Chapter 7, Problem 7.37QP

Which substance in each of the following pairs would you expect to have the higher boiling point: (a) Ne or Xe, (b) CO₂ or CS₂, (c) CH₄ or Cl₂. (d) F₂ or LiF, (e) NH₃ or PH₃? Explain why.

Interpretation Introduction

Interpretation:

The substance with higher boiling point in the given pairs of substances should be determined.

Concept introduction:

• Polarity of a bond is due to the difference in electro-negativities of atoms presented in it. The polarities of bonds are represented by using vectors.
• If the result of all bond polarities or vector sum is non-zero in a molecule, then the molecule is called as polar molecule.
• If the result of all bond polarities or vector sum is zero in a molecule, then the molecule is called as nonpolar molecule.
• Intermolecular force is the set of repulsive and attractive forces between molecules that result from the polarity between neighboring molecules. There are four types of intermolecular forces.
• Dipole – Dipole interaction: This force takes place between polar compounds.
• Hydrogen bonding is a type of dipole-dipole interaction of molecules when the hydrogen is bonded to strong electronegative atom (F, O, N, etc) in the molecules.
• Dispersion force is a weak force and this force is present in all compounds force.
• Boiling point is depending upon the strength of inter molecular forces. 

Answer to Problem 7.37QP

$\text{Xe}$ has higher boiling point than $\text{Ne}$.

Explanation of Solution

$\text{Xe}$ and $\text{Ne}$ are mono-atomic gases, so they are nonpolar molecules.

Therefore, only dispersion forces are presented in these molecules; dispersion forces is depends upon the molecular weight.  $\text{Xe}$ has greater molecular weight as compare to $\text{Ne}$

Boiling point is depending upon the strength of inter molecular forces. 

Hence,

$\text{Xe}$ has higher boiling point than $\text{Ne}$.

Interpretation Introduction

Interpretation:

The substance with higher boiling point in the given pairs of substances should be determined.

Concept introduction:

• Polarity of a bond is due to the difference in electro-negativities of atoms presented in it. The polarities of bonds are represented by using vectors.
• If the result of all bond polarities or vector sum is non-zero in a molecule, then the molecule is called as polar molecule.
• If the result of all bond polarities or vector sum is zero in a molecule, then the molecule is called as nonpolar molecule.
• Intermolecular force is the set of repulsive and attractive forces between molecules that result from the polarity between neighboring molecules. There are four types of intermolecular forces.
• Dipole – Dipole interaction: This force takes place between polar compounds.
• Hydrogen bonding is a type of dipole-dipole interaction of molecules when the hydrogen is bonded to strong electronegative atom (F, O, N, etc) in the molecules.
• Dispersion force is a weak force and this force is present in all compounds force.
• Boiling point is depending upon the strength of inter molecular forces. 

Answer to Problem 7.37QP

${\text{CS}}_{\text{2}}$ has higher boiling point than ${\text{CO}}_{\text{2}}$.

Explanation of Solution

In ${\text{CS}}_{\text{2}}$ molecule,

There are two C-S presented in ${\text{CS}}_{\text{2}}$ molecule. Sulfur atom has more electronegativity as compared to carbon atom; so all the bonds in ${\text{CS}}_{\text{2}}$ have polarities.

The result of all the bond polarities are the sum of all the vectors associated with each bonds.

The directions of C-S bond vectors are opposite to each other, so they cancel each other.

Hence,

The vector sum or the result of bond polarities for ${\text{CS}}_{\text{2}}$ molecule is zero, so ${\text{CS}}_{\text{2}}$ is a non-polar molecule.

Therefore,

Only dispersion forces are present in ${\text{CS}}_{\text{2}}$.

In ${\text{CO}}_{\text{2}}$ molecule,

There are two C-O presented in ${\text{CO}}_{\text{2}}$ molecule. Oxygen atom has more electronegativity as compared to carbon atom; so all the bonds in ${\text{CO}}_{\text{2}}$ have polarities.

