Question

Specify which of these systems can be classified as a buffer system: (a) \(\mathrm{KCl} / \mathrm{HCl}\), (b) \(\mathrm{NH}_{3} / \mathrm{NH}_{4} \mathrm{NO}_{3}\) (c) \(\mathrm{Na}_{2} \mathrm{HPO}_{4} / \mathrm{NaH}_{2} \mathrm{PO}_{4}\)

Short answer

The systems B (\(\mathrm{NH}_{3} / \(\mathrm{NH}_{4} \mathrm{NO}_{3}\)) and C (\(\mathrm{Na}_{2} \mathrm{HPO}_{4} / \(\mathrm{NaH}_{2} \mathrm{PO}_{4}\)) can be classified as buffer systems while system A (\(\mathrm{KCl} / \mathrm{HCl}\)) can not be classified as a buffer system.

Step-by-step solution

Identify the Acids and Bases

Acids donate protons in a chemical reaction, whereas bases accept protons. In the context of a buffer system, we are looking for a weak acid/base = > (a) \(\mathrm{KCl} / \mathrm{HCl}\) - Here, \( \mathrm{HCl} \) is a strong acid, not a weak one. So, this cannot be a buffer.(b) \(\mathrm{NH}_{3} / \(\mathrm{NH}_{4} \mathrm{NO}_{3}\) - Here, \( \mathrm{NH}_{3} \) is a weak base and its conjugate acid is \( \mathrm{NH}_{4}^{+}\), which is present in \(\mathrm{NH}_{4} \mathrm{NO}_{3}\). So, this could be a buffer.(c) \(\mathrm{Na}_{2} \mathrm{HPO}_{4} / \mathrm{NaH}_{2} \mathrm{PO}_{4}\) - Here, \(\mathrm{Na}_{2} \mathrm{HPO}_{4}\) is a weak base and \(\mathrm{NaH}_{2} \mathrm{PO}_{4}\) is its conjugate acid. So, this could be a buffer.

Conclusion

After analyzing each system, two of them - (b) \(\mathrm{NH}_{3} / \(\mathrm{NH}_{4} \mathrm{NO}_{3}\) and (c) \(\mathrm{Na}_{2} \mathrm{HPO}_{4} / \(\mathrm{NaH}_{2} \mathrm{PO}_{4}\) - meet the requirements of a buffer system because they include a weak acid/base and their respective conjugate base/acid. The first system (a) does not meet the requirement as it contains a strong acid, which is not suitable for a buffer system.

Key concepts

Weak Acid

A weak acid is a substance that partially dissociates into ions in an aqueous solution. This means only a small fraction of the acid molecules release protons (H\(^+\)) into the solution. Because of this partial ionization, weak acids maintain an equilibrium between their non-dissociated form and their ions. This equilibrium is crucial for buffer systems.

• Weak acids are often characterized by a larger pH range of effectiveness.
• Acetic acid (vinegar) and carbonic acid (found in carbonated beverages) are common examples.

These acids work in buffer systems by donating protons when the solution becomes too basic. They help to keep the pH stable, which is essential in many biological and chemical processes.

Conjugate Base

When a weak acid donates a proton, it transforms into its conjugate base. This process forms a conjugate acid-base pair. For example, the conjugate base of acetic acid is acetate ( ext{CH}_3 ext{COO}^-). In buffer solutions, the conjugate base plays a balancing act with its weak acid counterpart.

• The conjugate base can accept protons from the environment, effectively reducing acidity.
• Its presence alongside a weak acid ensures that the buffer can neutralize added acids or bases.

The conjugate base's ability to accept protons is vital in maintaining the buffer's range of pH stability.

Proton Donor

A proton donor is another term often used to describe acids, specifically in the context of acid-base reactions. In these reactions, the donor gives away a hydrogen ion (H\(^+\)) to another substance.

• In water, weak acids act as proton donors by releasing H\(^+\) ions into the solution.
• These protons can be accepted by the corresponding conjugate base in the buffer system.

The simplicity of donating protons allows weak acids to respond to pH changes effectively, making them indispensable in creating effective buffer systems that resist drastic changes in pH.

Weak Base

A weak base is a substance that only partially accepts protons in an aqueous solution. Unlike strong bases that completely dissociate, weak bases remain mostly intact, but they are capable of accepting protons if conditions demand it.

• Common examples include ammonia (NH\(_3\)) and baking soda (sodium bicarbonate).
• Weak bases play a critical role in buffer systems by accepting protons when the pH drops.

In a buffer system, the weak base works together with its conjugate acid to neutralize strong acids or bases that might otherwise alter the solution's pH. The balanced interplay between these weak bases and their conjugate acids or bases is what allows buffers to be so effective at maintaining stability.