of electrons, resulting in stronger London dispersion forces.

Choose substance with greatest heat of vaporization

Higher heat of vaporization corresponds to stronger molecular forces. Among these substances, \(\mathrm{H}_{2}\mathrm{CO}\), with the strongest London dispersion forces, will have the greatest heat of vaporization. #f. Smallest enthalpy of fusion#

Analyze the substances and their molecular forces

The substances are \(\mathrm{I}_{2}, \mathrm{CsBr},\) and \(\mathrm{CaO}.\) \(\mathrm{I}_{2}\) is a nonpolar molecule with London dispersion forces. \(\mathrm{CsBr}\) and \(\mathrm{CaO}\) are ionic compounds with strong ionic bonds.

Determine the strength of the molecular forces

The ionic bonds in \(\mathrm{CsBr}\) and \(\mathrm{CaO}\) are stronger than the London dispersion forces in \(\mathrm{I}_{2}.\)

Choose substance with smallest enthalpy of fusion

Lower enthalpy of fusion corresponds to weaker molecular forces. Among these substances, \(\mathrm{I}_{2}\), with the weakest London dispersion forces, will have the smallest enthalpy of fusion.

Key concepts

Molecular Forces

Molecular forces are the attractions between molecules that influence their physical properties. These forces can be categorized into several types:

• London Dispersion Forces: Present in all molecules, they are the weakest and arise from temporary shifts in electron density.
• Dipole-Dipole Interactions: Occur in polar molecules and are stronger than London forces due to permanent dipoles.
• Hydrogen Bonding: A strong type of dipole interaction occurring in molecules with hydrogen attached to electronegative atoms like N, O, or F.
• Ionic Bonds: The strongest forces, occurring in ionic compounds, such as salts.

Understanding these forces helps explain why substances have varying physical characteristics like boiling points and viscosities.

Boiling Point

The boiling point is the temperature where a liquid turns into vapor. It depends largely on the strength of molecular forces within the liquid:

• Stronger intermolecular forces lead to higher boiling points, as more energy is required to separate molecules.
• For instance, substances like \( \mathrm{CBr}_{4} \) have high boiling points due to strong London dispersion forces caused by large, heavy atoms.
• In contrast, molecules with weaker forces have lower boiling points.

This concept is crucial in determining the conditions under which different substances change states.

Vapor Pressure

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid phase at a given temperature.

• Low vapor pressure indicates strong intermolecular forces, as fewer molecules escape into the vapor phase.
• For example, \( \mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{OH} \) exhibits low vapor pressure due to hydrogen bonding, a strong intermolecular force.
• Substances with high vapor pressure will evaporate easily.

Vapor pressure is an essential concept to understand evaporation and condensation processes.

Viscosity

Viscosity describes a fluid's resistance to flow and is influenced by molecular forces.

• Liquids with strong intermolecular forces exhibit higher viscosity, as molecules resist movement.
• \( \mathrm{H}_{2}\mathrm{O}_{2} \), for example, has high viscosity due to robust hydrogen bonding.
• Conversely, substances with weaker forces flow more easily and have lower viscosity.

Understanding viscosity is vital in applications ranging from manufacturing to fluid dynamics.

Heat of Vaporization

The heat of vaporization is the energy required to turn a liquid into a vapor without a temperature change.

• It is greater for substances with stronger molecular forces, as more energy is needed to overcome these forces.
• \( \mathrm{H}_{2}\mathrm{CO} \), for example, has a high heat of vaporization due to its strong London dispersion forces.
• This concept is key in processes like distillation and cooling systems.

Knowing the heat of vaporization helps in understanding the energy demands for phase changes.

Enthalpy of Fusion

Enthalpy of fusion is the energy needed to change a solid into a liquid at its melting point.

• Weaker intermolecular forces lead to a lower enthalpy of fusion, as less energy is needed to separate particles.
• \( \mathrm{I}_{2} \), for instance, has a low enthalpy of fusion due to its weak London dispersion forces.
• Strong ionic compounds like \( \mathrm{CsBr} \) have high enthalpies of fusion due to strong bonds.

Understanding this concept is crucial for predicting the melting behavior of different substances.