Problem 36 - Chemistry and Chemical Reactivity

Question

Rank the following molecules in order of increasing intermolecular forces: \(\mathrm{CH}_{3} \mathrm{Cl}, \mathrm{HCO}_{2} \mathrm{H}\) (formic acid), and \(\mathrm{CO}_{2}.\)

Step-by-Step Solution

Answer

Rank: \(\mathrm{CO_2} < \mathrm{CH_3Cl} < \mathrm{HCO_2H}\).

Step 1: Identify Molecules and Their Types

First, identify the types of molecules. \(\mathrm{CH_3Cl}\) is chloromethane, \(\mathrm{HCO_2H}\) is formic acid, and \(\mathrm{CO_2}\) is carbon dioxide. Chloromethane is a polar molecule due to the presence of chlorine. Formic acid is also polar and can form hydrogen bonds due to the presence of H-O bonds. Carbon dioxide is a nonpolar molecule.

Step 2: Determine the Types of Intermolecular Forces Present

Next, determine the types of intermolecular forces each molecule can exhibit. \(\mathrm{CH_3Cl}\) has dipole-dipole interactions due to its polarity. \(\mathrm{HCO_2H}\) can have hydrogen bonding, which is stronger than dipole-dipole interactions. \(\mathrm{CO_2}\), being nonpolar, only has London dispersion forces, which are the weakest of the three.

Step 3: Rank the Molecules Based on Their Intermolecular Forces

Finally, rank the molecules from weakest to strongest intermolecular forces. \(\mathrm{CO_2}\) is ranked first as it has the weakest dispersion forces. \(\mathrm{CH_3Cl}\) is next with moderate dipole-dipole interactions. \(\mathrm{HCO_2H}\) is ranked last as it has the strongest intermolecular forces due to hydrogen bonding.

Key Concepts

Polar Molecules

Polar molecules have distinct positive and negative ends, or poles, due to their uneven distribution of electrons.
This occurs because of a difference in electronegativity between the atoms involved. In the case of chloromethane (\(\ce{CH3Cl}\)), chlorine is more electronegative than carbon.
Thus, it pulls the shared electrons toward itself, creating a dipole.
Formic acid (\(\ce{HCO2H}\)) is another example of a polar molecule.

• Polar molecules interact with other polar molecules through attractive forces known as dipole-dipole interactions.
• These interactions influence the physical properties of substances, such as boiling and melting points.
• Polar molecules tend to dissolve well in other polar substances, a principle famously encapsulated in the saying "like dissolves like."

Recognizing polar molecules can help predict how substances will interact in a mixture or solution.

Hydrogen Bonding

Hydrogen bonding represents one of the strongest types of intermolecular forces that occur.
It happens when hydrogen is covalently bonded to a highly electronegative element such as oxygen, nitrogen, or fluorine.
This bond leaves hydrogen with a partial positive charge, which then strongly attracts adjacent electronegative atoms.
Formic acid (\(\ce{HCO2H}\)) displays hydrogen bonding due to its -OH group.

• Hydrogen bonds are critical in determining the properties of water, a fundamental solvent in chemistry.
• They influence the high boiling and melting points of substances such as alcohol and formic acid.
• Hydrogen bonds are responsible for the unique structures of proteins and DNA.

Understanding hydrogen bonding offers insights into the realm of biological molecules and their complex behaviors.

Dipole-Dipole Interactions

Dipole-dipole interactions are present when polar molecules are close to each other.
These forces arise because the positive end of one dipole molecule is attracted to the negative end of another.
This kind of attraction explains why chloromethane (\(\ce{CH3Cl}\)) undergoes moderate dipole-dipole interactions.

• These interactions are stronger than London dispersion forces, yet weaker than hydrogen bonds.
• They contribute to the higher boiling points of polar substances compared to nonpolar ones.
• Dipole-dipole interactions are crucial in determining solubility and the physical state of molecular compounds.

Grasping the concept of dipole-dipole interactions is essential for understanding how molecules attract each other in solutions.

London Dispersion Forces

London dispersion forces, although the weakest of the intermolecular forces, are universal since they are present in all molecules, whether polar or nonpolar.
These forces arise due to temporary dipoles that occur when electron clouds fluctuate within molecules at close proximity.
Nonpolar molecules like carbon dioxide (\(\ce{CO2}\)) rely primarily on these forces.

• London dispersion forces are significant in large, heavy atoms or molecules as they have more electrons to form temporary dipoles.
• They give rise to the existence of solids and liquids of nonpolar substances at low temperatures.
• Larger molecules often have greater dispersion forces, affecting their boiling and melting points.

Understanding London dispersion forces reveals much about how substances behave in different physical states.