Problem 2 - Chemistry: The Central Science

Question

Methanol, \(\mathrm{CH}_{3} \mathrm{OH}\), and hydrogen chloride, \(\mathrm{HCl}\), are both molecular substances, yet an aqueous solution of methanol does not conduct an electrical current, whereas a solution of \(\mathrm{HCl}\) does conduct. Account for this difference. [Section 4.1]

Step-by-Step Solution

Answer

An aqueous solution of methanol, CH3OH, does not conduct electricity because methanol does not dissociate into ions when dissolved in water; it remains a neutral molecule interacting with water molecules via hydrogen bonds. In contrast, an aqueous solution of HCl conducts electricity because HCl dissociates into hydronium ions (H3O+) and chloride ions (Cl-) when it reacts with water: \[HCl + H_2O \rightarrow H_3O^+ + Cl^-\] The presence of mobile ions (H3O+ and Cl-) in the HCl solution enables the conduction of electricity, while the absence of ions in the methanol solution makes it non-conductive.

Step 1: Nature of Methanol in Aqueous Solution

When methanol is dissolved in water, it forms hydrogen bonds with water molecules but remains as a neutral molecule. In other words, methanol does not dissociate into ions. Since the conduction of electricity requires the movement of charged particles, and there are no ions in a solution of methanol in water, it does not conduct electricity.

Step 2: Nature of Hydrogen Chloride in Aqueous Solution

On the other hand, when hydrogen chloride (HCl) is dissolved in water, it undergoes a reaction to form hydronium ions (H3O+) and chloride ions (Cl-) as follows: \[HCl + H_2O \rightarrow H_3O^+ + Cl^-\] Since the solution now contains mobile ions (H3O+ and Cl-), it can conduct electricity as these charged particles can move and carry an electrical current.

Step 3: Comparison

The difference in the conductive properties of aqueous solutions of methanol and HCl arises from the fact that methanol does not dissociate into ions when dissolved in water, whereas HCl does. The presence of ions in the HCl solution allows it to conduct electricity, while the absence of ions in the methanol solution renders it non-conductive.

Key Concepts

Ionization

Ionization refers to the process in which a molecule or atom gains or loses an electron, resulting in the formation of ions. This concept is crucial in understanding why certain substances can conduct electricity when dissolved in water. When hydrogen chloride (HCl) is added to water, it undergoes ionization. This chemical reaction produces hydronium ions \[ HCl + H_2O \rightarrow H_3O^+ + Cl^- \] These ions are charged particles that move freely in solution.
This movement of ions allows the solution to conduct an electric current. In essence, ionization in water involves transforming a neutral molecule into charged particles, hence enabling conductivity.
In contrast, methanol does not undergo ionization in water. It remains as a neutral molecule, resulting in no charged particles to carry an electrical current. Therefore, solutions of methanol in water do not conduct electricity.

Hydrogen Bonding

Hydrogen bonding is a type of weak attraction that occurs between the positive charge of hydrogen atoms and the negative charge of other electronegative atoms, often oxygen or nitrogen. In the case of methanol, each molecule forms hydrogen bonds with water molecules. This molecular interaction is necessary for methanol to dissolve in water.
However, these hydrogen bonds don't result in ion formation. Instead, methanol remains neutral because hydrogen bonding does not involve electron transfer but rather a form of intermolecular attraction. Because of this, even though methanol is soluble in water, it won't conduct electricity.

• Methanol interacts with water through hydrogen bonding.
• No ionization means no charged particles for conductivity.

Hydrogen chloride, while also capable of forming hydrogen bonds when dissolved in water, undergoes ionization and results in ion formation, enabling electric conductivity.

Aqueous Solutions

An aqueous solution is a homogeneous mixture where water acts as the solvent. This type of solution can significantly alter the properties of solutes. An essential property affected is electrical conductivity.
When hydrogen chloride is dissolved in water, it creates an aqueous solution that conducts electricity due to produced ions. Electrolytes, like HCl, dissociate into ions that move and carry electrical charges.
In contrast, when methanol is dissolved in water to form an aqueous solution, it does not produce ions and remains electrically non-conductive.

• Aqueous solution simply means a substance is dissolved in water.
• Conductivity of an aqueous solution depends on the presence of ions.
• An electrolyte is a solute that forms ions in the solution.

Aqueous solutions can be either conductive, like in the case of ionized substances, or non-conductive, as seen with substances like methanol that do not ionize.