Question

1. which of the following are amphoteric in aqueous solution? for those that are amphoteric, write one equilibrium equation showing it acting as an acid and one equilibrium equation showing it acting as a base.

nh₄⁺, h₂po₄⁻, hcl, hc₂o₄⁻, ch₃nh₃⁺, hco₃⁻

Explanation:

Step1: Recall the definition of amphoteric

An amphoteric substance can act as both an acid (donate a proton) and a base (accept a proton).

Step2: Analyze each species

• $NH_4^+$: Can only donate a proton to act as an acid ($NH_4^+

ightleftharpoons NH_3 + H^+$), but it cannot easily accept a proton to act as a base. So it is not amphoteric.

• $H_2PO_4^-$: As an acid: $H_2PO_4^-

ightleftharpoons HPO_4^{2 -}+H^+$; as a base: $H_2PO_4^- + H^+
ightleftharpoons H_3PO_4$. It is amphoteric.

• $HCl$: Can only donate a proton ($HCl

ightleftharpoons H^++Cl^-$), it cannot act as a base. So it is not amphoteric.

• $HC_2O_4^-$: As an acid: $HC_2O_4^-

ightleftharpoons C_2O_4^{2 -}+H^+$; as a base: $HC_2O_4^-+H^+
ightleftharpoons H_2C_2O_4$. It is amphoteric.

• $CH_3NH_3^+$: Can only donate a proton ($CH_3NH_3^+

ightleftharpoons CH_3NH_2 + H^+$), but it cannot easily accept a proton to act as a base. So it is not amphoteric.

• $HCO_3^-$: As an acid: $HCO_3^-

ightleftharpoons CO_3^{2 -}+H^+$; as a base: $HCO_3^-+H^+
ightleftharpoons H_2CO_3$. It is amphoteric.

Answer:

The amphoteric species are $H_2PO_4^-$, $HC_2O_4^-$ and $HCO_3^-$.
For $H_2PO_4^-$: As an acid: $H_2PO_4^-
ightleftharpoons HPO_4^{2 -}+H^+$; as a base: $H_2PO_4^- + H^+
ightleftharpoons H_3PO_4$.
For $HC_2O_4^-$: As an acid: $HC_2O_4^-
ightleftharpoons C_2O_4^{2 -}+H^+$; as a base: $HC_2O_4^-+H^+
ightleftharpoons H_2C_2O_4$.
For $HCO_3^-$: As an acid: $HCO_3^-
ightleftharpoons CO_3^{2 -}+H^+$; as a base: $HCO_3^-+H^+
ightleftharpoons H_2CO_3$.