Question

Label each of the following as being a strong acid, a weak acid, or a species with negligible acidity. In each case write the formula of its conjugate base, and indicate whether the conjugate base is a strong base, a weak base, or a species with negligible basicity: \((\mathbf{a}) \mathrm{HCOOH}\), \((\mathbf{b})\mathrm{H}_{2},(\mathrm{c}) \mathrm{CH}_{4},(\mathbf{d}) \mathrm{HF},(\mathbf{e}) \mathrm{NH}_{4}^{+}\)

Short answer

a) \(\mathrm{HCOOH}\) is a weak acid, its conjugate base \(\mathrm{HCOO}^{-}\) is a weak base. b) \(\mathrm{H}_{2}\) has negligible acidity, its conjugate base \(\mathrm{H}^{-}\) is a strong base. c) \(\mathrm{CH}_{4}\) has negligible acidity, its conjugate base \(\mathrm{CH}_{3}^{-}\) has negligible basicity. d) \(\mathrm{HF}\) is a weak acid, its conjugate base \(\mathrm{F}^{-}\) is a weak base. e) \(\mathrm{NH}_{4}^{+}\) is a weak acid, its conjugate base \(\mathrm{NH}_{3}\) is a weak base.

Step-by-step solution

Identify the species and determine their acidity level

We have five species to analyze: \(\mathrm{HCOOH}\), \(\mathrm{H}_{2}\), \(\mathrm{CH}_{4}\), \(\mathrm{HF}\), and \(\mathrm{NH}_{4}^{+}\). 1. \(\mathrm{HCOOH}\) is formic acid, a weak acid. 2. \(\mathrm{H}_{2}\) is molecular hydrogen, a species with negligible acidity. 3. \(\mathrm{CH}_{4}\) is methane, a species with negligible acidity. 4. \(\mathrm{HF}\) is hydrofluoric acid, a weak acid. 5. \(\mathrm{NH}_{4}^{+}\) is the ammonium ion, a weak acid.

Write the formula of the conjugate base

We'll now write the formula of the conjugate base of each species: 1. \(\mathrm{HCOOH} \rightarrow \mathrm{HCOO}^{-}\) 2. \(\mathrm{H}_{2} \rightarrow \mathrm{H}^{-}\) 3. \(\mathrm{CH}_{4} \rightarrow \mathrm{CH}_{3}^{-}\) 4. \(\mathrm{HF} \rightarrow \mathrm{F}^{-}\) 5. \(\mathrm{NH}_{4}^{+} \rightarrow \mathrm{NH}_{3}\)

Identify the base strength of the conjugate base

Now we will determine whether each conjugate base is a strong base, weak base, or has negligible basicity: 1. \(\mathrm{HCOO}^{-}\) is the formate ion, a weak base. 2. \(\mathrm{H}^{-}\) is the hydride ion, a strong base. 3. \(\mathrm{CH}_{3}^{-}\) is the methyl anion, a species with negligible basicity. 4. \(\mathrm{F}^{-}\) is the fluoride ion, a weak base. 5. \(\mathrm{NH}_{3}\) is ammonia, a weak base. In summary, we have: a) \(\mathrm{HCOOH}\) is a weak acid, its conjugate base \(\mathrm{HCOO}^{-}\) is a weak base. b) \(\mathrm{H}_{2}\) has negligible acidity, its conjugate base \(\mathrm{H}^{-}\) is a strong base. c) \(\mathrm{CH}_{4}\) has negligible acidity, its conjugate base \(\mathrm{CH}_{3}^{-}\) has negligible basicity. d) \(\mathrm{HF}\) is a weak acid, its conjugate base \(\mathrm{F}^{-}\) is a weak base. e) \(\mathrm{NH}_{4}^{+}\) is a weak acid, its conjugate base \(\mathrm{NH}_{3}\) is a weak base.

Key concepts

Conjugate Base

When an acid donates a proton (H⁺) during a chemical reaction, it forms its conjugate base. The conjugate base is what remains of the acid once it loses the hydrogen ion. For example, formic acid (HCOOH) loses a proton to become its conjugate base, the formate ion (HCOO⁻).
In another example, when hydrofluoric acid (HF) loses its hydrogen ion, it forms the fluoride ion (F⁻) as its conjugate base.

• A strong acid produces a weak conjugate base, meaning the base does not tend to accept protons easily.
• A weak acid generates a relatively stronger conjugate base, which can more readily accept protons compared to a strong acid's conjugate base.
• Species with negligible acidity lead to conjugate bases that have significant basicity.

The concept of conjugate bases is essential in acid-base chemistry, as it helps predict the outcome and direction of chemical reactions that involve proton transfer.

Weak Acid

Weak acids do not completely dissociate into their ions in water. This means only a small fraction of acid molecules donate protons in a solution. Substances like hydrogfluoric acid (HF) and formic acid (HCOOH) are categorized as weak acids.

• Weak acids have higher equilibrium constants (Ka) than strong acids, but not high enough to dissociate completely in water.
• Their incomplete dissociation results in an equilibrium between the acid and its conjugate base in a solution.
• The conjugate base of a weak acid, such as formate ion (HCOO⁻) from formic acid, is often a weak base.

Understanding weak acids is crucial for grasping how buffers work and how acidity levels in solutions change.

Strong Base

A strong base readily accepts hydrogen ions (H⁺) in solution, often fully dissociating to produce hydroxide ions (OH⁻) and the corresponding cation. In the context of the original exercise, the hydride ion (H⁻) that forms from molecular hydrogen ( H₂ ), is considered a strong base.

• Strong bases have high pH values, usually above pH 12.
• They are efficient at accepting protons, making them very reactive in solution.
• Strong bases often neutralize acids effectively.

Knowing about strong bases is important because they are widely used in various industrial processes and chemical reactions.

Negligible Acidity

Species with negligible acidity, such as molecular hydrogen ( H₂ ) and methane ( CH₄ ), do not donate protons in solution. This characteristic makes them almost non-participatory in acid-base reactions.

• Negligible acidity scenarios often involve substances with very strong bonds to hydrogen, resisting ionization.
• These substances tend to form conjugate bases that exhibit significant basicity, such as the strong hydride ion (H⁻) from hydrogen.
• While they are not acids by traditional definitions, they provide insights into extremes of acidic and basic behavior in chemical systems.

Understanding species with negligible acidity offers a complete picture of the spectrum of acidity and basicity seen in chemical reactions.