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) \(\mathrm{HCO}_{3}^{-}\), (c) \(\mathrm{O}^{2-}\), (d) \(\mathrm{Cl}^{-}\) (e) \(\mathrm{NH}_{3} .\)

Short answer

(a) \(\mathrm{CH}_{3}\mathrm{COO}^{-}\) is a weak base with its conjugate acid \(\mathrm{CH}_{3}\mathrm{COOH}\) (a weak acid). (b) \(\mathrm{HCO}_{3}^{-}\) is a weak base with its conjugate acid \(\mathrm{H}_{2}\mathrm{CO}_{3}\) (a weak acid). (c) \(\mathrm{O}^{2-}\) is an extremely strong base with its conjugate acid \(\mathrm{OH}^{-}\) (a strong base). (d) \(\mathrm{Cl}^{-}\) has negligible basicity with its conjugate acid \(\mathrm{HCl}\) (a strong acid). (e) \(\mathrm{NH}_{3}\) is a weak base with its conjugate acid \(\mathrm{NH}_{4}^{+}\) (a weak acid).

Step-by-step solution

(a) CH3COO- - Base strength and conjugate acid

The \(\mathrm{CH}_{3}\mathrm{COO}^{-}\) ion is a conjugate base of acetic acid (\(\mathrm{CH}_{3}\mathrm{COOH}\)). Acetic acid is a weak acid, and therefore, \(\mathrm{CH}_{3}\mathrm{COO}^{-}\) is a weak base. Its conjugate acid is \(\mathrm{CH}_{3}\mathrm{COOH}\) (acetic acid), which is a weak acid.

(b) HCO3- - Base strength and conjugate acid

The \(\mathrm{HCO}_{3}^{-}\) ion (hydrogen carbonate or bicarbonate) is the conjugate base of carbonic acid (\(\mathrm{H}_{2}\mathrm{CO}_{3}\)). Carbonic acid is a weak acid, so \(\mathrm{HCO}_{3}^{-}\) is a weak base. Its conjugate acid is \(\mathrm{H}_{2}\mathrm{CO}_{3}\) (carbonic acid), which is a weak acid.

(c) O2- - Base strength and conjugate acid

The \(\mathrm{O}^{2-}\) ion (oxide ion) is a very strong base because it readily accepts protons (H+ ions) due to its high negative charge. Its conjugate acid is \(\mathrm{OH}^{-}\) (hydroxide ion) formed by accepting one proton. The conjugate acid, \(\mathrm{OH}^{-}\), is also a strong base, which means that \(\mathrm{O}^{2-}\) is an extremely strong base.

(d) Cl- - Base strength and conjugate acid

The \(\mathrm{Cl}^{-}\) ion (chloride ion) is the conjugate base of hydrochloric acid (\(\mathrm{HCl}\)), which is a strong acid. Therefore, \(\mathrm{Cl}^{-}\) has negligible basicity. Its conjugate acid is \(\mathrm{HCl}\) (hydrochloric acid), which is a strong acid.

(e) NH3 - Base strength and conjugate acid

Ammonia (\(\mathrm{NH}_{3}\)) is a weak base due to its ability to accept protons (H+ ions) and form the ammonium ion (\(\mathrm{NH}_{4}^{+}\)). Its conjugate acid is \(\mathrm{NH}_{4}^{+}\) (ammonium ion), which is produced when it accepts a proton (H+ ion). Ammonium, as the conjugate acid of a weak base, is a weak acid.

Key concepts

Conjugate Acid-Base Pairs

Understanding the concept of conjugate acid-base pairs is crucial to mastering acid-base chemistry. An acid and its conjugate base differ by one proton. When an acid donates a proton, it becomes its conjugate base; conversely, when a base accepts a proton, it becomes its conjugate acid.

For example, acetic acid (\( \text{CH}_3\text{COOH} \)) donates a proton to become the acetate ion (\( \text{CH}_3\text{COO}^- \)), its conjugate base. In reverse, the acetate ion, being a base, can accept a proton, reverting to acetic acid. This reversible transfer of protons encapsulates the dynamic essence of acid-base reactions and is central to understanding chemical equilibrium in such systems.

Examining Acid-Base Behavior

Differentiating between strong and weak acids or bases is pivotal. For instance, hydrochloric acid (\( \text{HCl} \)) is a strong acid, and its conjugate base, the chloride ion (\( \text{Cl}^- \)), has negligible basicity. That is because strong acids fully dissociate in water, leaving behind a conjugate base that is not prone to accept protons easily.

Base Strength

The base strength tells us how readily a base can accept a proton. This property is influenced by the structure of the base, electronegativity of the atoms involved, and the overall stability of the conjugate acid formed.

Strong bases like the oxide ion (\( \text{O}^{2-} \)) have a high affinity for protons due to available lone pairs of electrons and their high negativity, which easily form strong bases such as the hydroxide ion (\( \text{OH}^- \)) after accepting a proton. However, weak bases such as ammonia (\( \text{NH}_3 \)) accept protons less easily, having a less readily accessible electron pair and forming a less stable conjugate acid (\( \text{NH}_4^+ \)).

Understanding the Spectrum

Base strength varies across a spectrum, with strong bases like hydroxides on one end and species with negligible basicity on the other, which rarely, if ever, act as bases because they are the conjugate bases of strong acids or because they have little tendency to attract and bind protons.

Negligible Acidity

Some substances feature what's termed as negligible acidity. These are typically the conjugate bases of strong acids and exhibit almost no tendency to behave as acids themselves in aqueous solutions. This is because they are already stable and have a low affinity for adding a proton.

The chloride ion (\( \text{Cl}^- \)) is a prime example: it is produced when hydrochloric acid (\( \text{HCl} \)), a strong acid, dissociates. The high stability of \( \text{HCl} \) indicates that it's fully ionized in solution, and the resulting \( \text{Cl}^- \) ions are not interested in re-acquiring a proton. In practice, such ions do not contribute to acidity. Whenever we encounter negligible acidity in a compound, it's a cue that its conjugate acid is fiercely acidic and that the tendency for reverse action—acting as an acid—is minimal.