Answer: H2CO3 reacts to a strong base and NaHCO3 reacts to a strong acid in a carbonic acid/bicarbonate buffer system.
When a strong base is added to a buffer solution, the buffer pair in the system reacts with the strong base to form the conjugate acid of the weak base. For example, in a carbonic acid/bicarbonate buffer system, when a strong base like NaOH is added, it reacts with the carbonic acid component of the system to form bicarbonate ion and water: H2CO3 + NaOH → NaHCO3 + H2O
The bicarbonate ion acts as a weak base that acts to neutralize the added strong base and maintains the pH of the solution. On the other hand, when a strong acid is added to the buffer solution, the buffer pair in the system reacts with the strong acid to form the conjugate base of the weak acid. For example, in the same carbonic acid/bicarbonate buffer system, when a strong acid like HCl is added, it reacts with the bicarbonate ion component of the system to form carbonic acid: NaHCO3 + HCl → NaCl + H2CO3
The carbonic acid acts as a weak acid that acts to neutralize the added strong acid and maintains the pH of the solution. So to answer your question, H2CO3 reacts to a strong base and NaHCO3 reacts to a strong acid in a carbonic acid/bicarbonate buffer system.