When HCO3- acts as a Bronsted base, it donates a proton (H+) to another molecule or ion, resulting in the formation of carbonic acid (H2CO3) and water (H2O). The reaction can be represented as follows:

HCO3- + H2O ⇌ H2CO3 + OH-

In this reaction, HCO3- donates a proton to H2O, which acts as a Bronsted acid. The resulting products are carbonic acid, which is a weak acid, and hydroxide ions (OH-), which are a base.