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: (a) \(\mathrm{CH}_{3} \mathrm{COO}^{-}\), (b) \(\mathrm{HCO}_{3}^{-},(\mathrm{c}) \mathrm{O}^{2-},(\mathrm{d}) \mathrm{Cl}^{-},(\mathrm{e}) \mathrm{NH}_{3}\)

Short answer

(a) \(\mathrm{CH}_{3}\mathrm{COO}^{-}\): weak base, conjugate acid: \(\mathrm{CH}_{3}\mathrm{COOH}\) (weak acid). (b) \(\mathrm{HCO}_{3}^{-}\): weak base, conjugate acid: \(\mathrm{H}_{2}\mathrm{CO}_{3}\) (weak acid). (c) \(\mathrm{O}^{2-}\): strong base, conjugate acid: \(\mathrm{OH}^{-}\) (negligible acidity). (d) \(\mathrm{Cl}^{-}\): negligible base, conjugate acid: \(\mathrm{HCl}\) (strong acid). (e) \(\mathrm{NH}_{3}\): weak base, conjugate acid: \(\mathrm{NH}_{4}^{+}\) (weak acid).

Step-by-step solution

(a) \(\mathrm{CH}_{3}\mathrm{COO}^{-}\)

This is a known weak base because it partially dissociates in water. Its conjugate acid is formed by adding a proton (H+) to it: \(\mathrm{CH}_{3}\mathrm{COOH}\). This conjugate acid, acetic acid, is a known weak acid.

(b) \(\mathrm{HCO}_{3}^{-}\)

This is also a weak base, bicarbonate ion. Its conjugate acid is formed by adding a proton (H+) to it: \(\mathrm{H}_{2}\mathrm{CO}_{3}\), which is carbonic acid and is also a weak acid.

(c) \(\mathrm{O}^{2-}\)

This is oxide anion, a strong base, since it readily accepts protons (H+). Its conjugate acid is formed by adding a proton (H+) to it: \(\mathrm{OH}^{-}\), which is the hydroxide ion, a well-known strong base itself. It does not act as an acid, so it has negligible acidity.

(d) \(\mathrm{Cl}^{-}\)

This species is a negligible base because it doesn't significantly affect the pH of the solution. Its conjugate acid is formed by adding a proton (H+) to it: \(\mathrm{HCl}\), which is hydrochloric acid. HCl is a strong acid.

(e) \(\mathrm{NH}_{3}\)

This is ammonia, a weak base because it does not dissociate completely in water. Its conjugate acid is formed by adding a proton (H+) to it: \(\mathrm{NH}_{4}^{+}\), which is ammonium ion. Ammonium ion is a weak acid.

Key concepts

Conjugate Acids and Bases

In acid-base chemistry, when a base gains a proton (H+), it becomes a conjugate acid. Similarly, when an acid loses a proton, it becomes a conjugate base. This relationship between acids and bases is crucial for understanding their behavior in chemical reactions.

A practical example is the acetate ion (\( \mathrm{CH}_{3}\mathrm{COO}^{-} \)) which, upon gaining a proton, turns into acetic acid (\( \mathrm{CH}_{3}\mathrm{COOH} \)). Acetic acid is the conjugate acid of the acetate ion. In this case, neither the base nor its conjugate acid is strong, but both are weak.

Recognizing conjugate pairs helps predict the outcomes of acid-base reactions and understand the equilibria in solutions.

Strong and Weak Acids

The strength of an acid refers to its ability to donate protons. Strong acids like hydrochloric acid (\( \mathrm{HCl} \)) dissociate completely in water and release protons with ease. On the other hand, weak acids dissociate only partially, leading to an equilibrium state in solution.

For example, acetic acid (\( \mathrm{CH}_{3}\mathrm{COOH} \)) is a typical weak acid. It does not fully ionize in solution, making it less capable of donating protons compared to a strong acid like \( \mathrm{HCl} \). Understanding this distinction is fundamental to predicting how a solution will behave once an acid is added.

Strong and Weak Bases

Bases can also be classified as strong or weak, depending on their ability to accept protons (H+) or donate hydroxide ions (OH⁻). Strong bases, such as the oxide ion (\( \mathrm{O}^{2-} \)), readily accept protons and contribute significantly to the increase in \( \mathrm{OH}^{-} \) concentration in solution.

In contrast, weak bases like ammonia (\( \mathrm{NH}_{3} \)) do not completely dissociate in solution. They exist in equilibrium, accepting protons only to a limited extent. Knowing the difference between strong and weak bases is essential for anticipating the changes that occur when bases are added to a solution.

Negligible Acidity and Basicity

Some substances have negligible acidity or basicity, meaning they do not significantly alter the pH of a solution. They neither readily donate nor accept protons.

An excellent example of a species with negligible basicity is the chloride ion (\( \mathrm{Cl}^{-} \)). It doesn't affect the solution's pH significantly because it does not accept protons easily. Its conjugate acid, \( \mathrm{HCl} \), is a strong acid, and after losing its proton, \( \mathrm{Cl}^{-} \) remains inactive in terms of base activity.

Recognizing such species can help clarify why certain compounds do not participate actively in acid-base reactions.