Problem 21 - Chemistry The Central Science

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

Step-by-Step Solution

Answer

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

Step 1: Identify nature of species

This species \(\mathrm{CH}_{3}\mathrm{COO}^{-}\) is the conjugate base of acetic acid (\(\mathrm{CH}_{3}\mathrm{COOH}\)). It is a weak base because it is the conjugate base of a weak acid.

Step 2: Conjugate acid and its nature

The conjugate acid of this species is \(\mathrm{CH}_{3}\mathrm{COOH}\), which is a weak acid. (b) \(\mathrm{HCO}_{3}^{-}\)

Step 3: Identify nature of species

Bicarbonate ion (\(\mathrm{HCO}_{3}^{-}\)) is a weak base, as it is the conjugate base of carbonic acid (\(\mathrm{H}_{2}\mathrm{CO}_{3}\)).

Step 4: Conjugate acid and its nature

The conjugate acid of this species is \(\mathrm{H}_{2}\mathrm{CO}_{3}\), which is a weak acid. (c) \(\mathrm{O}^{2-}\)

Step 5: Identify nature of species

This species is a strong base. It is the oxide ion, formed when the stronger acid water (\(\mathrm{H}_{2}\mathrm{O}\)) loses two protons.

Step 6: Conjugate acid and its nature

The conjugate acid of this species is \(\mathrm{OH}^{-}\), which is a weak acid. (d) \(\mathrm{Cl}^{-}\)

Step 7: Identify nature of species

This species is the chloride ion and is considered to have negligible basicity, as it is the conjugate base of a strong acid (\(\mathrm{HCl}\)).

Step 8: Conjugate acid and its nature

The conjugate acid of this species is \(\mathrm{HCl}\), which is a strong acid. (e) \(\mathrm{NH}_{3}\)

Step 9: Identify nature of species

Ammonia (\(\mathrm{NH}_{3}\)) is a weak base because the nitrogen atom has a lone pair of electrons that it can use to bind a proton.

Step 10: Conjugate acid and its nature

The conjugate acid of this species is \(\mathrm{NH}_{4}^{+}\), which is a weak acid.

Key Concepts

Conjugate Acids and Bases

In acid-base chemistry, every acid has a conjugate base, and every base has a conjugate acid. When an acid donates a proton ( H^+ ), it forms its conjugate base. Conversely, when a base gains a proton, it forms its conjugate acid.
Let's explore some examples to understand this better:

• Acetic acid ( CH_{3}COOH ): Its conjugate base is CH_{3}COO^{-} . Since it's derived from a weak acid, it's a weak base.
• Ammonia ( NH_{3} ): Its conjugate acid is NH_{4}^{+} . Ammonia acts as a weak base, so its conjugate acid is also weak.
• Chloride ion ( Cl^{-} ): The conjugate acid is HCl , a strong acid. Therefore, the chloride ion has negligible basicity.

Understanding the relationship between acids and their conjugates can help predict chemical behavior, making it easier to balance reactions and predict products.

Weak and Strong Bases

Bases differ in their ability to accept protons, classified as weak or strong. Strong bases dissociate completely in water, while weak bases do not.

• Weak Bases: These include species like CH_{3}COO^{-} (acetate ion) and NH_{3} (ammonia). They partially accept protons, therefore not fully dissociating in water.
• Strong Bases: An example is O^{2-} (oxide ion), which strongly accepts protons, thus dissociates completely.
• Negligible Basicity: The Cl^{-} ion is an example, arising from HCl , a strong acid. It has virtually no ability to accept protons.

Understanding the strength of a base is crucial in predicting how it will behave in a reaction and in calculating pH values of solutions.

Acid-Base Reactions

Acid-base reactions are key chemical processes where an acid donates a proton to a base. The strength of acids and bases involved determines the extent of the reaction.
In these reactions:

• A strong acid like HCl fully dissociates, offering its H^+ to a base like OH^{-} .
• A weak acid, such as CH_{3}COOH , only partially donates H^+ , less fully reacting in solution.
• Reversible reactions often occur with weak acids and bases. For instance, NH_{3} reacting with water forms NH_{4}^{+} and OH^{-} , with an equilibrium established.

By analyzing the acid and base strengths, we can anticipate the products and whether the reaction will go to completion or reach equilibrium. This helps in planning chemical syntheses and understanding the natural chemical processes.