Problem 98 - Chemistry Matter and Change

Question

Write balanced chemical equations for the two successive ionizations of carbonic acid in water. Identify the conjugate-base pair in each of the equations.

Step-by-Step Solution

Answer

Carbonic acid ionizes to bicarbonate and carbonate, forming the pairs (H₂CO₃, HCO₃⁻) and (HCO₃⁻, CO₃²⁻).

Step 1: Understand Carbonic Acid Ionization

Carbonic acid is known to ionize in two steps. The first ionization involves the release of one hydrogen ion from the carbonic acid molecule. The chemical formula for carbonic acid is \( \text{H}_2\text{CO}_3 \).

Step 2: Write the First Ionization Equation

In the first ionization step, carbonic acid (\( \text{H}_2\text{CO}_3 \)) releases a proton (\( \text{H}^+ \)) to form bicarbonate (\( \text{HCO}_3^- \)). The balanced chemical equation is: \[ \text{H}_2\text{CO}_3 (aq) \rightleftharpoons \text{H}^+ (aq) + \text{HCO}_3^- (aq) \] In this reaction, the conjugate acid-base pair is \( \text{H}_2\text{CO}_3 \) and \( \text{HCO}_3^- \).

Step 3: Write the Second Ionization Equation

The bicarbonate ion (\( \text{HCO}_3^- \)), formed in the first step, can further ionize by losing another \( \text{H}^+ \) to form carbonate (\( \text{CO}_3^{2-} \)). The balanced chemical equation for the second ionization is: \[ \text{HCO}_3^- (aq) \rightleftharpoons \text{H}^+ (aq) + \text{CO}_3^{2-} (aq) \] Here, the conjugate acid-base pair is \( \text{HCO}_3^- \) and \( \text{CO}_3^{2-} \).

Key Concepts

Conjugate Acid-Base Pair

Every acid-base reaction features pairs known as conjugate acid-base pairs. When an acid donates a proton, it becomes a conjugate base. Similarly, when a base accepts a proton, it becomes a conjugate acid. In the ionization of carbonic acid in water, these conjugate pairs are crucial in understanding chemical reactions.
In the first ionization step:

• Carbonic acid (\( \text{H}_2\text{CO}_3 \)) acts as the acid, donating its proton to form bicarbonate (\( \text{HCO}_3^- \)), which is the conjugate base.

In the second ionization step:

• Bicarbonate (\( \text{HCO}_3^- \)) donates another proton to form carbonate (\( \text{CO}_3^{2-} \)), which is the conjugate base for this equation.

Understanding these pairs helps predict the direction of acid-base reactions, as acids seek to donate protons while bases aim to accept them.

Balanced Chemical Equations

A balanced chemical equation is crucial to reflect the conservation of mass. In these equations, the number of each type of atom on the reactant side must equal the number on the product side. This balance ensures the equation accurately represents the chemical reaction taking place.
For carbonic acid's ionization:

• First Ionization: \[ \text{H}_2\text{CO}_3 (aq) \rightleftharpoons \text{H}^+ (aq) + \text{HCO}_3^- (aq) \]
• Second Ionization: \[ \text{HCO}_3^- (aq) \rightleftharpoons \text{H}^+ (aq) + \text{CO}_3^{2-} (aq) \]

These equations show how carbonic acid systematically loses protons, becoming more negatively charged at each step. Balancing helps ensure that every element's quantity is accounted for during the reaction.

Bicarbonate Ion

The bicarbonate ion (\( \text{HCO}_3^- \)) is an intermediate form in the dissociation of carbonic acid. It acts as either an acid or a base, depending on the surrounding environment. In aqueous solutions, bicarbonate is essential for maintaining pH equilibrium, acting as a buffer.
Key characteristics:

• It is formed by the first ionization of carbonic acid as it loses one proton.
• Functions in physiological processes such as buffering systems in blood.
• Has the chemical formula \( \text{HCO}_3^- \) indicating it carries a single negative charge.

In the chemical equation, bicarbonate is involved both as a product of the first ionization and as a reactant in the second ionization.

Carbonate Ion

After bicarbonate, the next stage in carbonic acid ionization produces the carbonate ion (\( \text{CO}_3^{2-} \)).

• The carbonate ion carries a \( 2^{-} \) charge, reflecting its higher negative charge compared to bicarbonate.
• It results from the second ionization stage, when another hydrogen ion is released.

The carbonate ion is crucial in geological and biological systems, playing roles in:

• Mineral formation, such as limestone and other carbonate rocks.
• Being a part of aquatic ecosystems where it contributes to buffering capacities.

The presence and behavior of carbonate ions in various environments shed light on processes like acid rain impacts and ocean carbonation.