Question

Explain why the conjugate base of a strong acid is a weak base and the conjugate acid of a strong base is a weak acid.

Short answer

Strong acids have weak conjugate bases because they ionize completely. Strong bases have weak conjugate acids because they dissociate completely.

Step-by-step solution

Understanding Strong Acids and Bases

Strong acids are substances that completely ionize in solution, meaning they donate protons (H⁺) very easily. Similarly, strong bases are substances that completely dissociate in solution to donate hydroxide ions (OH⁻).

Defining Conjugate Base and Conjugate Acid

The conjugate base of an acid is what remains after the acid has donated a proton. The conjugate acid of a base is what forms after the base has accepted a proton.

Weak Nature of Conjugate Bases of Strong Acids

Since strong acids ionize completely, their conjugate bases have very low affinity to re-accept a proton, making them weak bases.

Weak Nature of Conjugate Acids of Strong Bases

Similarly, since strong bases dissociate completely, their conjugate acids have very low tendency to donate a proton, making them weak acids.

Key Takeaway

The strength of an acid or base is inversely related to the strength of its conjugate. A strong acid has a weak conjugate base, and a strong base has a weak conjugate acid.

Key concepts

Strong Acids

Strong acids are acids that completely ionize in water. This means that when a strong acid, like hydrochloric acid (HCl), is dissolved in water, it breaks down entirely into its ions:
Hydrogen ion, H⁺ and Chloride ion, Cl⁻.
A strong acid donates protons (H⁺) very easily, which means it has no trouble breaking its hydrogen bonds.
Examples of strong acids include:

• Hydrochloric acid (HCl)
• Sulfuric acid (H₂SO₄)
• Nitric acid (HNO₃)

Due to their complete ionization, the conjugate bases of strong acids are weak. This is because they have little tendency to re-accept the proton they have donated.

Strong Bases

Strong bases are substances that completely dissociate in water. When a strong base, like sodium hydroxide (NaOH), is dissolved in water, it breaks down completely into its ions:
Sodium ion, Na⁺ and Hydroxide ion, OH⁻.
A strong base easily donates hydroxide ions (OH⁻), which means it dissociates very readily.
Examples of strong bases include:

• Sodium hydroxide (NaOH)
• Potassium hydroxide (KOH)
• Calcium hydroxide (Ca(OH)₂)

Since strong bases dissociate completely, their conjugate acids are weak because they have a very low tendency to donate a proton.

Conjugate Acids

A conjugate acid forms when a base accepts a proton (H⁺). For example, if ammonia (NH₃) accepts a proton, it becomes its conjugate acid, ammonium (NH₄⁺).
Conjugate acids of strong bases are weak acids. This is because strong bases completely dissociate in water, and thus the conjugate acid formed has very low tendency to donate its proton back.
In essence, a strong base has a weak conjugate acid.

Conjugate Bases

A conjugate base is what remains after an acid has donated a proton. For instance, after hydrochloric acid (HCl) donates a proton, the chloride ion (Cl⁻) is formed as the conjugate base.
Conjugate bases of strong acids are weak bases. This is because strong acids completely ionize in water, and hence the conjugate base has very low tendency to accept the proton back.
To sum up, a strong acid has a weak conjugate base.

Ionization

Ionization is the process in which an atom or a molecule gains or loses an electron to form ions. For acids and bases, this involves donating or accepting protons (H⁺).
Strong Acids completely ionize in solution, meaning they split into hydrogen ions (H⁺) and their conjugate base.
Strong Bases completely dissociate into hydroxide ions (OH⁻) and their conjugate acid.
Understanding ionization is critical for grasping why the conjugate base of a strong acid is weak and the conjugate acid of a strong base is weak. The full ionization of strong acids and strong bases makes their conjugates have a low tendency to re-ionize.