Problem 4 - Chemistry in 30 days

Question

Of the given anions, the strongest Bronsted base is: (a) \(\mathrm{ClO}^{-}\) (b) \(\mathrm{ClO}_{2}^{-}\) (c) \(\mathrm{ClO}_{3}^{-}\) (d) \(\mathrm{ClO}_{4}^{-}\)

Step-by-Step Solution

Answer

The strongest Bronsted base is  ext{ClO}^- (option a).

Step 1: Understand Bronsted Bases

Bronsted bases are species that can accept a proton (H+). The strength of a Bronsted base is inversely related to the strength of its conjugate acid.

Step 2: Identify Conjugate Acids

For each anion provided, identify the conjugate acid: -  ext{HClO} -  ext{HClO}_2 -  ext{HClO}_3 -  ext{HClO}_4.

Step 3: Determine Acid Strength

The stronger the acid, the weaker its conjugate base. Generally, the more oxygens an acid has relative to its chlorine content, the stronger it is. Thus,  ext{HClO}_4 is strongest and  ext{HClO} is weakest.

Step 4: Rank the Conjugate Bases

Based on the acidity order from Step 3, rank the conjugate bases (anions) from weakest to strongest as follows: -  ext{ClO}_4^- -  ext{ClO}_3^- -  ext{ClO}_2^- -  ext{ClO}^-.

Step 5: Identify the Strongest Base

From the ordering,  ext{ClO}^- is the weakest conjugate acid, thus it is the strongest Bronsted base among the options given.

Key Concepts

Anion Strength

In the context of Bronsted bases, anions are the negatively charged ions that can accept protons. The strength of an anion as a base depends on its ability to attract and hold onto a proton. When considering oxyanions such as \( \text{ClO}^- \), \( \text{ClO}_2^- \), \( \text{ClO}_3^- \), and \( \text{ClO}_4^- \), the number of oxygens plays a crucial role. Generally, the presence of more oxygen atoms in an oxyanion indicates that the negative charge is more delocalized over the molecule. As a result, its strength as a Bronsted base decreases because it holds onto additional protons less effectively. In this scenario:

• \( \text{ClO}^- \) has the fewest oxygen atoms and thus holds its negative charge strongly, making it a stronger base.
• \( \text{ClO}_4^- \), with four oxygen atoms, spreads its negative charge over more atoms, weakening its basicity substantially.

Conjugate Acids

Conjugate acids are formed when a base accepts a proton. The concept of conjugate pairs is essential to understanding acid-base reactions. For each given anion, you can determine its conjugate acid by simply adding a proton \((\text{H}^+)\). For the anions presented:

• \( \text{ClO}^- \) converts to \( \text{HClO} \)
• \( \text{ClO}_2^- \) becomes \( \text{HClO}_2 \)
• \( \text{ClO}_3^- \) forms \( \text{HClO}_3 \)
• \( \text{ClO}_4^- \) turns into \( \text{HClO}_4 \)

This helps in identifying how strong a given conjugate acid is which directly affects the basicity of the original anion. A weaker conjugate acid implies a stronger conjugate base. Conversely, a strong conjugate acid will indicate a weak base.

Oxyacid Strength

Oxyacids are a particular type of acid with at least one hydrogen atom bonded to an oxygen atom. Their strength largely depends on the electronegativity and the number of oxygen atoms surrounding the central atom, generally affecting how easily the acid donates a proton.For the series of oxyacids derived from chloric ions, strength increases with the number of attached oxygens:

• \( \text{HClO} \) is the weakest since it has only one oxygen.
• \( \text{HClO}_4 \), with four oxygens, is the strongest oxyacid.

More oxygens help stabilize the negative charge by delocalization after a proton is donated, increasing acid strength. This order is crucial in determining the corresponding conjugate base strength, as stronger acids have weaker conjugate bases.

Acidity and Basicity Relationship

The relationship between acidity and basicity is inverse, meaning strong acids have weak conjugate bases and vice versa. This relationship is foundational in acid-base chemistry and helps predict the behavior of substances in reactions. When considering the given anions, we assess their conjugate acids to infer basicity. For instance:

• Sulfuric and nitric acids are strong, making their conjugate bases weak.
• Conversely, acetic acid is weaker, thus its conjugate base, acetate, is stronger.

In our specific case, \( \text{HClO}_{4} \) is the strongest acid, making \( \text{ClO}_{4}^- \) the weakest base. Understanding this balance aids in predicting which species will dominate in equilibrium reactions and under what conditions.