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) ${\mathrm{CH}}_3{\mathrm{COO}}^{-}$: 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 ${\mathrm{OH}}^{-}$: strong acid (d) ${\mathrm{Cl}}^{-}$: negligible basicity, conjugate acid $\mathrm{HCl}$: strong acid (e) ${\mathrm{NH}}_3$: weak base, conjugate acid ${\mathrm{NH}}_4^{+}$: weak acid

Step-by-step solution

Classify and identify the conjugate acids

To classify each species and identify their conjugate acids, follow these steps: 1. If the species is a base, it will accept a proton (H+) and become its conjugate acid. 2. If the species is an acid, it will donate a proton (H+) and become its conjugate base. Let's go through each species one by one: (a) ${\mathrm{CH}}_3{\mathrm{COO}}^{-}$

Classifying CH3COO- and its conjugate acid

${\mathrm{CH}}_3{\mathrm{COO}}^{-}$ is the conjugate base of the weak acid ${\mathrm{CH}}_3\mathrm{COOH}$ (acetic acid). Therefore, it is a weak base. We can find the conjugate acid by adding a proton (H+) to the base: ${\mathrm{CH}}_3{\mathrm{COO}}^{-}+{\mathrm{H}}^{+}\to {\mathrm{CH}}_3\mathrm{COOH}$ The conjugate acid, ${\mathrm{CH}}_3\mathrm{COOH}$, is a weak acid. (b) ${\mathrm{HCO}}_3^{-}$

Classifying HCO3- and its conjugate acid

${\mathrm{HCO}}_3^{-}$ is the conjugate base of the weak acid ${\mathrm{H}}_2{\mathrm{CO}}_3$ (carbonic acid) and can also act as an acid by donating a proton to become ${\mathrm{CO}}_3^{2-}$, a weaker conjugate base. Therefore, ${\mathrm{HCO}}_3^{-}$ is a weak base. We can find the conjugate acid by adding a proton: ${\mathrm{HCO}}_3^{-}+{\mathrm{H}}^{+}\to {\mathrm{H}}_2{\mathrm{CO}}_3$ The conjugate acid, ${\mathrm{H}}_2{\mathrm{CO}}_3$, is a weak acid. (c) ${\mathrm{O}}^{2-}$

Classifying O2- and its conjugate acid

${\mathrm{O}}^{2-}$ is a strong base, as it is willing to accept protons readily. We can find the conjugate acid by adding a proton: ${\mathrm{O}}^{2-}+{\mathrm{H}}^{+}\to {\mathrm{OH}}^{-}$ The conjugate acid, ${\mathrm{OH}}^{-}$, is a strong acid. (d) ${\mathrm{Cl}}^{-}$

Classifying Cl- and its conjugate acid

${\mathrm{Cl}}^{-}$ is the conjugate base of the strong acid $\mathrm{HCl}$ (hydrochloric acid). Consequently, it has negligible basicity. We can find the conjugate acid by adding a proton: ${\mathrm{Cl}}^{-}+{\mathrm{H}}^{+}\to \mathrm{HCl}$ The conjugate acid, $\mathrm{HCl}$, is a strong acid. (e) ${\mathrm{NH}}_3$

Classifying NH3 and its conjugate acid

${\mathrm{NH}}_3$ (ammonia) is a weak base, as it readily accepts protons but does not dissociate completely in water. We can find the conjugate acid by adding a proton: ${\mathrm{NH}}_3+{\mathrm{H}}^{+}\to {\mathrm{NH}}_4^{+}$ The conjugate acid, ${\mathrm{NH}}_4^{+}$ (ammonium ion), is a weak acid. In summary, we have classified each species and their corresponding conjugate acids in terms of acidity and basicity: (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 ${\mathrm{OH}}^{-}$: strong acid (d) ${\mathrm{Cl}}^{-}$: negligible basicity, conjugate acid $\mathrm{HCl}$: strong acid (e) ${\mathrm{NH}}_3$: weak base, conjugate acid ${\mathrm{NH}}_4^{+}$: weak acid

Key concepts

Conjugate Acids

In an acid-base reaction, when a base receives a proton (H${}^{+}$), it transforms into what is known as its conjugate acid. This is an essential concept because it helps us understand the balance and roles in chemical reactions. For example:

• Acetate ion ${\mathrm{CH}}_3{\mathrm{COO}}^{-}$ becomes acetic acid ${\mathrm{CH}}_3\mathrm{COOH}$ upon gaining a proton.
• The bicarbonate ion ${\mathrm{HCO}}_3^{-}$ becomes carbonic acid ${\mathrm{H}}_2{\mathrm{CO}}_3$.

In these transformations, the original base accepts a hydrogen ion to form its conjugate acid, demonstrating how dynamic and interactive these chemical species are. Each paired conjugate acid and base can decide whether a reverse reaction might occur, reflecting the potential directionality of acid-base reactions.

Acid Strength

Acid strength refers to the degree to which an acid dissociates in water. The stronger the acid, the more it releases H${}^{+}$ ions into the solution.
There are many factors affecting acid strength, including the bond energy between the hydrogen and the rest of the acid molecule, and the molecule's stability after losing a proton:

• Strong acids like hydrochloric acid (HCl) dissociate almost completely in water, releasing significant amounts of hydrogen ions.
• Weak acids like acetic acid (${\mathrm{CH}}_3\mathrm{COOH}$) do not fully dissociate, leaving most of the acid molecules intact in solution.

Understanding acid strength is crucial for predicting the behavior of acids in chemical reactions, including their ability to donate protons to bases.

Base Strength

Base strength, on the other hand, refers to a base's ability to accept protons. Just like acids, bases vary widely in their proton-accepting capacity:

• Strong bases, such as the hydroxide ion (OH${}^{-}$), efficiently accept protons, turning into water when they react with hydrogen ions.
• Weak bases, like ammonia (NH${}_3$), accept protons but do less so compared to strong bases.

The base's strength is influenced by the atom's electronegativity, the ability of its conjugate acid, and its structural features. This strength can also impact reaction speed and equilibrium by altering how readily a base can compete for positive hydrogen ions.

Strong Bases

Strong bases are characterized by their complete dissociation in water.This means they can readily attract and bond with hydrogen ions. A common example is the oxide ion (O${}^{2-}$), which will quickly accept a hydrogen ion to become hydroxide (OH${}^{-}$).
These bases:

• Have high pH values,
• Are powerful conductors of electricity in aqueous solutions, and
• Are often used in industrial processes due to their strong reactivity.

Recognizing strong bases is essential for applications where neutralization and pH adjustment are necessary.

Weak Bases

Weak bases are compounds that do not fully accept protons in water. They react partially, leaving behind a significant proportion of the unreacted base molecules. Ammonia (${\mathrm{NH}}_3$) is a classic example of a weak base.
Key characteristics include:

• Relatively lower pH than strong bases, closer to 7 but above it.
• Incomplete electrical conductivity in solution because only some molecules ionize.

These bases are often utilized in processes where a mild reaction is necessary, such as in biological systems or where less corrosive behavior is desired. Recognizing weak bases helps in adjusting reactions to prevent overly aggressive chemical interactions.