Problem 118 - Introductory Chemistry Atoms First

Question

When water undergoes autoionization, is it serving as an acid, a base, or neither? Explain.

Step-by-Step Solution

Answer

During autoionization, water serves as both an acid and a base. This is because one water molecule donates a proton (H⁺) to another water molecule, acting as an acid, while the second water molecule accepts the proton (H⁺), acting as a base. The autoionization of water can be represented by the equation: \(2H_2O \rightleftharpoons H_3O^+ + OH^-\).

Step 1: Define autoionization of water

Autoionization of water is a chemical equilibrium process in which water molecules dissociate into hydronium ions (H₃O⁺) and hydroxide ions (OH⁻). The process can be represented by the following equation: \[2H_2O \rightleftharpoons H_3O^+ + OH^-\]

Step 2: Define acid and base

According to the Brønsted-Lowry theory, an acid is a substance that donates a proton (H⁺) to another substance, while a base is a substance that accepts a proton (H⁺) from another substance.

Step 3: Determine the role of water in autoionization

In the autoionization equation, we can see that one water molecule donates a proton (H⁺) to another water molecule: \[H_2O + H_2O \rightleftharpoons H_3O^+ + OH^-\] This proton transfer results in the formation of a hydronium ion (H₃O⁺) and a hydroxide ion (OH⁻).

Step 4: Identify water as both an acid and a base

During autoionization, one water molecule acts as an acid because it donates a proton (H⁺) to another water molecule. The second water molecule acts as a base because it accepts the proton (H⁺) from the first water molecule. Therefore, water serves as both an acid and a base during autoionization.

Key Concepts

Brønsted-Lowry Theory

The Brønsted-Lowry theory is an essential concept in acid-base chemistry. According to this theory, an acid is a substance that donates a proton ( H⁺ ) to another substance, while a base is a substance that accepts a proton. This idea broadened the previous definitions by not restricting acids and bases to only aqueous solutions.

In the context of autoionization of water, this theory helps us understand the roles played by water molecules. When two water molecules interact, one water molecule donates a proton, acting as an acid. The other water molecule accepts this proton, playing the role of a base.

This dual functionality explains the unique nature of water. It can behave both as an acid and a base, depending on the reaction conditions.

Hydronium Ion

The hydronium ion ( H₃O⁺ ) is a vital player in the chemistry of acids. It forms when water ( H₂O ) accepts a proton ( H⁺ ), making it a proxy for the presence of protons in a solution. This ion is often used to represent the acidity of a solution in a more tangible form.

In the process of water's autoionization, one of the water molecules donates a proton to another, creating the hydronium ion.

• This transfer stabilizes the otherwise free proton.
• By forming H₃O⁺ , the solution can conduct electricity.

Understanding the role of hydronium ions clarifies why a pH scale is used to measure acidity. The concentration of these ions determines whether a solution is acidic, neutral, or basic.

Hydroxide Ion

The hydroxide ion ( OH⁻ ) is commonly associated with basicity in an aqueous solution. It forms when a water molecule loses a proton ( H⁺ ), leaving behind an OH⁻ ion. This process is central to the concept of alkalinity or basicity.

During the autoionization of water, when a water molecule donates a proton, the remaining part turns into a hydroxide ion.

• OH⁻ contributes to solutions becoming basic because it can neutralize protons.
• It's crucial in determining the pH scale, where a higher concentration of OH⁻ means the solution is more basic.

These ions play a crucial role in reactions, especially in neutralizations where acids and bases meet. This interplay maintains a balance in chemical solutions.

Chemical Equilibrium

Chemical equilibrium occurs when the forward and reverse reactions happen at the same rate. This balance means that the concentrations of reactants and products remain constant over time.

In the case of water's autoionization, the following equilibrium is established:\[2H_2O \rightleftharpoons H_3O^+ + OH^-\]

• The system remains balanced because as soon as hydronium and hydroxide ions form, some recombine to become water again.
• This dynamic state leads to the concept that even in pure water, both H₃O⁺ and OH⁻ are always present, albeit in tiny amounts.

Understanding chemical equilibrium helps us grasp how seemingly neutral substances, like pure water, have intrinsic acidic and basic properties. This realization is fundamental to many chemical processes.