The result of all the bond polarities are the sum of all the vectors associated with each bonds.

The directions of C-O bond vectors are opposite to each other, so they cancel each other.

Hence,

The vector sum or the result of bond polarities for ${\text{CO}}_{\text{2}}$ molecule is zero, so ${\text{CO}}_{\text{2}}$ is a non-polar molecule.

Therefore,

Only dispersion forces are present in ${\text{CO}}_{\text{2}}$.

Dispersion forces is depends upon the molecular weight. ${\text{CS}}_{\text{2}}$ has greater molecular weight as compare to ${\text{CO}}_{\text{2}}$. So ${\text{CS}}_{\text{2}}$ has greater dispersion forces as compare to ${\text{CO}}_{\text{2}}$

Boiling point depends upon the strength of inter molecular forces.

Hence,

${\text{CS}}_{\text{2}}$ has higher boiling point than ${\text{CO}}_{\text{2}}$.

Interpretation Introduction

Interpretation:

The substance with higher boiling point in the given pairs of substances should be determined.

Concept introduction:

• Polarity of a bond is due to the difference in electro-negativities of atoms presented in it. The polarities of bonds are represented by using vectors.
• If the result of all bond polarities or vector sum is non-zero in a molecule, then the molecule is called as polar molecule.
• If the result of all bond polarities or vector sum is zero in a molecule, then the molecule is called as nonpolar molecule.
• Intermolecular force is the set of repulsive and attractive forces between molecules that result from the polarity between neighboring molecules. There are four types of intermolecular forces.
• Dipole – Dipole interaction: This force takes place between polar compounds.
• Hydrogen bonding is a type of dipole-dipole interaction of molecules when the hydrogen is bonded to strong electronegative atom (F, O, N, etc) in the molecules.
• Dispersion force is a weak force and this force is present in all compounds force.
• Boiling point is depending upon the strength of inter molecular forces. 

Answer to Problem 7.37QP

${\text{Cl}}_{\text{2}}$ has higher boiling point than ${\text{CH}}_4$.

Explanation of Solution

The Cl-Cl bond in the ${\text{Cl}}_{\text{2}}$ molecule has no bond polarity. So ${\text{Cl}}_{\text{2}}$ molecule is a nonpolar molecule.

Therefore,

Only dispersion forces are present in ${\text{Cl}}_{\text{2}}$.

In ${\text{CH}}_4$ molecule,

There are four C-H presented in ${\text{CH}}_4$ molecule. Carbon atom has more electronegativity as compared to hydrogen; so all the bonds in ${\text{CH}}_4$ have polarities.

The result of all the bond polarities are the sum of all the vectors associated with each bonds.

The directions of C-H bond vectors are opposite to each other, so they cancel each other.

Hence,

The vector sum or the result of bond polarities for ${\text{CH}}_4$ molecule is zero, so ${\text{CH}}_4$ is a non-polar molecule.

Since the ${\text{CH}}_4$ is a nonpolar molecule, it exhibit only one type of intermolecular force, which is dispersion forces.

Dispersion forces is depends upon the molecular weight. ${\text{Cl}}_{\text{2}}$ has greater molecular weight as compare to ${\text{CH}}_4$. So ${\text{Cl}}_{\text{2}}$ have greater dispersion forces as compare to ${\text{CH}}_4$

Boiling point is depending upon the strength of inter molecular forces.

Hence,

${\text{Cl}}_{\text{2}}$ has higher boiling point than ${\text{CH}}_4$.

Interpretation Introduction

Interpretation:

The substance with higher boiling point in the given pairs of substances should be determined.

