Problem 32 - Chemistry and Chemical Reactivity

Question

Select the substance in each of the following pairs that should have the higher boiling point: (a) \(\mathrm{Br}_{2}\) or \(\mathrm{ICl}\) (b) neon or krypton (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) (ethanol) or \(\mathrm{C}_{2} \mathrm{H}_{4} \mathrm{O}\) (ethylene oxide, structure below) (EQUATION CAN'T COPY)

Step-by-Step Solution

Answer

(a) ICl; (b) krypton; (c) ethanol.

Step 1: Understanding Boiling Points

The boiling point of substances depends on the type and strength of intermolecular forces present. The stronger the intermolecular forces, the higher the boiling point of the substance.

Step 2: Compare Bromine and Iodine Chloride

In pair (a), \(\mathrm{Br}_{2}\) is a non-polar molecule with London dispersion forces. \(\mathrm{ICl}\) is a polar molecule that can exhibit dipole-dipole interactions. Since dipole-dipole interactions are typically stronger than dispersion forces, \(\mathrm{ICl}\) should have a higher boiling point than \(\mathrm{Br}_{2}\).

Step 3: Compare Neon and Krypton

For pair (b), both neon and krypton are noble gases and exhibit London dispersion forces, which get stronger with increasing atomic mass. Krypton ( oble gas with higher atomic mass) will have stronger dispersion forces and thus a higher boiling point than neon.

Step 4: Compare Ethanol and Ethylene Oxide

In pair (c), \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) (ethanol) can form hydrogen bonds due to the presence of the hydroxyl (OH) group, while \(\mathrm{C}_{2} \mathrm{H}_{4} \mathrm{O}\) (ethylene oxide) primarily exhibits dipole-dipole interactions. Hydrogen bonding is a stronger intermolecular force than dipole-dipole interactions, making ethanol have a higher boiling point than ethylene oxide.

Key Concepts

Intermolecular Forces

Boiling points are a fascinating aspect of chemistry that are deeply tied to the concept of intermolecular forces. These forces are attractions between molecules that determine many physical properties of substances, including their boiling points. The stronger these forces are, the more energy is needed to separate the molecules when a substance boils. There are several types of intermolecular forces, each varying in strength:

• London Dispersion Forces: The weakest forces, acting between all atoms and molecules.
• Dipole-Dipole Interactions: Stronger than dispersion forces, occurring between polar molecules.
• Hydrogen Bonding: The strongest type, occurring when hydrogen is bound to highly electronegative atoms like nitrogen, oxygen, or fluorine.

Understanding these forces helps in predicting and rationalizing the boiling points of different substances. Thus, knowing the types of intermolecular forces present in a molecule provides valuable insight into its physical properties.

Dipole-Dipole Interactions

Dipole-dipole interactions are a significant type of intermolecular force that occurs between polar molecules. These interactions result from the attraction between the positive end of one polar molecule and the negative end of another. Polar molecules have permanent dipoles due to the unequal sharing of electrons in their polar bonds. This inequality causes one end of the molecule to be slightly positive and the other to be slightly negative. Key Characteristics:

• Stronger than London dispersion forces but weaker than hydrogen bonds.
• Impact the boiling point by increasing it, as these interactions require more energy to break.
• Found in molecules like iodine chloride (ICl), which has a higher boiling point than bromine (Br₂), due to its ability to engage in dipole-dipole interactions.

By understanding dipole-dipole interactions, we gain insight into how molecular polarity influences boiling points and other physical properties.

Hydrogen Bonding

Hydrogen bonding is an exceptionally strong type of intermolecular force that significantly impacts the boiling points of substances. It occurs when hydrogen is covalently bonded to highly electronegative atoms, such as nitrogen, oxygen, or fluorine, creating a strong dipole. What makes hydrogen bonding so unique is its strength. Even though it is technically a dipole-dipole interaction, the specific bond configuration between hydrogen and these electronegative atoms makes for very strong attractions. Noteworthy Effects:

• Substantially raises the boiling points of compounds that exhibit it, like ethanol (CH₃CH₂OH), which can hydrogen bond due to the presence of the hydroxyl group.
• Molecules with hydrogen bonds have higher boiling points than those without, due to more energy needed to break these strong interactions.

Recognizing when hydrogen bonding can occur helps to explain why certain substances have higher boiling points than you might predict based solely on molecular weight.

London Dispersion Forces

The most universal type of intermolecular force, London dispersion forces, occur between all molecules, whether polar or nonpolar. They are the result of momentary charge imbalances caused by the movement of electrons, which induce temporary dipoles. Despite being the weakest of the intermolecular forces, London dispersion forces play a vital role in determining the boiling points of nonpolar substances. Important Aspects:

• The strength of these forces increases with molecular size and mass, as larger atoms or molecules have more electrons that can contribute to temporary dipoles.
• For example, in noble gases like neon and krypton, the difference in boiling points can be attributed to the greater mass and larger number of electrons in krypton, resulting in stronger London dispersion forces compared to neon.
• These forces, though weaker than other types, are crucial for understanding the behavior of nonpolar molecules and elements.

Understanding London dispersion forces is key to grasping how even very weak interactions can have significant effects on the physical properties of substances.