Problem 21 - Chemistry The Central Science In Si Units

Question

Label each of the following as being a strong base, a weak base, or a species with negligible basicity. In each case write the formula of its conjugate acid, and indicate whether the conjugate acid is a strong acid, a weak acid, or a species with negligible acidity: \((\mathbf{a}) \mathrm{F}^{-}(\mathbf{b}) \mathrm{Br}^{-}(\mathbf{c}) \mathrm{HS}^{-}(\mathbf{d}) \mathrm{ClO}_{4}^{-}(\mathbf{e}) \mathrm{HCOO}^{-}\)

Step-by-Step Solution

Answer

(a) F⁻: Weak base, conjugate acid is weak; (b) Br⁻: Negligible, conjugate acid is strong; (c) HS⁻: Weak base, conjugate acid is weak; (d) ClO₄⁻: Negligible, conjugate acid is strong; (e) HCOO⁻: Weak base, conjugate acid is weak.

Step 1: Analyze Fluoride Ion (F⁻)

The fluoride ion, \(\text{F}^-\), is considered a weak base because it is the conjugate base of hydrofluoric acid (\(\text{HF}\)), a weak acid. Thus, \(\text{F}^-\) has some ability to accept protons. Its conjugate acid, \(\text{HF}\), is classified as a weak acid.

Step 2: Analyze Bromide Ion (Br⁻)

The bromide ion, \(\text{Br}^-\), is a species with negligible basicity because it is the conjugate base of hydrobromic acid (\(\text{HBr}\)), a strong acid. Therefore, \(\text{Br}^-\) does not significantly attract protons. Its conjugate acid, \(\text{HBr}\), is a strong acid.

Step 3: Analyze Hydrogen Sulfide Ion (HS⁻)

The hydrogen sulfide ion, \(\text{HS}^-\), is a weak base, as it is the conjugate base of hydrogen sulfide (\(\text{H}_2\text{S}\)), a weak acid. Thus, \(\text{HS}^-\) can accept protons to some extent. Its conjugate acid, \(\text{H}_2\text{S}\), is a weak acid.

Step 4: Analyze Perchlorate Ion (ClO₄⁻)

The perchlorate ion, \(\text{ClO}_4^-\), is a species with negligible basicity as it is the conjugate base of perchloric acid (\(\text{HClO}_4\)), which is a strong acid. Thus, \(\text{ClO}_4^-\) does not attract protons. Its conjugate acid, \(\text{HClO}_4\), is a strong acid.

Step 5: Analyze Formate Ion (HCOO⁻)

The formate ion, \(\text{HCOO}^-\), is a weak base since it is the conjugate base of formic acid (\(\text{HCOOH}\)), which is a weak acid. Therefore, \(\text{HCOO}^-\) can accept protons moderately. Its conjugate acid, \(\text{HCOOH}\), is a weak acid.

Key Concepts

Strong Acids and Bases

Strong acids and bases are powerful players in the acid-base world. When you hear "strong" in this context, it means they completely dissociate in water, which means that every molecule splits into ions. For acids, this results in a high concentration of hydrogen ions
(H⁺) and for bases, a high concentration of hydroxide ions (OH⁻).

Some common strong acids include:

• Hydrochloric acid (HCl)
• Sulfuric acid (H₂SO₄)
• Nitric acid (HNO₃)

On the flip side, some strong bases you might encounter are:

• Sodium hydroxide (NaOH)
• Potassium hydroxide (KOH)
• Calcium hydroxide (Ca(OH)₂)

The key point about strong acids and bases is their ability to completely ionize in water. This makes them very reactive and efficient at conducting electricity in a solution. For instance, hydrobromic acid (HBr) is a strong acid, meaning its conjugate base, bromide ion (Br⁻), has negligible basicity.

Knowing whether a substance is a strong acid or base helps predict how it will behave in a chemical reaction.

Weak Acids and Bases

Weak acids and bases are quite different from the strong ones. They partially dissociate in water, meaning only a small fraction of the molecules break into ions. This results in a lower concentration of hydrogen ions
for acids or hydroxide ions for bases in solution.

Consider the case of acetic acid (CH₃COOH), a common weak acid. Only a small percentage of acetic acid molecules will release H⁺ ions in solution. This partial ionization results in a relatively weak acidic behavior. Similarly, ammonia (NH₃) is a weak base, which means it only partially accepts hydrogen ions to form ammonium (NH₄⁺).

When we think about fluoride ion (F⁻), it’s a weak base because it’s linked to hydrofluoric acid (HF), a weak acid. This means fluoride ion can indeed attract hydrogen ions, just not very strongly. Keep in mind that weak acids and bases are in equilibrium with their ionized forms, meaning they readily return to their original state.

Conjugate Acid-Base Pairs

Conjugate acid-base pairs are like chemical partners. They consist of two species that transform into each other by the gain or loss of a proton (H⁺). It’s a concept at the heart of the Bronsted-Lowry acid-base theory.

Here is how it works: - The acid part of the pair donates a proton and becomes the conjugate base. - Conversely, the base part accepts a proton and transforms into its conjugate acid.
For instance, consider the transformation of hydrogen sulfide (HS⁻) and hydrosulfide (H₂S). Here, HS⁻ acts as a base by accepting a proton to become H₂S, its conjugate acid.

A notable factor in conjugate acid-base pairs is their relative strengths:

• A strong acid has a weak conjugate base, meaning the base doesn't hold protons tightly.
• A weak acid has a relatively stronger conjugate base, which can hold on to protons a bit more.

Understanding conjugate pairs helps us foresee reaction pathways and the dynamism of acid-base equilibria. Recognizing relationships like C_l_O₄⁻ and H C_l_O₄ is central to mastering chemical behavior and reactivity.