Bronsted Lowry and Lewis Acid-Base theory

Edited By Shivani Poonia | Updated on Jul 02, 2025 06:07 PM IST

The theories of Bronsted Lowry and Lewis acid-base theories were not developed by any single scientist at a particular time. These evolve over time . The Bronsted Lowry acid-base theory was developed by the scientists Johannes Nicolaus Bronsted and Thomas Martin Lowry. and their theories were published in the year of 1923. As compared to the Arrhenius theory these theories provide the broader context of acids and bases. According to the Bronsted theories the acids are those Which can donate the proton and bases are those that are the good acceptors of protons.

Bronsted-Lowry Acids and Bases

According to this concept, an acid and a base can defined as follows :
Acid: It is a substance that can donate a proton.
Base: It is a substance that can accept a proton.

Some examples include:

When HCl is dissolved in water, it donates a proton to H₂O which behaves as a base.HCl(aq)+H2O(ℓ)→Cl−(aq)+H₃O−(aq)
When HCO₃^-is dissolved in water, it donates a proton to NH₃ which behaves as a base.HCO₃⁻(aq)+NH₃(aq)⇌CO₃²⁻(aq)+NH₄⁺(aq)

The base formed from an acid is known as the conjugate base of the acid. Correspondingly, the acid formed from a base is called the conjugate acid of the base.
HCl+NH₃⇌Cl⁻+NH₄⁺
In the above reaction, Cl^- is the conjugate base of HCl and NH₄⁺is the conjugate acid NH₃.

Strength of Bronsted-Lowry Acid and Bases:

The strength of an acid or base is measured by its tendency to lose or gain a proton. A strong acid is a substance that loses a proton easily to a base. Consequently, the conjugate base of a strong acid is weak.

HCl ong Acid +H₂O⟶Cl− Weak base +H₃O

HS⁻+H₂O⇌H₃O⁺+S₂
The ability of an acid to lose a proton is experimentally measured by its equilibrium constant known as K_a. The larger the value of K_a, the more complete the reaction or the higher the concentration of H₃O⁺ and the stronger the acid. Similarly, for bases, we have the equilibrium constant, K_b which determines the extent of the completion of the reaction.

Amphiprotic Compounds:
The compounds that can act either as acids or as bases, NaSH, NaHCO_3, etc are some of the examples.

Lewis Acid and Bases:

Acid: It is a substance that can form a covalent bond by accepting a shared pair of electrons.
Base: It is a substance that possesses at least one unshared pair of electrons.

Substances that are based on the Bronsted system are also based on the Lewis concept. However, the Lewis definition of an acid considerably expands the number of substances that are classified as acid. A Lewis acid must have an empty orbital capable of receiving the electron pair of the base.
Lewis acids include molecules or atoms that have incomplete octets. For example molecules like BF₃, AlCl₃, etc. act as Lewis Acid.

Many simple cations can act as Lewis acids: Cu²⁺+4NH₃→Cu(NH₃)₄²⁺

Some metal atoms can function as acids in the formation of compounds such as: Ni+4CO→Ni(CO)₄

Compounds that have central atoms capable of expanding their valence shells are Lewis acids in reactions in which this expansion occurs

SnCl₄+2Cl⁻→SnCl₆²⁻

Some Solved Examples

1. Which of the following salts is the most basic in aqueous solution?

1) Pb(CH₃COO)₂
2) Al(CN)₃
3) (correct) CH₃COOK
4) FeCl₃

Solution

As we learned from the concept
CH₃COOK+H₂O⇌CH₃COOH+KOH Out of the given salts, this is the only one which is the salt of a strong base and hence, it is the most basic.

Hence, the answer is an option (3).

2. The following equilibrium is established when HClO₄ is dissolved in weak acid HF.

HF+HClO₄⇌ClO₄⁻+H₂ F⁺

Which of the following is the correct set of conjugate acid-base pairs?

1) HF and HClO₄
2) HF and ClO4⁻
3) (correct) HF and H₂ F⁺
4) HClO₄ and H₂F⁺

Solution

Conjugate acid-base pair -

The acid-base pair that differ only by one proton is called a conjugate acid-base pair.

HCl+H₂O⇌H₃O⁺+Cl⁻

Cl^- is the conjugate base of HCl & H₃O⁺ is the conjugate acid of H₂O.

In the given case,

HF⁺→H+H² F⁺
(Acid) (base)

Hence, the answer is the option (3).

3. Which of the following is a Lewis acid?

1) PH₃
2) (correct) B(CH₃)³
3) NaH
(4) NF₃

Solution

Lewis acids and bases -

Lewis defined an acid as a species that accepts an electron pair and a base that donates an electron pair.

e.g BF₃

BF₃+NH₃→BF₃:NH₃

In the given question,

It is electron deficient, so it can accept a lone pair of electrons and behave as Lewis acid.

4. Which of the following are Lewis acids?

1) (correct)BCl₃ and AlCl₃

2)PH₃ and BCl₃

3) AlCl₃and SiCl₄

4) PH₃and SiCl₄

Solution

Lewis acids and bases -

Lewis defined an acid as a species that accepts an electron pair and a base that donates an electron pair.

