Question

Would you expect pure acetic acid to be a strongly hydrogenbonded substance? How do the melting and boiling points of the substance \(\left(16.7^{\circ} \mathrm{C}\right.\) and \(\left.118^{\circ} \mathrm{C}\right)\) support your answer?

Short answer

Yes, pure acetic acid is a strongly hydrogen-bonded substance. This is evident from its comparatively higher melting (16.7 °C) and boiling points (118 °C) compared to similar substances like acetone, which has a melting point of -94.7 °C and a boiling point of 56.05 °C. These higher values indicate stronger intermolecular forces, consistent with the presence of hydrogen bonding in acetic acid facilitated by the -OH group.

Step-by-step solution

Identify Hydrogen Bonding

Hydrogen bonding occurs when a hydrogen atom bonded to a highly electronegative atom (like nitrogen, oxygen, or fluorine) attracts another electronegative atom, thus forming a weak bond. In the case of acetic acid (CH3COOH), the hydrogen atom in the -OH group can form hydrogen bonds with other electronegative atoms, such as oxygen.

Analyze Melting and Boiling Points

In general, substances with strong hydrogen bonding have higher melting and boiling points compared to substances without hydrogen bonding or those with weaker bonds. This is because hydrogen bonding leads to additional intermolecular forces holding the substance together, and these forces need to be overcome to change the state of the substance from solid to liquid or from liquid to gas. In the case of acetic acid, the melting point is 16.7 °C, and the boiling point is 118 °C. Let's analyze these values to determine if acetic acid is strongly hydrogen-bonded.

Compare with Similar Substances

To determine if acetic acid is strongly hydrogen-bonded, we can compare its melting and boiling points with those of other similar substances that do not form hydrogen bonding. A common substance for comparison is acetone (CH3COCH3), which has a melting point of -94.7 °C and a boiling point of 56.05 °C.

Draw Conclusions on Hydrogen Bonding and Melting/Boiling Points

Comparing the melting and boiling points of acetic acid to those of acetone, we can see that acetic acid has higher values for both points. This indicates that there are stronger intermolecular forces at play in acetic acid, which is consistent with the presence of hydrogen bonding. Based on this comparison and the presence of a -OH group in acetic acid, we can conclude that acetic acid is indeed a strongly hydrogen-bonded substance. The higher melting and boiling points of acetic acid compared to acetone serve as supporting evidence for this conclusion.

Key concepts

Acetic Acid

Acetic acid, known chemically as CH₃COOH, is an essential molecule in chemistry, especially when discussing hydrogen bonding. It's a simple carboxylic acid found in household vinegar, where it gives vinegar its sour taste and pungent smell. The molecular structure of acetic acid includes a carbonyl group (C=O) and a hydroxyl group (-OH), making it a polar molecule.

This polarity is key because it allows acetic acid to participate in hydrogen bonding, one of the most vital intermolecular forces. In the structure of acetic acid, the hydrogen atom of the -OH group can form temporary bonds with other electronegative atoms such as oxygen from other acetic acid molecules. This interaction leads to the formation of linked chains or dimers, significantly impacting its boiling and melting points.

Melting Point

The melting point is the temperature at which a solid turns into a liquid. For acetic acid, this temperature is 16.7°C. At this point, the molecules gain enough energy to overcome some intermolecular forces, specifically, those due to hydrogen bonding, which keeps the molecules in a stable, fixed position in a solid.

A higher melting point usually suggests stronger intermolecular forces, such as the hydrogen bonds in acetic acid, requiring more energy for these bonds to be broken. The melting point of acetic acid is considerably higher than that of other similar-sized molecules that do not engage in significant hydrogen bonding, like acetone. This high melting point is indicative of the stronger interactions between acetic acid molecules.

Boiling Point

The boiling point of a substance is the temperature at which its liquid form becomes a gas. For acetic acid, this temperature is 118°C. This comparatively high boiling point reflects the strong hydrogen bonding between acetic acid molecules. These molecules need substantial energy input to break away from the liquid form completely.

In liquids like acetic acid, intermolecular hydrogen bonds must be overcome; this does not occur until sufficiently high temperatures provide the needed kinetic energy. Against reference substances like acetone (boiling at 56.05°C), acetic acid's boiling point is high. This largely reinforces the idea that acetic acid doesn't just form hydrogen bonds, but forms rather strong ones, markedly impacting its phase change points.

Intermolecular Forces

Intermolecular forces are interactions that occur between molecules, influencing their physical properties, such as melting and boiling points. Hydrogen bonding is a particularly potent type of intermolecular force present in molecules with polar bonds where hydrogen is involved, such as in acetic acid.

These forces are crucial in determining the state and stability of substances. In the case of acetic acid, hydrogen bonds keep molecules closely knit, resulting in higher melting and boiling points compared to substances without such strong forces, like acetone. Ultimately, these bonds impact everything from solubility to the viscosity of liquids, making them a central concept in understanding molecular behavior.