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Chapter 12: Problem 51

Which substance has the lower boiling point? Explain. (a) \(\mathrm{LiCl}\) or \(\mathrm{HCl}\) (b) \(\mathrm{NH}_{3}\) or \(\mathrm{PH}_{3}\) (c) Xe or I \(_{2}\)

Short Answer

Expert verified
HCl, PH3, and Xe have lower boiling points as compared to LiCl, NH3, and I2 respectively.

Step by step solution

01

- Understanding Intermolecular Forces

First, identify the types of intermolecular forces present in each substance. The strength and type of these forces determine the boiling point.
02

- Compare LiCl and HCl

LiCl is an ionic compound with strong ionic bonds, whereas HCl is a covalent compound with dipole-dipole interactions. Ionic bonds are generally stronger than dipole-dipole interactions, thus requiring more energy (higher temperature) to break.
03

- Determine Boiling Point of LiCl and HCl

Since LiCl has stronger ionic bonds, it has a higher boiling point compared to HCl. Therefore, HCl has the lower boiling point.
04

- Compare NH3 and PH3

NH3 forms hydrogen bonds, which are strong intermolecular forces. PH3 does not form hydrogen bonds and only has weaker van der Waals forces.
05

- Determine Boiling Point of NH3 and PH3

Hydrogen bonds in NH3 lead to a higher boiling point than the van der Waals forces in PH3. Therefore, PH3 has the lower boiling point.
06

- Compare Xe and I2

Xe is a noble gas with weak van der Waals forces (London dispersion forces), while I2 is a diatomic molecule with stronger van der Waals forces due to its larger size and higher polarizability.
07

- Determine Boiling Point of Xe and I2

I2, with its larger size, has stronger dispersion forces, necessitating more energy to boil. Therefore, Xe has the lower boiling point.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Intermolecular Forces
Intermolecular forces are the forces of attraction between molecules. These forces determine many physical properties, such as boiling and melting points. There are several types of intermolecular forces: ionic bonds, hydrogen bonds, dipole-dipole interactions, and van der Waals forces (London dispersion forces). Each of these forces varies in strength.
Ionic Bonds
Ionic bonds occur between positively and negatively charged ions. These bonds are very strong, as they involve the transfer of electrons from one atom to another, creating ions. For example, in \( \text{LiCl} \), lithium donates an electron to chlorine, forming Li+ and Cl- ions. The electrostatic attraction between these ions results in a strong bond.
Ionic compounds, like LiCl, generally have high boiling points because a lot of energy is required to break these strong bonds.
Hydrogen Bonds
Hydrogen bonds are a type of dipole-dipole interaction that occurs when a hydrogen atom is bonded to a highly electronegative atom, like nitrogen, oxygen, or fluorine. This creates a significant dipole as the electrons are pulled toward the electronegative atom.
For instance, in \( \text{NH}_3 \) (ammonia), hydrogen bonds are formed between hydrogen and nitrogen atoms. These bonds are stronger than other dipole-dipole interactions, increasing the boiling point of hydrogen-bonded compounds.
Van der Waals Forces
Van der Waals forces, also known as London dispersion forces, are the weakest intermolecular forces. They result from temporary dipoles that occur when the electron distribution within an atom or molecule becomes asymmetrical.
For example, in \( \text{PH}_3 \), the van der Waals forces are the primary interactions because PH3 does not form hydrogen bonds. These forces are much weaker than hydrogen bonds and ionic bonds, resulting in lower boiling points for substances that exhibit only van der Waals forces.
Polarizability
Polarizability refers to how easily the electron cloud around a molecule or atom can be distorted to form a temporary dipole. Larger atoms or molecules with more electrons are generally more polarizable.
For example, \( \text{I}_2 \) (iodine) is more polarizable than \( \text{Xe} \) (xenon) due to its larger size. This increased polarizability leads to stronger van der Waals forces, resulting in a higher boiling point for I2 compared to Xe.

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Most popular questions from this chapter

Which species in each pair has the greater polarizability? Explain. (a) \(\mathrm{Br}^{-}\) or \(\mathrm{I}^{-}\) (b) \(\mathrm{CH}_{2}=\mathrm{CH}_{2}\) or \(\mathrm{CH}_{3}-\mathrm{CH}_{3}\) (c) \(\mathrm{H}_{2} \mathrm{O}\) or \(\mathrm{H}_{2} \mathrm{Se}\)

Which forces are intramolecular and which intermolecular? (a) Those allowing fog to form on a cool, humid evening (b) Those allowing water to form when \(\mathrm{H}_{2}\) is sparked (c) Those allowing liquid benzene to crystallize when cooled (d) Those responsible for the low boiling point of hexane

What is the strongest interparticle force in each substance? (a) \(\mathrm{CH}_{3} \mathrm{Cl}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{3}\) (c) \(\mathrm{NH}_{3}\)

Polytetrafluoroethylene (Teflon) has a repeat unit with the formula \(\mathrm{F}_{2} \mathrm{C}-\mathrm{CF}_{2}\). A sample of the polymer consists of fractions with the following distribution of chains: $$ \begin{array}{ccc} & \text { Average Number } & \text { Amount (mol) } \\ \text { Fraction } & \text { of Repeat Units } & \text { of Polymer } \\ \hline 1 & 273 & 0.10 \\ 2 & 330 & 0.40 \\ 3 & 368 & 1.00 \\ 4 & 483 & 0.70 \\ 5 & 525 & 0.30 \\ 6 & 575 & 0.10 \end{array} $$ (a) Determine the molar mass of each fraction. (b) Determine the number-average molar mass of the sample. (c) Another type of average molar mass of a polymer sample is called the weight-average molar mass, \(\mathscr{H}_{\mathrm{w}}\) : \(\mathscr{M}_{\mathrm{w}}=\frac{\Sigma(\mathscr{M} \text { of fraction } \times \text { mass of fraction })}{\text { total mass of all fractions }}\) Calculate the weight-average molar mass of the sample of polytetrafluoroethylene.

What type of forces, intramolecular or intermolecular, (a) Prevent ice cubes from adopting the shape of their container? (b) Are overcome when ice melts? (c) Are overcome when liquid water is vaporized? (d) Are overcome when gaseous water is converted to hydrogen gas and oxygen gas?
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