Problem 71 - Chemistry Matter and Change

Question

Disinfectant Hypochlorous acid is an industrial disinfectant. Write the chemical equation and the \(K_{\text {a }}\) expression for the ionization of hypochlorous acid in water.

Step-by-Step Solution

Answer

The chemical equation is \( \text{HClO} (aq) \rightleftharpoons \text{H}^+ (aq) + \text{ClO}^- (aq) \) and the \( K_a \) expression is \( K_a = \frac{[\text{H}^+][\text{ClO}^-]}{[\text{HClO}]} \).

Step 1: Identify the Chemical Formula

Hypochlorous acid is represented by the chemical formula \( ext{HClO} \).

Step 2: Write the Ionization Equation

When hypochlorous acid \( ext{HClO} \) is dissolved in water, it partially ionizes into hydrogen ions \( ext{H}^+ \) and hypochlorite ions \( ext{ClO}^- \). The chemical equation for this process is: \[ \text{HClO} (aq) \rightleftharpoons \text{H}^+ (aq) + \text{ClO}^- (aq) \]

Step 3: Write the Expression for the Acid Dissociation Constant \( K_a \)

The expression for the acid dissociation constant \( K_a \) involves the concentrations of the products and the reactant at equilibrium. It is given by: \[ K_a = \frac{[\text{H}^+][\text{ClO}^-]}{[\text{HClO}]} \] where \([\text{H}^+]\), \([\text{ClO}^-]\), and \([\text{HClO}]\) are the equilibrium concentrations of the hydrogen ions, hypochlorite ions, and hypochlorous acid, respectively.

Key Concepts

Acid Dissociation Constant

In the realm of chemistry, the acid dissociation constant, often represented as \( K_a \), is a crucial concept. It provides insight into the strength of an acid in a solution.
This constant quantifies the degree to which an acid dissociates into its ions in water. In simpler terms, the larger the \( K_a \), the stronger the acid, as more molecules ionize to release hydrogen ions.
Effectively, \( K_a \) can be understood through the following formula for a general acid \( HA \):

• The equilibrium equation is: \[ HA (aq) \rightleftharpoons H^+ (aq) + A^- (aq) \]
• The \( K_a \) expression is: \[ K_a = \frac{[H^+][A^-]}{[HA]} \]

The concentration brackets \([ ]\) indicate the molarity of each species at equilibrium.
For strong acids, which fully dissociate, \( K_a \) tends to be large. Conversely, weaker acids, which partially dissociate, have smaller \( K_a \) values, exemplifying incomplete ionization.

Hypochlorous Acid

Hypochlorous acid, whose chemical formula is \( ext{HClO} \), is a fascinating compound. It's widely used as an industrial disinfectant due to its potent antimicrobial properties.
Unlike some other acids, hypochlorous acid does not completely dissociate in solution, categorizing it as a weak acid. This characteristic is important in understanding its behavior and effects in different environments.
As a weak acid, when \( ext{HClO} \) is dissolved in water, it only partially ionizes. The ionization equation demonstrates this partial dissociation process:

• \[ \text{HClO} (aq) \rightleftharpoons \text{H}^+ (aq) + \text{ClO}^- (aq) \]

In this reaction, hypochloric acid separates into hydrogen ions (\( ext{H}^+ \)) and hypochlorite ions (\( ext{ClO}^- \)).
This partial dissociation is one of the aspects that makes hypochlorous acid an effective yet safe disinfectant, as it remains relatively stable in solution.

Ionization Equation

The ionization equation offers a snapshot of how acids like hypochlorous acid dissociate in aqueous solutions. It visually represents the reversible process of dissociation that occurs for many weak acids.

• The equation for hypochlorous acid is: \[ \text{HClO} (aq) \rightleftharpoons \text{H}^+ (aq) + \text{ClO}^- (aq) \]

Here, the double arrow (\( \rightleftharpoons \)) indicates a dynamic equilibrium, reflecting the balance between the forward and reverse reactions.
The forward reaction shows the acid \(\text{HClO}\) releasing a hydrogen ion, forming a hypochlorite ion, while the reverse reaction entails the recombination of these ions to form the undissociated acid.
This dynamic balance is essential for maintaining stable pH levels in solutions and is pivotal in reactions involving buffers.