Which species is the conjugate acid of HCO3-?
A. CO32-
B. H2CO3
C. CO2
D. H2O
The Correct Answer and Explanation is:
The conjugate acid of ( \text{HCO}_3^- ) (bicarbonate) is ( \text{H}_2\text{CO}_3 ) (carbonic acid). Therefore, the correct answer is B. ( \text{H}_2\text{CO}_3 ).
To understand why ( \text{H}_2\text{CO}_3 ) is the conjugate acid of ( \text{HCO}_3^- ), we must first grasp the concept of conjugate acids and bases within the Brønsted-Lowry acid-base theory. In this context, an acid is defined as a proton (H(^+)) donor, while a base is a proton acceptor. When an acid donates a proton, it transforms into its conjugate base; conversely, when a base accepts a proton, it becomes its conjugate acid.
In the case of bicarbonate, ( \text{HCO}_3^- ) can act as a base. It has the ability to accept a proton (H(^+)) to form carbonic acid ( \text{H}_2\text{CO}_3 ). The chemical reaction can be represented as:
[
\text{HCO}_3^- + \text{H}^+ \rightarrow \text{H}_2\text{CO}_3
]
This process illustrates how bicarbonate accepts a proton, thereby becoming its conjugate acid.
Examining the other options:
- A. ( \text{CO}_3^{2-} ) (carbonate) is the conjugate base of ( \text{HCO}_3^- ), formed when bicarbonate loses a proton.
- C. ( \text{CO}_2 ) (carbon dioxide) is not a conjugate acid or base in this context but rather a product formed when carbonic acid dissociates.
- D. ( \text{H}_2\text{O} ) (water) is a neutral molecule and does not relate directly to the conjugate acid-base pair involving bicarbonate.
Thus, the relationship between ( \text{HCO}_3^- ) and ( \text{H}_2\text{CO}_3 ) highlights the fundamental principles of acid-base chemistry, with bicarbonate acting as a base that can accept a proton to become carbonic acid, its conjugate acid.