Final answer:
According to Brønsted-Lowry theory, acids are proton donors and bases are proton acceptors. NH3 and H2PO4 can act as either, depending on the reaction. H3O+ and HCl are examples of Brønsted-Lowry acids, while in the reaction HNO2 +- OH-, HNO2 acts as the acid and OH- as the base.
Step-by-step explanation:
The Brønsted-Lowry theory characterizes acids as proton donors and bases as proton acceptors. A conjugate acid-base pair consists of two species that transform into each other by the gain or loss of a proton (H+). For instance, when ammonia (NH3) acts as a base, it accepts a proton to form its conjugate acid, ammonium (NH4+). Conversely, the ammonium ion (NH4+) can donate a proton to revert to its conjugate base, NH3. When H2PO4- acts as an acid, it donates a proton to become HPO4^2-, its conjugate base; it can also act as a base by accepting a proton to become H3PO4, its conjugate acid.
In the case of H3O+ and HCl, both can act as Brønsted-Lowry acids by donating a proton. For example, the hydronium ion (H3O+) donates a proton to water (H2O) to form the conjugate acid-base pair of H3O+ (acid) and H2O (base), while HCl donates a proton to H2O to yield Cl- (conjugate base) and H3O+ (conjugate acid).
Examining the specific reaction of HNO2 with OH-, HNO2 functions as a Brønsted-Lowry acid by donating a proton to OH-, resulting in the production of NO2- (conjugate base) and H2O (conjugate acid).