Problem 4 - Chemistry The Molecular Science

Question

When \(\mathrm{OH}^{-}\) is the base in a conjugate acid-base pair, the acid is ________ ; when \(\mathrm{OH}^{-}\) is the acid, the base is __________.

Step-by-Step Solution

Answer

When OH- is the base, the conjugate acid is H2O; when OH- is the acid, the base is O^2-.

Step 1: Identify OH- as a Base

When OH- acts as a base in a conjugate acid-base pair, it accepts a proton to form the acid.

Step 2: Determine the Conjugate Acid of OH- as a Base

When OH- accepts a proton (H+), it becomes water ( H2O). Therefore, when OH- is the base, the conjugate acid is H2O.

Step 3: Identify OH- as an Acid

When OH- acts as an acid, it donates a proton (H+), transforming into its conjugate base.

Step 4: Determine the Conjugate Base of OH- as an Acid

When OH- donates a proton, it forms the base O^2-. Therefore, when OH- is the acid, the conjugate base is O^2-.

Key Concepts

Hydroxide Ion

The hydroxide ion, denoted as \( \mathrm{OH}^- \), is a fundamental component in chemistry, particularly in acid-base reactions. It is composed of one oxygen and one hydrogen atom, carrying an overall negative charge. This charge arises because the oxygen atom has one more electron than it has protons.

In the context of acid-base chemistry, \( \mathrm{OH}^- \) plays a dual role. It can either act as a base by accepting protons or as an acid by donating protons, depending on the reaction in which it is involved.

• As a base, when \( \mathrm{OH}^- \) accepts a proton (\( \mathrm{H}^+ \)), it transforms into water (\( \mathrm{H_2O} \)). This reaction changes the hydroxide ion into its conjugate acid.
• Conversely, as an acid, \( \mathrm{OH}^- \) can donate a proton, forming the oxide ion \( \mathrm{O}^{2-} \). This showcases its role as an amphoteric species, which can either donate or accept protons under different conditions.

Acid-Base Equilibrium

Acid-Base Equilibrium is a central theme in chemistry, describing how acids and bases interact in reversible reactions. At equilibrium, the rate of the forward reaction (acid donating a proton or base accepting one) equals the rate of the reverse reaction, leading to a constant concentration of reactants and products.

The balance between an acid and its conjugate base, or a base and its conjugate acid, can be described by equilibrium constants. For the hydroxide ion acting as a base, the equilibrium can be represented as:

\[ \mathrm{OH}^- + \mathrm{H}^+ \leftrightarrow \mathrm{H_2O} \]

• The equilibrium constant for this reaction is the inverse of the ion product of water.
• This indicates how readily \( \mathrm{OH}^- \) ions react with \( \mathrm{H}^+ \) ions to form water.

Maintaining equilibrium is essential in various chemical processes and is integral in understanding the inherent pH of a solution, emphasizing the neutralizing effect \( \mathrm{OH}^- \) has on acidic environments.

Proton Transfer Reactions

Proton transfer reactions are fundamental in many chemical processes. These reactions involve the transfer of protons (\( \mathrm{H}^+ \) ions) between molecules.

In the case of hydroxide ions, they engage in proton transfer reactions in two distinctive manners:

• As a base: \( \mathrm{OH}^- \) accepts \( \mathrm{H}^+ \) to form \( \mathrm{H_2O} \).
• As an acid: \( \mathrm{OH}^- \) can donate \( \mathrm{H}^+ \) to become \( \mathrm{O}^{2-} \).

These proton exchanges exemplify the concept of conjugate acid-base pairs. When \( \mathrm{OH}^- \) accepts a proton, it pairs with its conjugate acid, \( \mathrm{H_2O} \). Conversely, upon losing a proton, \( \mathrm{OH}^- \) becomes \( \mathrm{O}^{2-} \), demonstrating its ability to act in both roles. Understanding these reactions aids in grasping more complex chemical systems and biological processes, as proton transfer is a critical component in energy transfer and metabolism.