A bronsted acid/base will have a corresponding conjugate base/acid.

Explanation:

In order to determine which species can act as both a Bronsted acid and base, we need to understand the concept of Bronsted acids and bases.

Bronsted Acid:
A Bronsted acid is a species that donates a proton (H+) in a chemical reaction. It is a species that can easily lose a hydrogen ion.

Bronsted Base:
A Bronsted base is a species that accepts a proton (H+) in a chemical reaction. It is a species that can easily gain a hydrogen ion.

Now, let's analyze each of the given species to determine if they can act as both a Bronsted acid and base:

1. H2O (water):
- Water can act as a Bronsted acid by donating a proton to a base. For example, in the reaction: H2O + NH3 -> NH4+ + OH-, water donates a proton to ammonia (NH3) and acts as an acid.
- Water can also act as a Bronsted base by accepting a proton from an acid. For example, in the reaction: HCl + H2O -> H3O+ + Cl-, water accepts a proton from hydrochloric acid (HCl) and acts as a base.
- Therefore, water can act as both a Bronsted acid and base.

2. NH2- (amide ion):
- The amide ion can act as a Bronsted base by accepting a proton. For example, in the reaction: NH2- + HCl -> NH3 + Cl-, the amide ion accepts a proton from hydrochloric acid and acts as a base.
- However, the amide ion cannot act as a Bronsted acid because it does not have a readily available hydrogen ion to donate.
- Therefore, the amide ion can only act as a Bronsted base, not as an acid.

3. NH3 (ammonia):
- Ammonia can act as a Bronsted base by accepting a proton. For example, in the reaction: NH3 + HCl -> NH4+ + Cl-, ammonia accepts a proton from hydrochloric acid and acts as a base.
- Ammonia can also act as a Bronsted acid by donating a proton. For example, in the reaction: NH3 + H2O -> NH4+ + OH-, ammonia donates a proton to water and acts as an acid.
- Therefore, ammonia can act as both a Bronsted acid and base.

4. HCO3- (bicarbonate ion):
- The bicarbonate ion can act as a Bronsted base by accepting a proton. For example, in the reaction: HCO3- + HCl -> H2CO3 + Cl-, the bicarbonate ion accepts a proton from hydrochloric acid and acts as a base.
- However, the bicarbonate ion can also act as a Bronsted acid by donating a proton. For example, in the reaction: HCO3- + OH- -> CO3^2- + H2O, the bicarbonate ion donates a proton to hydroxide ion and acts as an acid.
- Therefore, the bicarbonate ion can act as both a Bronsted acid and base.

Conclusion:
Among the given species, the amide ion (NH2-) cannot act as both a Bronsted acid and base. It can only act as a Bronsted base