Question

Which equilibrium system has HCO3- acting as a Brønsted-Lowry base? HCO3- H+ + CO3 2- HCO3-  :  
 + HS- H2S + CO32-  HCO3-  :
+ H2S H2CO3 + HS- :  
     HCO3- + H2O H3O+ + CO32-  : 
          HCO3- + O2- CO32- + OH-

Answer 1

Answer is: HCO₃⁻ + H₂S → H₂CO3 + HS⁻.

According to  Bronsted-Lowry theory acid are donor of protons and bases are acceptors of protons (the hydrogen cation or H⁺).  

The bicarbonate anion (HCO₃⁻) is Bronsted base and it can accept proton and become conjugate acid H₂CO₃.

H₂S (hydrogen sulfide) is Bronsted-Lowry acid because it lost proton.

Answer 2

Answer : The equilibrium system in which $HCO3^-$ acting as a Brønsted-Lowry base is, $HCO_3^-+H_2S\rightleftharpoons H_2CO_3+HS^-$

Explanation :

According to the Bronsted-Lowry conjugate acid-base concept, an acid is a substance that donates protons and forming conjugate base and a base is a substance that accepts protons and forming conjugate acid.

$HCO_3^-+H_2S\rightleftharpoons H_2CO_3+HS^-$

In this equilibrium system, $HCO3^-$ acting as a Brønsted-Lowry base which accepts protons and forming conjugate acid $H_2CO_3$.

$HCO_3^-\rightleftharpoons H^++CO_3^{2-}\\\\HCO_3^-+HS^-\rightleftharpoons H_2S+CO_3^{2-}\\\\HCO_3^-+ H_2O\rightleftharpoons H_3O^++CO_3^{2-}\\\\HCO_3^-+O^{2-}\rightleftharpoons CO_3^{2-}+OH^-$

While in these equilibrium system, $HCO3^-$ acting as a Brønsted-Lowry acid which donates protons and forming conjugate base $CO_3^{2-}$.

Hence, the equilibrium system in which $HCO3^-$ acting as a Brønsted-Lowry base is, $HCO_3^-+H_2S\rightleftharpoons H_2CO_3+HS^-$