Question

What is the Lowry-Bensted acid in (a) HCl dissolved in water; (b) HC1 (unionized) dissolved in benzene? (c) Which solution is the more strongly acidic?

Short answer

The Lowry-Bensted acid is HCl in both (a) HCl dissolved in water and (b) HCl (unionized) dissolved in benzene. However, HCl dissolved in water is more strongly acidic due to its nearly 100% ionization and formation of hydronium ions (H3O+), while HCl remains mostly unionized in benzene and doesn't have a strong acidic nature. Therefore, the more strongly acidic solution is HCl dissolved in water (solution a).

Step-by-step solution

Identify the Lowry-Bensted acids in each solution.

(a) In a solution of HCl dissolved in water, the Lowry-Bensted acid is HCl. (b) In an HCl (unionized) dissolved in benzene solution, the Lowry-Bensted acid is also HCl.

Compare the acidic properties of the two solutions.

To determine which solution is more strongly acidic, we need to consider the extent of proton (H+) ionization in each solution. (a) In the case of HCl dissolved in water, it ionizes nearly 100% to produce H+ (as hydronium ions, H3O+) and Cl- ions: \[ HCl_{(aq)} + H_2O_{(l)} \rightarrow H_3O^+_{(aq)} + Cl^-_{(aq)} \] This means the HCl in this solution acts as a strong acid. (b) In the case of HCl dissolved in benzene (a nonpolar solvent), the HCl remains unionized, meaning it doesn't dissociate into its constituent ions: \[ HCl_{(benzene)} \rightleftharpoons H^+_{(benzene)} + Cl^-_{(benzene)} \] Since HCl remains largely unionized in benzene, it doesn't act as a strong acid in this solution.

Determine the more strongly acidic solution.

Comparing the two solutions, HCl dissolved in water exhibits a more strongly acidic character due to its nearly 100% ionization and formation of hydronium ions (H3O+). On the other hand, HCl remains mostly unionized in benzene and doesn't have a strong acidic nature. So, the more strongly acidic solution is HCl dissolved in water (solution a).

Key concepts

Acidic Properties

The acidic properties of a substance are primarily defined by its ability to donate protons (H⁺ ions) to another substance. This characteristic is explained by the Bronsted-Lowry theory of acids and bases, where an acid is defined as a proton donor and a base is a proton acceptor.

When HCl is dissolved in water, it essentially splits into its ions, releasing H⁺ ions which attach to water molecules to form hydronium ions \(H_3O^+\). This process is an indicator of acidic strength because a high concentration of hydronium ions in solution corresponds to stronger acidic properties.

In the case of the HCl in benzene, a nonpolar solvent, the typical ionization does not occur, meaning HCl remains largely unionized. Its ability to donate protons is severely restricted, which makes it exhibit weak acidic properties in this context.

Ionization in Solvents

Ionization refers to the process where molecules split into ions when dissolved in a solvent. The extent to which a compound like HCl ionizes can greatly depend on the nature of the solvent it's dissolved in. For example, in water, HCl nearly completely ionizes:

• Water is a polar solvent that stabilizes the ions produced, facilitating the dissociation of HCl into H⁺ and Cl⁻ ions.
• The complete ionization in water means almost all HCl molecules dissociate, leading to a high concentration of free hydronium ions (\(H_3O^+\)).

In contrast, benzene is a nonpolar solvent and does not stabilize the ions effectively.

• This results in HCl only partially ionizing or not ionizing much at all, keeping HCl mostly in its molecular form.
• The lack of ionization in benzene means the acid cannot show its full acidic nature in this solvent.

Strong vs Weak Acids

Acids are categorized as strong or weak depending on their ability to ionize fully in a solution. A strong acid like HCl, when dissolved in an appropriate solvent such as water, typically ionizes almost completely.

Strong Acids:

• In water, HCl ionizes nearly 100%, indicating that it is a strong acid. It releases a significant amount of H⁺ ions, leading to a powerful acidic effect.
• Strong acids typically have a lower pH due to high hydronium ion concentration.

Weak Acids:

• When HCl is placed in benzene, it acts like a weak acid. This is because benzene doesn't support the ionization of HCl.
• With less ionization, fewer H⁺ ions are available, resulting in a weaker acid presence.

It's important to remember that the same acid can behave differently depending on the solvent, highlighting the sensitivity of acidic behavior to environmental conditions.