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: (a) \(\mathrm{HNO}_{2}\), (b) \(\mathrm{H}_{2} \mathrm{SO}_{4},(\mathrm{c}) \mathrm{HPO}_{4}{ }^{2-}\), (d) \(\mathrm{CH}_{4}\) (e) \(\mathrm{CH}_{3} \mathrm{NH}_{3}{ }^{+}\) (an ion related to \(\mathrm{NH}_{4}{ }^{+}\) ).

Short answer

(a) HNO2 is a weak acid with conjugate base NO2⁻, which is a weak base. (b) H2SO4 is a strong acid with conjugate base HSO4⁻, which is a weak base. (c) HPO4²⁻ is a weak acid with conjugate base PO4³⁻, which is a weak base. (d) CH4 has negligible acidity with conjugate base CH3⁻, which is a strong base. (e) CH3NH3⁺ is a weak acid with conjugate base CH3NH2, which is a weak base.

Step-by-step solution

a) HNO2

HNO2 is a weak acid because it does not completely dissociate in water. Its conjugate base can be determined by removing one proton (H⁺) from the acid: HNO2 → NO2⁻. NO2⁻ is a weak base, as it is the conjugate base of a weak acid.

b) H2SO4

H2SO4 is a strong acid because it completely dissociates in water. Its conjugate base can be determined by removing one proton (H⁺) from the acid: H2SO4 → HSO4⁻. HSO4⁻ is a weak base since it is moderately basic, but not completely dissociated, and is the conjugate base of a strong acid.

c) HPO4²⁻

HPO4²⁻ is a weak acid, as it donates protons, but not completely, in water. Its conjugate base can be determined by removing one proton (H⁺) from the acid: HPO4²⁻ → PO4³⁻. PO4³⁻ is a weak base, as it is the conjugate base of a weak acid.

d) CH4

CH4 has negligible acidity, as it does not readily donate protons in water. Its conjugate base can be determined by removing one proton (H⁺) from the species: CH4 → CH3⁻. CH3⁻ is a strong base, as it is highly reactive and is the conjugate base of a species with negligible acidity.

e) CH3NH3⁺

CH3NH3⁺ is a weak acid, as it donates protons, but not completely, in water. This is similar to the behavior of NH4⁺. Its conjugate base can be determined by removing one proton (H⁺) from the acid: CH3NH3⁺ → CH3NH2. CH3NH2 is a weak base, as it is the conjugate base of a weak acid.

Key concepts

Strong Acid

A strong acid is an acid that completely dissociates into its ions in water. This means that when the acid is dissolved, it breaks apart into its respective hydrogen ions (H⁺) and anions. For example, sulfuric acid (\(\mathrm{H}_2\mathrm{SO}_4\)) is a strong acid because it fully dissociates to produce \(\mathrm{HSO}_4^-\) and \(\mathrm{H}^+\) ions.
A key characteristic of strong acids is their ability to lower the pH of a solution significantly because all the acidic protons are available to increase the concentration of hydrogen ions. Strong acids include hydrochloric acid (\(\mathrm{HCl}\)), nitric acid (\(\mathrm{HNO}_3\)), and sulfuric acid. The conjugate base of a strong acid is usually a weak base or has negligible basicity because the ion left behind (anion) is stable and does not readily reaccept a proton.

Weak Acid

Weak acids only partially dissociate in solution, meaning not all acid molecules donate their protons to the solution. For example, nitrous acid (\(\mathrm{HNO}_2\)) is a weak acid because it does not completely dissociate into nitrite ions (\(\mathrm{NO}_2^−\)) and hydrogen ions.Weak acids usually establish an equilibrium between the undissociated and dissociated forms. This equilibrium feature makes weak acids less active in changing the pH than strong acids. Examples of weak acids include acetic acid (\(\mathrm{CH}_3\mathrm{COOH}\)) and phosphoric acid (\(\mathrm{H}_3\mathrm{PO}_4\)). The conjugate base of a weak acid, like \(\mathrm{NO}_2^−\) from \(\mathrm{HNO}_2\), tends to be a weak base, indicating mild reactivity in accepting protons back.

Conjugate Base

A conjugate base is formed when an acid donates a proton (\(\mathrm{H}^+\)). The conjugate base can potentially accept a proton, making it a base. For instance, if you remove a proton from \(\mathrm{HPO}_4^{2-}\), you get its conjugate base \(\mathrm{PO}_4^{3-}\).
The strength of a conjugate base often depends on its parent acid. Strong acids have weak conjugate bases because the original acid ionizes completely, leaving a conjugate base that has little tendency to re-associate with a proton. In contrast, weak acids have stronger conjugate bases because the partial dissociation means the conjugate base is more willing to accept the proton back.
Identifying the conjugate base helps understand acid-base behavior because it reveals the substance's ability to affect the equilibrium balance in a chemical reaction.

Negligible Acidity

Negligible acidity refers to species that are poor proton donors. They hardly ionize to donate hydrogen ions in solution. Methane (\(\mathrm{CH}_4\)) is an example, with no tendency to dissociate in water to form \(\mathrm{H}^+\) ions.
Species with negligible acidity often form strong bases when they lose a proton because the corresponding conjugate base has a strong affinity for protons. For example, removing \(\mathrm{H}^+\) from \(\mathrm{CH}_4\) yields \(\mathrm{CH}_3^-\), a highly reactive and strong base.
This concept is crucial in predicting the reactions and interactions of different substances in both experimental and theoretical chemistry.

Weak Base

A weak base is a substance that does not fully ionize or accept protons in an aqueous solution. They are characterized by a less intense reaction with acids. For example, ammonia (\(\mathrm{NH}_3\)) is a common weak base that partially accepts protons to form \(\mathrm{NH}_4^+\).
In acid-base chemistry, weak bases are often formed from weak acids, such as \(\mathrm{CH}_3\mathrm{NH}_2\), which is the conjugate base of \(\mathrm{CH}_3\mathrm{NH}_3^+\). This amine is a weak base because it weakly associates with protons.
Understanding weak bases helps in determining their role in neutralization reactions and their effect on pH, which is less drastic than that of strong bases. Weak bases are pivotal in processes where control over pH levels is necessary, such as in biological systems or in buffering solutions.