Question

Pure acetic acid is a viscous liquid, with high melting and boiling points \(\left(16.7^{\circ} \mathrm{C}\right.\) and \(\left.118^{\circ} \mathrm{C}\right)\) compared to compounds of similar molecular weight. Suggest an explanation.

Short answer

The high melting and boiling points of pure acetic acid compared to compounds of similar molecular weight can be attributed to its strong intermolecular forces, including both dipole-dipole interactions and hydrogen bonding. The presence of these strong forces means that more energy is required to overcome them, thus resulting in elevated melting and boiling points. Comparatively, compounds like ethyl ether, with weaker intermolecular forces, exhibit lower boiling points.

Step-by-step solution

Identify the molecular structure of acetic acid

Acetic acid has the molecular formula CH3COOH. The structure of acetic acid includes a carboxyl group (-COOH) bonded to a methyl group (CH3). In the carboxyl group, there's an oxygen atom double bonded to a carbon atom, and a hydroxyl group (-OH) bonded to the same carbon atom. This structure plays an important role in the intermolecular forces it exhibits.

Identify the intermolecular forces acting in acetic acid

Intermolecular forces are the forces between molecules that hold them together. There are three main types of intermolecular forces: London dispersion forces, dipole-dipole interactions, and hydrogen bonding. Due to the presence of polar bonds (C=O and O-H) in the acetic acid molecule, it exhibits dipole-dipole interactions. Also, the presence of a hydroxyl group (-OH) allows for strong hydrogen bonding interactions.

Explain the effect of intermolecular forces on physical properties

Substances with strong intermolecular forces require more energy to overcome these forces, leading to higher melting and boiling points. In the case of acetic acid, both dipole-dipole and hydrogen bonding interactions contribute to the strength of the intermolecular forces, leading to high melting and boiling points.

Compare acetic acid with compounds of similar molecular weight

Let's compare acetic acid to a compound with a similar molecular weight that doesn't have strong intermolecular forces, such as ethyl ether (C2H5OCH2CH3). Ethyl ether has a lower boiling point of 34.6°C due to weaker London dispersion forces and minor dipole-dipole interactions. There is no hydrogen bonding present in ethyl ether, making its intermolecular forces weaker than those in acetic acid.

Conclusion

The high melting and boiling points of pure acetic acid compared to compounds of similar molecular weight can be explained by the presence of strong intermolecular forces in acetic acid, particularly hydrogen bonding, which require more energy to overcome.

Key concepts

Hydrogen Bonding in Acetic Acid

Hydrogen bonding is a highly significant type of intermolecular force, especially for molecules that contain hydrogen linked to electronegative atoms, such as oxygen or nitrogen. In the case of acetic acid, which has the chemical structure CH3COOH, hydrogen bonding plays a crucial role.
Acetic acid contains a hydroxyl group (-OH). In this group, the oxygen is electronegative, creating a partial negative charge, while the hydrogen acquires a partial positive charge.
This polarity allows the hydrogen to form strong bonds with other oxygen atoms in nearby acetic acid molecules.
Key Features of Hydrogen Bonding:

• Occurs when hydrogen is bonded to electronegative atoms like oxygen.
• Results in significant attraction between molecules.
• Leads to higher boiling and melting points as more energy is needed to break these bonds.

The extensive hydrogen bonds in acetic acid mean that a lot of energy is required to overcome these forces, which results in the compound's relatively high boiling and melting points when compared to molecules of similar size.

Dipole-Dipole Interactions

Dipole-dipole interactions occur between two polar molecules. These interactions arise because polar molecules feature regions of partial positive and negative charges, creating an electrostatic attraction.
Acetic acid is a molecule with a notable amount of dipole-dipole interactions due to its polar bonds, specifically the carbon-oxygen double bond (C=O) in the carboxyl group, and the hydroxyl group (O-H).
This makes acetic acid significantly polar.
Characteristics of Dipole-Dipole Interactions:

• Involve the attraction of opposite charges from adjacent molecules.
• Are stronger than London dispersion forces, but weaker than hydrogen bonds.
• They influence the physical state and solubility of the molecules.

Though not as strong as hydrogen bonds, dipole-dipole interactions still contribute to the overall stronger intermolecular forces in acetic acid, enhancing its physical properties such as increased boiling and melting points.

Physical Properties of Acetic Acid

The physical properties of acetic acid, such as its high boiling and melting points, are directly tied to its molecular interactions. At the heart of these properties are the strong intermolecular forces—mainly, hydrogen bonding and dipole-dipole interactions.
For acetic acid, these interactions create a tightly bound molecular network, resisting phase changes like melting or boiling.
Impact on Physical Properties:

• High boiling point: At 118°C, it's significantly higher than many similar molecules without substantial hydrogen bonding.
• High melting point: With a melting point of 16.7°C, acetic acid is a solid at relatively low temperatures, emphasizing the strength of its bonds.
• Increased viscosity: The molecule’s ability to form pairs (dimers) through hydrogen bonds increases the liquid’s thickness.

Comparatively, acetic acid is more resistant to changes in its physical state, especially when juxtaposed against molecules with weaker intermolecular forces, such as ethyl ether.