Problem 52 - 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: \(\mathrm{CO}_2\), \(\mathrm{CH}_3\mathrm{Cl}\), \(\mathrm{HCO}_2\mathrm{H}\).

Step 1: Identify the Types of Intermolecular Forces

Determine the predominant intermolecular forces for each molecule. \(\mathrm{CH}_3\mathrm{Cl}\) has dipole-dipole interactions due to its polar nature. \(\mathrm{HCO}_2\mathrm{H}\) is capable of hydrogen bonding because of the presence of both an acidic proton and an electronegative oxygen atom. \(\mathrm{CO}_2\) is a nonpolar molecule, experiencing London dispersion forces only.

Step 2: Assign Strength to Each Type of Force

Understand the relative strength of the intermolecular forces: London dispersion forces (weakest), dipole-dipole interactions (moderately strong), and hydrogen bonding (strongest). Based on this, hydrogen bonding is stronger than dipole-dipole interactions, which are stronger than London dispersion forces.

Step 3: Rank Molecules by Intermolecular Force Strength

Based on the identified forces, rank the molecules from weakest to strongest intermolecular forces: \(\mathrm{CO}_2\), due to dispersion forces only, \(\mathrm{CH}_3\mathrm{Cl}\), which has dipole-dipole interactions, and \(\mathrm{HCO}_2\mathrm{H}\), capable of hydrogen bonding.

Key Concepts

Dipole-Dipole Interactions

Dipole-dipole interactions occur between polar molecules where there is a partial positive charge on one side and a partial negative charge on the other due to differences in electronegativity. These molecules align themselves such that the positive side of one molecule attracts the negative side of another, much like magnets.
Your typical example of a molecule that experiences dipole-dipole interactions is \(\mathrm{CH}_3\mathrm{Cl}\) (chloromethane). Here, the chlorine atom is much more electronegative than carbon, which creates a dipole moment. This causes the molecules to align in a way where opposite charges attract.

• These forces are moderate in strength compared to other intermolecular forces.
• They greatly influence boiling and melting points, making polar substances boil at higher temperatures compared to nonpolar ones.
• They are essential for understanding how substances mix or repel each other based on polarity.

Hydrogen Bonding

Hydrogen bonding represents one of the strongest forms of intermolecular attraction. However, it's not a true chemical bond. It occurs specifically in molecules where hydrogen is covalently bonded to highly electronegative atoms like nitrogen, oxygen, or fluorine.
A perfect case is formic acid (\(\mathrm{HCO}_2\mathrm{H}\)). Here, the hydrogen atom is bound to an oxygen atom, which due to its high electronegativity, pulls the electron cloud towards itself, leaving hydrogen slightly positive. It's this positive hydrogen that can attract nearby electronegative atoms forming a hydrogen bond.

• This interaction is particularly significant in biological structures, like DNA, where it holds the helical structure together.
• Hydrogen bonds explain water's high boiling point and heat capacity.
• These interactions are pivotal in determining the properties and structure of many organic and inorganic compounds.

London Dispersion Forces

London dispersion forces, sometimes known as Van der Waals forces, are the weakest intermolecular forces but are present in all molecules, whether polar or non-polar. These forces arise due to temporary fluctuations in electron density in a molecule, leading to temporary dipoles.
Consider \(\mathrm{CO}_2\) (carbon dioxide), a linear molecule that is nonpolar. Even in such molecules, electrons are constantly moving, and at any instant, the electron cloud might be denser on one side, creating a temporary dipole.

• These forces increase in strength with the number of electrons—larger molecules exhibit stronger dispersion forces.
• They are crucial in nonpolar molecular interactions, influencing the liquefaction and solidification of noble gases.
• While weak, they are significant because they enable gaseous molecules to form liquids and solids under low temperatures.