Which Of The Following Is A Conjugate Acid Base Pair

Conjugate acid-base pairs form the cornerstone of understanding acid-base chemistry, playing a vital role in various chemical reactions and biological processes. Identifying these pairs accurately is crucial for predicting reaction outcomes, designing experiments, and comprehending the behavior of chemical systems.

Understanding Acids and Bases: A Brief Review

Before diving into conjugate acid-base pairs, it's essential to revisit the fundamental definitions of acids and bases. Several theories define acids and bases, but the Brønsted-Lowry definition is particularly relevant in this context.

• Brønsted-Lowry Acid: A substance that donates a proton (H⁺).
• Brønsted-Lowry Base: A substance that accepts a proton (H⁺).

In essence, an acid-base reaction involves the transfer of a proton from an acid to a base.

What is a Conjugate Acid-Base Pair?

A conjugate acid-base pair consists of two species that differ by the presence or absence of a proton (H⁺). When an acid donates a proton, the remaining species becomes its conjugate base. Conversely, when a base accepts a proton, the resulting species becomes its conjugate acid.

• Acid ⇌ Conjugate Base + H⁺
• Base + H⁺ ⇌ Conjugate Acid

The acid and its conjugate base, or the base and its conjugate acid, form a conjugate pair.

How to Identify Conjugate Acid-Base Pairs

Identifying conjugate acid-base pairs involves looking for substances that differ by a single proton (H⁺). Here's a step-by-step guide:

1. Identify the Acid and Base: In a given reaction or equilibrium, determine which substance is donating a proton (acid) and which is accepting a proton (base).
2. Determine the Products: After the proton transfer, identify the resulting species. The species formed from the acid is its conjugate base, and the species formed from the base is its conjugate acid.
3. Check for the H⁺ Difference: Confirm that the acid and its conjugate base (or the base and its conjugate acid) differ by only one proton (H⁺). The charge will also differ by one unit.
4. Write the Pairs: Express the conjugate acid-base pairs clearly, indicating which is the acid and which is the base.

Examples of Conjugate Acid-Base Pairs

Let's examine several examples to illustrate how to identify conjugate acid-base pairs:

1. Reaction of Hydrochloric Acid (HCl) with Water (H₂O)

   • Reaction: HCl (aq) + H₂O (l) ⇌ H₃O⁺ (aq) + Cl⁻ (aq)
   • Acid and Base:
     • HCl is the acid because it donates a proton.
     • H₂O is the base because it accepts a proton.
   • Conjugate Pairs:
     • HCl (acid) and Cl⁻ (conjugate base)
     • H₂O (base) and H₃O⁺ (conjugate acid)
   • Explanation: HCl loses a proton to become Cl⁻, and H₂O gains a proton to become H₃O⁺.

2. Reaction of Ammonia (NH₃) with Water (H₂O)

   • Reaction: NH₃ (aq) + H₂O (l) ⇌ NH₄⁺ (aq) + OH⁻ (aq)
   • Acid and Base:
     • H₂O is the acid because it donates a proton.
     • NH₃ is the base because it accepts a proton.
   • Conjugate Pairs:
     • H₂O (acid) and OH⁻ (conjugate base)
     • NH₃ (base) and NH₄⁺ (conjugate acid)
   • Explanation: NH₃ gains a proton to become NH₄⁺, and H₂O loses a proton to become OH⁻.

3. Reaction of Acetic Acid (CH₃COOH) with Water (H₂O)

   • Reaction: CH₃COOH (aq) + H₂O (l) ⇌ H₃O⁺ (aq) + CH₃COO⁻ (aq)
   • Acid and Base:
     • CH₃COOH is the acid because it donates a proton.
     • H₂O is the base because it accepts a proton.
   • Conjugate Pairs:
     • CH₃COOH (acid) and CH₃COO⁻ (conjugate base)
     • H₂O (base) and H₃O⁺ (conjugate acid)
   • Explanation: CH₃COOH loses a proton to become CH₃COO⁻, and H₂O gains a proton to become H₃O⁺.

4. Reaction of a Polyprotic Acid: Phosphoric Acid (H₃PO₄)

   • Reaction 1: H₃PO₄ (aq) + H₂O (l) ⇌ H₂PO₄⁻ (aq) + H₃O⁺ (aq)
     • H₃PO₄ (acid) and H₂PO₄⁻ (conjugate base)
     • H₂O (base) and H₃O⁺ (conjugate acid)
   • Reaction 2: H₂PO₄⁻ (aq) + H₂O (l) ⇌ HPO₄²⁻ (aq) + H₃O⁺ (aq)
     • H₂PO₄⁻ (acid) and HPO₄²⁻ (conjugate base)
     • H₂O (base) and H₃O⁺ (conjugate acid)
   • Reaction 3: HPO₄²⁻ (aq) + H₂O (l) ⇌ PO₄³⁻ (aq) + H₃O⁺ (aq)
     • HPO₄²⁻ (acid) and PO₄³⁻ (conjugate base)
     • H₂O (base) and H₃O⁺ (conjugate acid)
   • Explanation: Phosphoric acid can donate up to three protons in a stepwise manner, each step forming a new conjugate acid-base pair.

Amphoteric Substances

Some substances can act as both acids and bases, depending on the reaction conditions. These substances are called amphoteric. Water is a classic example of an amphoteric substance.

• Water as a Base: In the reaction with HCl, water acts as a base by accepting a proton to form H₃O⁺.
• Water as an Acid: In the reaction with NH₃, water acts as an acid by donating a proton to form OH⁻.

Other amphoteric substances include ions like H₂PO₄⁻ and HCO₃⁻, which can either donate or accept a proton depending on the chemical environment.

