Question

When methanol, \(\mathrm{CH}_{3} \mathrm{OH},\) is dissolved in water, a nonconducting solution results. When acetic acid, \(\mathrm{CH}_{3} \mathrm{COOH},\) dissolves in water, the solution is weakly conducting and acidic in nature. Describe what happens upon dissolution in the two cases, and account for the different results.

Short answer

When methanol (CH3OH) dissolves in water, it forms hydrogen bonds with water molecules without creating any ions, resulting in a nonconducting and nonacidic solution. However, when acetic acid (CH3COOH) dissolves in water, it partially ionizes, releasing a percentage of hydrogen (H+) ions and acetate (CH3COO-) ions. These ions contribute to the weak electric conductivity and acidic nature of the solution. In summary, the different behaviors of these two substances upon dissolution in water can be attributed to the presence or absence of ion formation.

Step-by-step solution

1. Understanding Methanol Dissolution

Methanol (CH3OH) is a simple alcohol. When it dissolves in water, the polar nature of the molecule (specifically the O-H bond) allows it to interact with the water molecules through hydrogen bonding. However, methanol does not form any ions in this process, which means that it does not contribute to the electric conductivity of the solution.

2. Understanding Acetic Acid Dissolution

Acetic acid (CH3COOH) is a weak acid. When it dissolves in water, it partially ionizes, meaning that it releases a percentage of hydrogen(H+) ions and acetate (CH3COO-) ions: \[CH_3COOH_{(aq)} \rightleftharpoons H^+_{(aq)} + CH_3COO^-_{(aq)}\] Conversely to methanol, these ions contribute to the electric conductivity of the solution, although to a comparatively minor extent due to the partial ionization of acetic acid.

3. Accounting for Acidity

The acidic nature of an aqueous solution can be attributed to the presence of hydrogen ions (H+) in that solution. In the case of acetic acid, the partial ionization results in the release of H+ ions, rendering the solution acidic. On the other hand, methanol does not release any such ions, and hence its solution with water does not exhibit acidic properties.

4. Comparing the Two Cases

In summary, the dissolution of methanol (CH3OH) in water results in a nonconducting and nonacidic solution, as the water molecules form hydrogen bonds with methanol molecules without creating any ions. Conversely, the dissolution of acetic acid (CH3COOH) in water leads to a weakly conducting and acidic solution because of its partial ionization that generates H+ and CH3COO- ions.

Key concepts

Methanol Dissolution in Water

Methanol, commonly known as wood alcohol, is a polar substance due to its O-H group. When methanol is mixed with water, each methanol molecule forms hydrogen bonds with surrounding water molecules. This process is known as dissolution.

During methanol dissolution, the molecules disperse evenly throughout the water, but crucially, they stay as intact CH3OH molecules rather than breaking down into ions. Due to not forming ions, methanol in water doesn't increase the water's ability to conduct electricity, leading to a nonconducting solution. This keeps the electrical conductivity of the solution fairly similar to that of pure water.

Acetic Acid Ionization

Unlike methanol, acetic acid is a weak acid and it partially ionizes in water. This ionization can be represented by the equilibrium:
\[CH_3COOH_{(aq)} \rightleftharpoons H^+_{(aq)} + CH_3COO^-_{(aq)}\]

This means that some of the acetic acid molecules lose a hydrogen ion (H+), which becomes a free proton in solution, and in turn leaves behind an acetate ion (CH3COO-). Since the solution gains these charged particles, H+ and CH3COO-, it conducts electricity. However, because acetic acid is a weak acid, it doesn't ionize completely, so the solution is only weakly conductive.

Conductivity of Solutions

The ability of a solution to carry an electric current is known as its conductivity. This property is directly related to the presence of free ions in the solution.

Substances that dissolve to form free ions, like salts, strong acids, and strong bases, make a solution highly conductive. However, if a substance does not produce ions, such as sugar or methanol, or only partially dissociates into ions, like acetic acid, the resulting solution will have low conductivity. As conductivity is crucial in many applications, understanding the ionization behavior of solutes is vital.

Hydrogen Bonding

Hydrogen bonding is a specific type of dipole-dipole interaction that occurs between a hydrogen atom bonded to a highly electronegative atom, such as oxygen or nitrogen, and another electronegative atom with a lone pair of electrons.

H4. Why Hydrogen Bonds Are Special
In methanol and water, hydrogen bonds form due to the attraction between the slightly positive hydrogen atoms and the slightly negative oxygen atoms. These bonds are responsible for the high boiling point of water, the solubility of many polar compounds in water, and the complex three-dimensional structures of proteins and DNA. While not as strong as covalent or ionic bonds, hydrogen bonds are stronger than van der Waals forces and are crucial in the dissolution process.

Weak Acids

Weak acids, such as acetic acid, do not fully dissociate into their constituent ions in solution. They are in a dynamic equilibrium between the unionized acid and the ions it forms.

H4. The Nature of Weak Acid Equilibrium
This equilibrium is usually far to the left, with only a small portion of the weak acid molecules ionizing. Weak acids produce a lower concentration of H+ ions compared to strong acids at the same concentration, resulting in a higher pH level for their solutions. Their acidity can be quantified using a value known as the acid dissociation constant (Ka), which gives insight into the strength of the acid in aqueous solutions.