In this article, Bronsted-Lowry acid-base concept is described including examples, Brønsted-Lowry acid-base reactions, and more.
- Bronsted-Lowry Concept of Acid and Base
- Bronsted-Lowry Acid
- Bronsted-Lowry Base
- Bronsted-Lowry Acid-Base Reaction
- Bronsted-Lowry Acids and their Conjugate Bases
- Bronsted-Lowry Bases and their Conjugate Acids
- Limitations
- References
Bronsted-Lowry Concept of Acid and Base
In 1923, a new concept of acid and base was proposed independently by the Danish chemist Johannes Nicolaus Brønsted and the English chemist Thomas Martin Lowry. According to this concept, an acid is a substance that donates one or more protons (H+), and a base is a substance that accepts the proton (H+). Unlike Arrhenius’s theory of acid-base, the Brønsted-Lowry concept is independent of the solvent.
Bronsted-Lowry Acid
A Bronsted-Lowry acid is a substance that is capable of donating a proton. For example, HCl is Bronsted-Lowry acid as it has the ability to donate a proton. When a Bronsted-Lowry acid donates a proton, a base is formed, and this base is called the conjugate base. For example, the conjugate base of the HCl molecule is the chloride ion (Cl-).
HCl ⇌ H+ + Cl–
Acid⇌Proton+Conjugatebase
The strength of the acid can be determined by measuring the acid-dissociation equilibrium constant, Ka. A strong acid gives Ka relatively large than the Ka of a weak acid.
A strong acid shows a higher tendency to donate the protons or a lower tendency to accept the protons of its corresponding conjugate base. Hence, in the case of a strong acid, the reaction highly favors the forward direction and gives the Ka relatively large than the Ka of a weak acid. It is also clear that the stronger the acid, the weaker its conjugate base.
Bronsted-Lowry Base
A Bronsted-Lowry base is a substance that is capable of accepting a proton. For example, NH3 is a Bronsted-Lowry base as it has the ability to accept a proton. When a Bronsted-Lowry base accepts a proton, an acid is formed, and this acid is called the conjugate acid. For example, the conjugate acid of the NH3 molecule is the ammonium cation (NH4+).
NH3 + H+ ⇌ NH4+
Base+Proton⇌ Conjugate acid
The strength of the base can be determined by measuring the equilibrium constant, Kb. A strong base gives Kb relatively large than the Kb of a weak base.
A strong base shows a higher tendency to accept the protons or a lower tendency to donate protons of its corresponding conjugate acid. Hence, in the case of a strong base, the reaction highly favors the forward direction and gives the Kb relatively large than the Kb of a weak base. It is also clear that the stronger the base, the weaker its conjugate acid.
Bronsted-Lowry Acid-Base Reaction
The Brønsted-Lowry acid-base reaction involves the transfer of protons from an acid to a base. In this process, after donating a proton, the original acid becomes a conjugate base, and after accepting a proton, the original base becomes a conjugate acid. Thus, any Bronsted-Lowry acid-base reaction involves two acids and two bases, forming conjugate acid‐base pairs. For example, a reaction between a Brønsted-Lowry acid HCl (hydrochloric acid) and a Brønsted-Lowry base NH3 (Ammonia) as follows,
HCl + NH3 ⇌ NH4+ + Cl–
Acid1 + Base2 ⇌ Acid2 + Base1
Acid2 is the Conjugateacid of Base2 and Base1 is the Conjugatebase of Acid1.
Some other examples of Bronsted-Lowry acid-base reactions are given below:
H2O + NH3 ⇌ NH4+ + OH–
Acid1 + Base2 ⇌ Acid2 + Base1
CH3COOH + NH3 ⇌ NH4+ + CH3COO–
Acid1 + Base2 ⇌ Acid2 + Base1
H2SO4– + OH– ⇌ H2O + SO42-
Acid1 + Base2 ⇌ Acid2 + Base1
H2O + F– ⇌ HF + OH–
Acid1 + Base2 ⇌ Acid2 + Base1
H2O + CH3NH2 ⇌ CH3NH3+ + OH–
Acid1 + Base2 ⇌ Acid2 + Base1
H2O + CH3NH2 ⇌ CH3NH3+ + OH–
Acid1 + Base2 ⇌ Acid2 + Base1
HNO3 + H2O ⇌ H3O+ + NO3–
Acid1 + Base2 ⇌ Acid2 + Base1
NH4+ + H2O ⇌ H3O+ + NH3
Acid1 + Base2 ⇌ Acid2 + Base1
In the above reactions, it is seen that in the presence of a stronger acid (e.g., HNO3) than water, the water acts as a base, but in the presence of a stranger base (e.g., NH3) than water, the water acts as an acid. Thus, water has the ability to both accept and donate a proton and it behaves as both Brønsted-Lowry acid and base. It is also known as an amphoteric or amphiprotic substance.
Bronsted-Lowry Acids and their Conjugate Bases
Bronsted-Lowry Acid | Conjugate Base |
H2SO4 | HSO4– |
NH4+ | NH3 |
HNO3 | NO3– |
CH3COOH | CH3COO– |
CH3OH | CH3O– |
H2O | OH– |
HCl | Cl– |
HI | I– |
H2CO3 | HCO3– |
HNO2 | NO2– |
Bronsted-Lowry Bases and their Conjugate Acids
Bronsted-Lowry Base | Conjugate Acid |
NH3 | NH4+ |
H2O | H3O+ |
CH3NH2 | CH3NH3+ |
SO42- | HSO4– |
CIO4– | HCIO4 |
NH2– | NH3 |
(CH3)3N | (CH3)3NH+ |
OH– | H2O |
H2PO4− | H3PO4 |
NO3− | HNO3 |
C5H5N | C5H5NH+ |
Limitation
- Bronsted-Lowry Theory can not explain the acidic and basic behaviors of non-protonic substances like SO2, CO2, BF3, AlCl3, etc.
- It can not explain the acid-base reaction of non-protonic substances. For example, the reaction between CaO and SO3 is as follows,
CaO + SO3→CaSO4
References
- Brönsted, J. N. “Einige Bemerkungen über den Begriff der Säuren und Basen” [Some observations about the concept of acids and bases]. Recueil des Travaux Chimiques des Pays-Bas. 42 (8): 718–728 (1923).
- Lowry, T. M. “The uniqueness of hydrogen”. Journal of the Society of Chemical Industry. 42 (3): 43–47 (1923).