1. 11.2 Brønsted–Lowry Acids and Bases
According to the Brønsted–Lowry theory,
an acid is a substance that donates H+.
a base is a substance that accepts H+.
Learning Goal Identify conjugate acid–base pairs for Brønsted–Lowry acids and bases.

2. NH3, a Brønsted–Lowry Base
In the reaction of ammonia and water,
NH3 acts as the base that accepts H+.
H2O acts as the acid that donates H+.
Because the nitrogen atom of NH3 has a stronger attraction for H+ than oxygen, water acts as an acid by donating H+.

3. Study Check
In each of the following equations, identify the Brønsted–Lowry acid and base in the reactants:
A. HNO3(aq) + H2O(l) H3O+(aq) + NO3−(aq)
B. HF(aq) + H2O(l) H3O+ (aq) + F−(aq)

4. Solution
In each of the following equations, identify the Brønsted–Lowry acid and base in the reactants:
A. HNO3(aq) + H2O(l) H3O+(aq) + NO3−(aq)
Acid Base
B. HF(aq) + H2O(l) H3O+ (aq) + F−(aq)
Acid Base

5. Study Check Identify each as a characteristic of
A. an acid or B. a base.
____ 1. has a sour taste
____ 2. produces OH− in aqueous solutions
____ 3. has a chalky taste
____ 4. is an electrolyte
____ 5. produces H+ in aqueous solutions

6. Solution Identify each as a characteristic of A. an acid or B. a base.
A 1. has a sour taste
B 2. produces OH− in aqueous solutions
B 3. has a chalky taste
A, B 4. is an electrolyte
A 5. produces H+ in aqueous solutions

7. Conjugate Acid–Base Pairs
In any acid–base reaction, there are two conjugate acid–base pairs.
Each pair is related by the loss and gain of H+.
One pair occurs in the forward direction.
One pair occurs in the reverse direction.
Acid and conjugate base pair 1
HA + B A− + BH+
Base and conjugate acid pair 2

8. Conjugate Acid–Base Pairs
In this acid–base reaction,
the first conjugate acid–base pair is HF, which donates H+ to form its conjugate base, F−.
the other conjugate acid–base pair is H2O, which accepts H+ to form its conjugate acid, H3O+.
each pair is related by a loss and gain of H+.

9. Conjugate Acid–Base Pairs
In the reaction of NH3 and H2O,
one conjugate acid–base pair is NH3/NH4+.
the other conjugate acid–base pair is H2O/H3O+.
Core Chemistry Skill Identifying Conjugate Acid–Base Pairs

10. Study Check
1. Write the conjugate base for each of the following acids:
A. HBr
B. H2S
C. H2CO3
2. Write the conjugate acid of each of the following bases:
A. NO2−
B. NH3
C. OH−

11. Solution 1. Write the conjugate base for each of the following acids:
A. HBr H Br−
B. H2S H HS−
C. H2CO3 H HCO3−
2. Write the conjugate acid of each of the following bases: (Add an H+ to each base to get the conjugate acid.)
A. NO2− + H HNO2
B. NH3 + H NH4+
C. OH− + H H2O

12. Study Check Identify the sets that contain acid–base conjugate pairs.
1. HNO2, NO2−
2. H2CO3, CO32−
3. HCl, ClO4−
4. HS−, H2S
5. NH3, NH4+

13. Solution Identify the sets that contain acid–base conjugate pairs.
1. HNO2, NO2− acid, conjugate base
2. H2CO3, CO32− not acid–base conjugate pair
3. HCl, ClO4− not acid–base conjugate pair
4. HS−, H2S base, conjugate acid
5. NH3, NH4+ base, conjugate acid

14. Amphoteric Substances
Substances that can act as both acids and bases are amphoteric or amphiprotic.
For water, the most common amphoteric substance, the acidic or basic behavior depends on the other reactant.
Water donates H+ when it reacts with a stronger base.
Water accepts H+ when it reacts with a stronger acid.

15. Guide to Writing Conjugate Acid–Base Pairs

16. Study Check
Identify the conjugate acid–base pairs in the following reaction:
HNO3(aq) + NH3(aq) NO3−(aq) + NH4+(aq)

17. Solution
Identify the conjugate acid–base pairs in the following reaction:
HNO3(aq) + NH3(aq) NO3−(aq) + NH4+(aq)
STEP 1 Identify the reactant that loses H+ as the acid.
In the reaction, HNO3 donates H+ to NH3.
STEP 2 Identify the reactant that gains H+ as the base.
In the reaction, NH3 gains H+ to form NH4+. Thus,
NH3 is the base and NH4+ is its conjugate acid.
HBr is the acid and Br− is its conjugate base.

18. Solution
Identify the conjugate acid–base pairs in the following reaction:
HNO3(aq) + NH3(aq) NO3−(aq) + NH4+(aq)
STEP 3 Write the conjugate acid–base pairs.
HBr/Br− is the acid and conjugate base pair.
NH3/NH4+ is the base and conjugate acid pair.