Problem 81 - Introductory Chemistry Atoms First

Question

Arrange in order of increasing boiling point: \(\mathrm{CO}_{2}, \mathrm{SO}_{2}, \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}, \mathrm{Al}\)

Step-by-Step Solution

Answer

The substances in order of increasing boiling points are: \(\mathrm{CO}_{2}\), \(\mathrm{SO}_{2}\), \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\), and \(\mathrm{Al}\).

Step 1: Identify the molecular structure and intermolecular forces for each substance

1. CO2: Linear molecule, symmetric, non-polar. Its primary intermolecular force is London dispersion forces. 2. SO2: Bent molecule, asymmetrical with sulfur double bonded to 2 oxygens. It has a polar structure, leading to dipole-dipole forces. 3. CH3CH2OH (Ethanol): Ethanol molecule contains a polar O-H group, allowing for hydrogen bonding in addition to dipole-dipole forces. 4. Al: Metallic bonding and as a pure metal, is held together by ion-dipole forces

Step 2: Compare the strength of their intermolecular forces

Rank the strength of the intermolecular forces from weakest to strongest: 1. London dispersion force: CO2 2. Dipole-Dipole force: SO2 3. Hydrogen bonding: CH3CH2OH 4. Ion-Dipole force: Al

Step 3: Arrange the substances in order of increasing boiling points

Since the boiling point depends on the strength of the intermolecular forces, the substances can be arranged using the order of intermolecular forces we determined in step 2: 1. CO2 (London dispersion force) 2. SO2 (Dipole-Dipole force) 3. CH3CH2OH (Hydrogen bonding) 4. Al (Ion-Dipole force) Thus, the substances in order of increasing boiling points are: CO2, SO2, CH3CH2OH, Al

Key Concepts

Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion which act between neighboring particles (molecules, atoms, or ions). These forces directly influence the physical properties of substances, including boiling points. Higher intermolecular forces equate to higher boiling points as more energy is required to overcome these forces and change the substance from a liquid to a vapor. Typically, the stronger the intermolecular forces, the higher the boiling point of the substance.

There are several types of intermolecular forces:

• London Dispersion Forces: These are the weakest and are present in all molecules, whether they are polar or non-polar. They are the only forces present in non-polar substances like \(\mathrm{CO}_{2}\).
• Dipole-Dipole Forces: These occur when there is an electronegativity difference between bonded atoms, creating a permanent dipole. Molecules such as \(\mathrm{SO}_{2}\) exhibit these forces.
• Hydrogen Bonding: A special, stronger form of dipole-dipole interactions, hydrogen bonds occur specifically between hydrogen atoms and strongly electronegative atoms like N, O, or F. Ethanol (\(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{OH}\)) is a classic example featuring hydrogen bonding.
• Ion-Dipole Forces: Present between ions and polar molecules, these are among the strongest intermolecular forces. In metals such as Aluminum (\(\mathrm{Al}\)), these forces significantly raise the boiling point.

Molecular Structure

The molecular structure of a substance greatly influences its physical properties like boiling point. The arrangement and shape of molecules affect how they interact with each other, determining the available intermolecular forces. For instance, \(\mathrm{CO}_{2}\) has a linear and symmetric structure, which makes it non-polar and only susceptible to London dispersion forces. This weak interaction leads to a lower boiling point.

In contrast, \(\mathrm{SO}_{2}\) has a bent structure due to electron pair repulsion, creating an uneven charge distribution and making the molecule polar. This enables stronger dipole-dipole interactions, raising its boiling point compared to non-polar molecules. Similarly, the structure of ethanol includes a hydroxyl group (OH) that facilitates hydrogen bonding, considerably increasing its boiling point. Understanding the structure provides crucial insight into the nature and strength of interactions possible within and between molecules.

Hydrogen Bonding

Hydrogen bonding is a pivotal concept in understanding chemical properties, as it significantly influences the boiling points of substances. This type of bond forms when a hydrogen atom, covalently bonded to an electronegative atom like oxygen, nitrogen, or fluorine, experiences attraction to another electronegative atom. Even though they are not true bonds, these interactions are considerably stronger than van der Waals forces.

In ethanol (\(CH_{3}CH_{2}OH\)), the presence of an -OH group makes it capable of forming hydrogen bonds due to the hydrogen's polarity. This results in stronger attraction between molecules, requiring more energy to vaporize them, thus leading to a higher boiling point. Compared to other molecules like \(\mathrm{CO}_{2}\) and \(\mathrm{SO}_{2}\), ethanol's ability to hydrogen bond gives it a distinguishing feature that manifests in its significantly higher boiling point. This not only highlights the strength of hydrogen bonds but also provides an avenue to predict and explain discrepancies in boiling points among different substances.