An amphiprotic substance is a chemical species that can act as both a Brønsted-Lowry acid (donating a proton) and a Brønsted-Lowry base (accepting a proton). Understanding this dual capability is key to identifying such compounds.
Key Characteristics of Amphiprotic Substances
To determine if a substance is amphiprotic, it must meet two fundamental criteria related to its ability to interact with protons ($\text{H}^+$):
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Ability to Donate a Proton (Act as an Acid):
- The substance must contain a hydrogen atom that is capable of being released as a proton ($\text{H}^+$). This typically means the hydrogen is bonded to a highly electronegative atom (like oxygen or nitrogen), making the bond polar and the hydrogen relatively acidic.
- In essence, the substance must be "willing to lend its protons to other compounds."
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Ability to Accept a Proton (Act as a Base):
- The substance must be capable of accepting protons from other compounds. This usually implies the presence of a lone pair of electrons or a negative charge that can form a bond with the incoming proton.
Therefore, an amphiprotic substance possesses both an acidic hydrogen and a basic site (like a lone pair or negative charge) that allows it to react differently depending on the environment.
Common Examples of Amphiprotic Substances
Several well-known chemical species demonstrate amphiprotic behavior:
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Water ($\text{H}_2\text{O}$)
- As an acid: $\text{H}_2\text{O}$ can donate a proton to form a hydroxide ion ($\text{OH}^-$): $\text{H}_2\text{O} \rightleftharpoons \text{H}^+ + \text{OH}^-$
- As a base: $\text{H}_2\text{O}$ can accept a proton to form a hydronium ion ($\text{H}_3\text{O}^+$): $\text{H}_2\text{O} + \text{H}^+ \rightleftharpoons \text{H}_3\text{O}^+$
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Bicarbonate Ion ($\text{HCO}_3^-$)
- As an acid: The bicarbonate ion can donate its proton to form a carbonate ion ($\text{CO}_3^{2-}$): $\text{HCO}_3^- \rightleftharpoons \text{H}^+ + \text{CO}_3^{2-}$
- As a base: The bicarbonate ion can accept a proton to form carbonic acid ($\text{H}_2\text{CO}_3$): $\text{HCO}_3^- + \text{H}^+ \rightleftharpoons \text{H}_2\text{CO}_3$
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Dihydrogen Phosphate Ion ($\text{H}_2\text{PO}_4^-$)
- As an acid: It can donate a proton to form hydrogen phosphate ($\text{HPO}_4^{2-}$): $\text{H}_2\text{PO}_4^- \rightleftharpoons \text{H}^+ + \text{HPO}_4^{2-}$
- As a base: It can accept a proton to form phosphoric acid ($\text{H}_3\text{PO}_4$): $\text{H}_2\text{PO}_4^- + \text{H}^+ \rightleftharpoons \text{H}_3\text{PO}_4$
Understanding the fundamental Brønsted-Lowry acid-base theory is essential for recognizing these proton donation and acceptance capabilities.
