Problem 79 - Chemistry The Molecular Nature of Matter

Question

Write equations that illustrate the amphiprotic nature of the bicarbonate ion.

Step-by-Step Solution

Answer

The bicarbonate ion (HCO3-) acts as an acid by donating a proton to form carbonate (CO3^2-) and as a base by accepting a proton to form carbonic acid (H2CO3).

Step 1: Define Amphiprotic Nature

An amphiprotic molecule is one that can both donate and accept a proton (H+). This means it can act as both an acid and a base according to the Brønsted-Lowry acid-base theory.

Step 2: Write the Acid Equation

As an acid, the bicarbonate ion (HCO3-) can donate a proton. The equation showing bicarbonate acting as an acid is: HCO3- (aq) → CO3^2- (aq) + H+ (aq).

Step 3: Write the Base Equation

As a base, the bicarbonate ion can accept a proton. The equation showing bicarbonate acting as a base is: HCO3- (aq) + H+ (aq) → H2CO3 (aq).

Key Concepts

Brønsted-Lowry acid-base theory

The Brønsted-Lowry acid-base theory provides a framework to understand acid-base reactions. It defines an acid as a substance that can donate a proton (H⁺), and a base as a substance that can accept a proton. This theory broadens the scope of acid-base chemistry beyond the simple transfer of hydrogen ions as understood in original theories.

This theory is particularly helpful when analyzing the behavior of amphiprotic substances like bicarbonate. The amphiprotic nature means that these substances can switch roles, acting as either an acid or a base depending on the reaction they are involved in. Understanding how bicarbonate behaves in acid-base reactions is crucial for interpreting various physiological and environmental processes, such as blood buffering systems and carbon dioxide dissolving in ocean water.

Proton donor

In the context of the Brønsted-Lowry acid-base theory, a proton donor is any species that releases a hydrogen ion (H⁺) when reacting with a base. An acid is, therefore, essentially a proton donor. For example, the bicarbonate ion (HCO₃^-) can act as an acid and donate a proton to become carbonate (CO₃^2-), as shown in the acid equation:

HCO₃^- (aq) → CO₃^2- (aq) + H⁺ (aq).

Recognizing the substance that acts as a proton donor in an acid-base reaction is crucial for discerning the directional flow of protons and thus the direction of the reaction itself.

Proton acceptor

The inverse of a proton donor is a proton acceptor, which in the Brønsted-Lowry theory is defined as a base. A proton acceptor is a substance capable of binding with an available hydrogen ion, often resulting in a neutralization reaction. In the case of bicarbonate acting as a base, it accepts a proton to form carbonic acid (H₂CO₃):

HCO₃^- (aq) + H⁺ (aq) → H₂CO₃ (aq).

This process exemplifies bicarbonate's role as a proton acceptor, which is pivotal in many buffering solutions that maintain pH balance in a variety of conditions.

Acid-base reactions

Acid-base reactions are fundamental chemical processes where protons (H⁺) are exchanged between substances. These reactions are characterized by the transfer of protons from acids (proton donors) to bases (proton acceptors). The amphiprotic nature of bicarbonate allows it to participate in acid-base reactions as either a donor or acceptor of protons:

As an acid, releasing a proton:HCO₃^- (aq) → CO₃^2- (aq) + H⁺ (aq);
As a base, receiving a proton:HCO₃^- (aq) + H⁺ (aq) → H₂CO₃ (aq).

These dual roles are essential in maintaining the delicate acid-base equilibrium in numerous natural and industrial systems. Understanding these reactions allows for better comprehension of biochemical cycles and industrial processes that utilize or regulate pH levels.