Bronsted-Lowry Theory of Acid and Base

In this article, Bronsted-Lowry acid-base concept is described including examples, Brønsted-Lowry acid-base reactions, and more.

1. Bronsted-Lowry Concept of Acid and Base
2. Bronsted-Lowry Acid
3. Bronsted-Lowry Base
4. Bronsted-Lowry Acid-Base Reaction
5. Bronsted-Lowry Acids and their Conjugate Bases
6. Bronsted-Lowry Bases and their Conjugate Acids
7. Limitations
8. 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, K_a. A strong acid gives K_a relatively large than the K_a 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, K_b. A strong base gives K_b relatively large than the K_b 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 K_b relatively large than the K_b 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 + SO₄^2-
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).