Common Mistakes to Avoid

When identifying conjugate acid-base pairs, be aware of common mistakes:

• Confusing Acids and Bases: Ensure you correctly identify which substance is donating the proton (acid) and which is accepting the proton (base).
• Incorrectly Counting Protons: The acid and its conjugate base must differ by exactly one proton (H⁺).
• Ignoring Charges: When a proton is gained or lost, the charge of the species also changes by one unit. Pay attention to the charges of the species involved.
• Misidentifying Amphoteric Substances: Remember that amphoteric substances can act as either acids or bases depending on the reaction.

Significance of Conjugate Acid-Base Pairs

Understanding conjugate acid-base pairs is crucial for several reasons:

1. Predicting Reaction Direction: The strength of an acid and its conjugate base are inversely related. Strong acids have weak conjugate bases, and strong bases have weak conjugate acids. This relationship helps predict the direction of an acid-base reaction. Reactions tend to favor the formation of weaker acids and bases.
2. Buffer Solutions: Buffer solutions are mixtures of a weak acid and its conjugate base (or a weak base and its conjugate acid). These solutions resist changes in pH upon the addition of small amounts of acid or base. The buffering capacity depends on the concentrations of the weak acid and its conjugate base.
3. Titration Calculations: In acid-base titrations, understanding conjugate acid-base pairs is essential for calculating the pH at different points in the titration and determining the equivalence point.
4. Biological Systems: Acid-base chemistry is fundamental to many biological processes. For example, the bicarbonate buffer system (H₂CO₃/HCO₃⁻) in blood helps maintain a stable pH, which is crucial for enzyme activity and cellular function.
5. Industrial Processes: Many industrial processes, such as chemical synthesis and wastewater treatment, rely on acid-base reactions. Understanding conjugate acid-base pairs is essential for optimizing these processes.

Factors Affecting Acid and Base Strength

The strength of an acid or base depends on its ability to donate or accept protons, respectively. Several factors influence acid and base strength:

1. Electronegativity: For acids with the general formula H-A, the acidity increases as the electronegativity of A increases. A more electronegative atom A pulls electron density away from the H-A bond, making it easier to release H⁺.
2. Bond Strength: Stronger H-A bonds are more difficult to break, making the acid weaker. Weaker H-A bonds are easier to break, making the acid stronger.
3. Size of the Atom: For acids with the general formula H-A, within the same group, acidity increases as the size of A increases. Larger atoms have weaker H-A bonds, making it easier to release H⁺.
4. Resonance: Resonance stabilization of the conjugate base increases the acidity of the acid. The more resonance structures a conjugate base has, the more stable it is, and the stronger the corresponding acid.
5. Inductive Effects: Electron-withdrawing groups near the acidic proton increase acidity by stabilizing the conjugate base. Electron-donating groups decrease acidity by destabilizing the conjugate base.

Examples of Identifying Conjugate Pairs in Complex Molecules

Identifying conjugate acid-base pairs can become more challenging with complex organic molecules. Here are some examples:

1. Protonation of an Amine:

   • Reaction: R-NH₂ + H₂O ⇌ R-NH₃⁺ + OH⁻
   • Conjugate Pairs:
     • H₂O (acid) and OH⁻ (conjugate base)
     • R-NH₂ (base) and R-NH₃⁺ (conjugate acid)

2. Deprotonation of a Carboxylic Acid:

   • Reaction: R-COOH + H₂O ⇌ R-COO⁻ + H₃O⁺
   • Conjugate Pairs:
     • R-COOH (acid) and R-COO⁻ (conjugate base)
     • H₂O (base) and H₃O⁺ (conjugate acid)

3. Protonation of an Alcohol:

   • Reaction: R-OH + H₂SO₄ ⇌ R-OH₂⁺ + HSO₄⁻
   • Conjugate Pairs:
     • H₂SO₄ (acid) and HSO₄⁻ (conjugate base)
     • R-OH (base) and R-OH₂⁺ (conjugate acid)

Practice Questions

To test your understanding, try to identify the conjugate acid-base pairs in the following reactions:

1. HCN (aq) + H₂O (l) ⇌ H₃O⁺ (aq) + CN⁻ (aq)
2. HCO₃⁻ (aq) + OH⁻ (aq) ⇌ CO₃²⁻ (aq) + H₂O (l)
3. H₂SO₄ (aq) + H₂O (l) ⇌ H₃O⁺ (aq) + HSO₄⁻ (aq)

Solutions to Practice Questions

1. HCN (aq) + H₂O (l) ⇌ H₃O⁺ (aq) + CN⁻ (aq)

   • HCN (acid) and CN⁻ (conjugate base)
   • H₂O (base) and H₃O⁺ (conjugate acid)

2. HCO₃⁻ (aq) + OH⁻ (aq) ⇌ CO₃²⁻ (aq) + H₂O (l)

   • H₂O (acid) and OH⁻ (conjugate base)
   • HCO₃⁻ (acid) and CO₃²⁻ (conjugate base)

3. H₂SO₄ (aq) + H₂O (l) ⇌ H₃O⁺ (aq) + HSO₄⁻ (aq)

   • H₂SO₄ (acid) and HSO₄⁻ (conjugate base)
   • H₂O (base) and H₃O⁺ (conjugate acid)

Conclusion

Mastering the identification of conjugate acid-base pairs is fundamental to understanding and predicting acid-base reactions. By following a systematic approach and understanding the factors that influence acid and base strength, you can confidently navigate the complexities of acid-base chemistry. This knowledge is essential for various applications, from predicting reaction outcomes to designing buffer solutions and understanding biological processes.