Concept introduction:

• Polarity of a bond is due to the difference in electro-negativities of atoms presented in it. The polarities of bonds are represented by using vectors.
• If the result of all bond polarities or vector sum is non-zero in a molecule, then the molecule is called as polar molecule.
• If the result of all bond polarities or vector sum is zero in a molecule, then the molecule is called as nonpolar molecule.
• Intermolecular force is the set of repulsive and attractive forces between molecules that result from the polarity between neighboring molecules. There are four types of intermolecular forces.
• Dipole – Dipole interaction: This force takes place between polar compounds.
• Hydrogen bonding is a type of dipole-dipole interaction of molecules when the hydrogen is bonded to strong electronegative atom (F, O, N, etc) in the molecules.
• Dispersion force is a weak force and this force is present in all compounds force.
• Boiling point is depending upon the strength of inter molecular forces. 

Answer to Problem 7.37QP

$\text{LiF}$ has higher boiling point than ${\text{F}}_{\text{2}}$.

Explanation of Solution

The F-F bond in the ${\text{F}}_{\text{2}}$ molecule has no bond polarity. So ${\text{F}}_{\text{2}}$ molecule is a nonpolar molecule.

Therefore,

Only dispersion forces are present in ${\text{F}}_{\text{2}}$.

$\text{LiF}$ is an ionic compound, in which ${\text{Li}}^{\text{+}}$ and $\text{F}$ ions are electrostatically attracted each other.

Therefore,

Ionic forces are present in $\text{LiF}$ prevalently.

Since ionic forces stronger than dispersion forces, then $\text{LiF}$ has greater intermolecular forces as compared to ${\text{F}}_{\text{2}}$.

Boiling point depends upon the strength of inter molecular forces.

Hence,

$\text{LiF}$ has higher boiling point than ${\text{F}}_{\text{2}}$.

Interpretation Introduction

Interpretation:

The substance with higher boiling point in the given pairs of substances should be determined.

Concept introduction:

• Polarity of a bond is due to the difference in electro-negativities of atoms presented in it. The polarities of bonds are represented by using vectors.
• If the result of all bond polarities or vector sum is non-zero in a molecule, then the molecule is called as polar molecule.
• If the result of all bond polarities or vector sum is zero in a molecule, then the molecule is called as nonpolar molecule.
• Intermolecular force is the set of repulsive and attractive forces between molecules that result from the polarity between neighboring molecules. There are four types of intermolecular forces.
• Dipole – Dipole interaction: This force takes place between polar compounds.
• Hydrogen bonding is a type of dipole-dipole interaction of molecules when the hydrogen is bonded to strong electronegative atom (F, O, N, etc) in the molecules.
• Dispersion force is a weak force and this force is present in all compounds force.
• Boiling point is depending upon the strength of inter molecular forces. 

Answer to Problem 7.37QP

${\text{NH}}_{\text{3}}$ has higher boiling point than ${\text{PH}}_{\text{3}}$.

Explanation of Solution

In ammonia (${\text{NH}}_{\text{3}}$) molecule,

Three N-H bonds are presented and due to the difference in electronegativities of nitrogen and hydrogen, it has bond polarity. So ${\text{NH}}_{\text{3}}$ molecule is a polar molecule.

Polar molecules exhibit dipole-dipole interactions.

Since the hydrogen atom is bonded to nitrogen, then hydrogen bonding will be presented in between ${\text{NH}}_{\text{3}}$ molecules.

In ${\text{PH}}_{\text{3}}$ molecule,

Three P-H bonds are presented and due to the difference in electronegativities of phosphorus and hydrogen, it has bond polarity. So ${\text{PH}}_{\text{3}}$ molecule is a polar molecule.

Polar molecules exhibit dipole-dipole interactions.

${\text{NH}}_{\text{3}}$ has dipole-dipole interaction with Hydrogen bonding; but ${\text{PH}}_{\text{3}}$ has only dipole-dipole interactions.

Boiling point depends upon the strength of inter molecular forces.

Hence,

${\text{NH}}_{\text{3}}$ has higher boiling point than ${\text{PH}}_{\text{3}}$.

Conclusion

The molecules of higher boiling point in the given pairs of molecules are determined according to the polarities or molecular weights of molecules.