Which One Of The Following Is A Diprotic Acid

Which One of the Following is a Diprotic Acid? Understanding Acid Strength and Polyprotic Acids

Determining whether a given acid is diprotic requires a solid understanding of acid-base chemistry, specifically the concept of polyprotic acids. This article will thoroughly explore diprotic acids, contrasting them with monoprotic and triprotic acids, providing clear examples, and explaining how to identify them. We'll also delve into the implications of diproticity in various chemical reactions and applications.

Understanding Acids and Their Properties

Before diving into diprotic acids, let's establish a foundation in acid-base chemistry. An acid is a substance that donates a proton (H⁺ ion) in a chemical reaction. The strength of an acid depends on its ability to donate this proton. Strong acids completely dissociate in water, while weak acids only partially dissociate. This dissociation is crucial in determining the acid's properties and behavior.

Defining Monoprotic, Diprotic, and Triprotic Acids

Acids are categorized based on the number of protons they can donate per molecule:

• Monoprotic Acids: These acids can donate only one proton per molecule. Examples include hydrochloric acid (HCl), nitric acid (HNO₃), and acetic acid (CH₃COOH). Their dissociation in water is represented by a single equilibrium reaction. For example, for HCl:

  HCl(aq) + H₂O(l) ⇌ H₃O⁺(aq) + Cl⁻(aq)

• Diprotic Acids: These acids can donate two protons per molecule. They undergo a two-step dissociation process, each with its own equilibrium constant (Ka). This is a key characteristic we will explore in detail throughout this article.

• Triprotic Acids: These acids can donate three protons per molecule. They undergo a three-step dissociation process, each with its own Ka value. Phosphoric acid (H₃PO₄) is a classic example.

Identifying Diprotic Acids: Key Characteristics and Examples

The defining feature of a diprotic acid is its ability to donate two protons. Let's look at several examples and examine their dissociation:

1. Sulfuric Acid (H₂SO₄): This is a strong diprotic acid, meaning its first proton dissociation is essentially complete. The dissociation proceeds in two steps:

• First Dissociation (Complete): H₂SO₄(aq) → H⁺(aq) + HSO₄⁻(aq)

• Second Dissociation (Partial): HSO₄⁻(aq) + H₂O(l) ⇌ H₃O⁺(aq) + SO₄²⁻(aq)

Note that the second dissociation is an equilibrium reaction, indicating that HSO₄⁻ is a weaker acid than H₂SO₄.

2. Carbonic Acid (H₂CO₃): This is a weak diprotic acid, important in the buffering systems of blood. Its dissociation is also a two-step process:

• First Dissociation: H₂CO₃(aq) + H₂O(l) ⇌ H₃O⁺(aq) + HCO₃⁻(aq)

• Second Dissociation: HCO₃⁻(aq) + H₂O(l) ⇌ H₃O⁺(aq) + CO₃²⁻(aq)

Both dissociation steps are equilibria, reflecting the weak acidic nature of carbonic acid.

3. Oxalic Acid (C₂H₂O₄): This is another weak diprotic acid commonly found in plants. Its dissociation follows a similar pattern:

• First Dissociation: C₂H₂O₄(aq) + H₂O(l) ⇌ H₃O⁺(aq) + C₂HO₄⁻(aq)

• Second Dissociation: C₂HO₄⁻(aq) + H₂O(l) ⇌ H₃O⁺(aq) + C₂O₄²⁻(aq)

Differentiating Diprotic Acids from Other Polyprotic Acids

The key to differentiating diprotic acids from monoprotic and triprotic acids lies in the number of acidic protons and the corresponding dissociation steps. Monoprotic acids only have one ionizable proton, while triprotic acids have three. Diprotic acids occupy the middle ground, possessing two ionizable protons leading to a two-step dissociation process. Examining the chemical formula is a crucial starting point. Acids with two replaceable hydrogen atoms (often written at the beginning of the formula) are likely to be diprotic. However, it is crucial to verify the actual dissociation behavior.

The Significance of Ka Values in Diprotic Acids

The equilibrium constants (Ka) for each dissociation step provide crucial information about the relative strengths of the acid in each step. For a diprotic acid, we have Ka₁ (for the first dissociation) and Ka₂ (for the second dissociation). Generally, Ka₁ > Ka₂. This means the first proton is easier to donate than the second. This is because the negatively charged conjugate base (formed after the first dissociation) holds the remaining proton more tightly due to electrostatic attraction.

The Ka values are essential in calculating the pH of diprotic acid solutions and understanding the relative concentrations of the various species present at equilibrium.

Applications of Diprotic Acids

Diprotic acids play significant roles in various applications, including:

• Biological Systems: Carbonic acid is vital in maintaining blood pH. Other diprotic acids are involved in metabolic processes and enzyme function.

• Industrial Processes: Sulfuric acid, a strong diprotic acid, is one of the most important industrial chemicals, used in various processes like fertilizer production and metal refining.

• Analytical Chemistry: Diprotic acids are used in titrations and other analytical techniques to determine the concentration of bases or other analytes.

• Food and Beverage Industry: Many food additives and preservatives are diprotic acids or their salts. They are often used for buffering or regulating acidity.

Solving Problems: Identifying Diprotic Acids

Let's consider a practical example. Given the following list of acids: HCl, H₂SO₄, H₃PO₄, CH₃COOH, which ones are diprotic?

• HCl (Hydrochloric acid): Monoprotic

• H₂SO₄ (Sulfuric acid): Diprotic

• H₃PO₄ (Phosphoric acid): Triprotic

• CH₃COOH (Acetic acid): Monoprotic

Therefore, only sulfuric acid (H₂SO₄) is a diprotic acid from this list.

Conclusion: Understanding the Nuances of Diprotic Acids

This comprehensive exploration of diprotic acids clarifies their defining characteristics, highlighting their differences from monoprotic and triprotic acids. Understanding the stepwise dissociation, the significance of Ka values, and their diverse applications is critical for anyone studying chemistry, biochemistry, or related fields. By mastering these concepts, you'll gain a deeper understanding of acid-base chemistry and its implications in various scientific disciplines. Remember to always carefully analyze the chemical formula and dissociation behavior to correctly classify an acid as monoprotic, diprotic, or triprotic. The number of ionizable protons is the key!