does a mixture of carbonic acid, H2CO3, a weak acid, and sodium bicarbonate, NaHCO3 in water constitute a buffer? If no explain why
Yes, a mixture of carbonic acid, H2CO3, and sodium bicarbonate, NaHCO3, in water constitutes a buffer system.
To understand why, let's first discuss what a buffer is. A buffer is a solution that resists changes in its pH when small amounts of acid or base are added to it. Buffers are usually made up of a weak acid and its conjugate base or a weak base and its conjugate acid.
In this case, carbonic acid, H2CO3, is a weak acid, and sodium bicarbonate, NaHCO3, is the conjugate base.
When carbonic acid is dissolved in water, it partially dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3-):
H2CO3 ⇌ H+ + HCO3-
The bicarbonate ions can act as a base and can accept additional hydrogen ions to form carbonic acid:
HCO3- + H+ ⇌ H2CO3
Now, let's consider what happens when a small amount of acid or base is added to the mixture:
1. If an acid is added: The additional hydrogen ions (H+) will react with the bicarbonate ions (HCO3-) in the solution, shifting the equilibrium towards the formation of more carbonic acid (H2CO3). This reaction helps to neutralize the added acid and minimize the change in pH.
2. If a base is added: The additional hydroxide ions (OH-) from the base will react with the carbonic acid (H2CO3), forming bicarbonate ions (HCO3-) and water. This reaction helps to neutralize the added base and minimize the change in pH.
Therefore, the combination of carbonic acid and sodium bicarbonate in water forms a buffer because it can resist changes in pH by adjusting the concentrations of carbonic acid and bicarbonate ions in response to the addition of acid or base.