Many acids in Lewis acid do not have protons.

e.g. BF₃

BF₃+NH₃→BF₃:NH₃

(i) BCl₃ and AlCl₃ have vacant p-orbitals

(ii) AlCl₃has a vacant p-orbital, while SiCl₄ has a vacant d-orbital.

Hence, the answer is an option (1).

5. Species acting as both Bronsted acid and base is

1) (correct)(HSO₄)^-1

2)Na₂CO₃

3)NH₃

4)OH^-1

Solution

Bronsted-Lowry Acids and Bases-

According to Bronsted Lowry's theory, acid is a substance that is capable of donating a hydrogen ion H⁺ and bases are a substance that is capable of accepting H⁺ ions.

NH₃+H₂O⇌NH₄⁺+O―HNH₄⁺→ add proton O―H→ lose proton

The substance that can donate as well as accept ions can act as bronsted acid as well as bronsted base

Example HSO₄^-HSO₄⁻+H⁺→H₂SO₄ HSO₄⁻→H⁺+SO₄²⁻

Hence, the answer is the option (1).

Summary

Lewis's acid-base theory extends the concept of acids and bases more than their definitions cover. It includes reactions that are not covered by the Brønsted-Lowry or Arrhenius definitions, such as those involving compounds that do not donate or accept protons. It applies to a wide range of chemical reactions, including those in non-aqueous solvents and complex coordination chemistry. It helps in understanding reactions involving metal ions, ligands, and catalysts. Lewis acid-base reactions include the formation of complex ions, coordination compounds, and other processes where electron pair transfer occurs. It is widely used in organic chemistry, catalysis, and materials science.

Frequently Asked Questions (FAQs)

1. What is the key difference between Brønsted-Lowry and Lewis acid-base theories?

The Brønsted-Lowry theory defines acids and bases in terms of proton transfer, while the Lewis theory focuses on electron pair donation and acceptance. Brønsted-Lowry acids donate protons, and bases accept them. Lewis acids accept electron pairs, and Lewis bases donate electron pairs.

2. Can a substance be both a Brønsted-Lowry acid and a Lewis acid?

Yes, a substance can be both a Brønsted-Lowry acid and a Lewis acid. For example, H+ is a Brønsted-Lowry acid (proton donor) and also a Lewis acid (electron pair acceptor). This dual nature highlights the complementary relationship between these acid-base theories.

3. Why is water considered both a Brønsted-Lowry acid and base?

Water is amphoteric, meaning it can act as both a Brønsted-Lowry acid and base. It can donate a proton (H+) to act as an acid or accept a proton to act as a base. This property allows water to participate in acid-base reactions with various substances, making it a versatile solvent.

4. What is a conjugate acid-base pair in the Brønsted-Lowry theory?

A conjugate acid-base pair consists of two species that differ by a single proton. When a Brønsted-Lowry acid donates a proton, it becomes its conjugate base. Similarly, when a Brønsted-Lowry base accepts a proton, it becomes its conjugate acid. This concept is crucial for understanding acid-base equilibria.

5. How does the strength of a Brønsted-Lowry acid relate to its conjugate base?

There's an inverse relationship between the strength of a Brønsted-Lowry acid and its conjugate base. A strong acid has a weak conjugate base, as it readily donates protons and has little tendency to accept them back. Conversely, a weak acid has a strong conjugate base, as it holds onto its protons more tightly.

6. How does the Lewis theory expand our understanding of acid-base reactions?

The Lewis theory expands our understanding by including reactions that don't involve proton transfer. It defines acids as electron pair acceptors and bases as electron pair donors, encompassing a broader range of reactions, including those involving metal ions and molecules without hydrogen atoms.

7. What role do lone pairs play in Lewis acid-base reactions?

Lone pairs are crucial in Lewis acid-base reactions. Lewis bases use their lone pairs to donate electrons to Lewis acids, which accept these electron pairs. This electron donation forms a coordinate covalent bond between the acid and base, a key feature of Lewis acid-base chemistry.

8. How does the concept of Lewis acids and bases apply to coordination compounds?

In coordination compounds, metal ions act as Lewis acids by accepting electron pairs from ligands, which act as Lewis bases. This electron donation forms coordinate covalent bonds, creating complex ions. The Lewis acid-base concept is fundamental to understanding the formation and properties of coordination compounds.

9. What is meant by a "hard" or "soft" Lewis acid or base?

Hard and soft refer to the polarizability of Lewis acids and bases. Hard acids and bases are small, highly charged, and weakly polarizable. Soft acids and bases are larger, less charged, and more polarizable. This concept helps predict the strength and stability of acid-base interactions in various chemical systems.

10. Why can some compounds act as both Lewis acids and Lewis bases?

Some compounds can act as both Lewis acids and bases due to their molecular structure. They may have electron-deficient sites that can accept electron pairs (acting as acids) and lone pairs that can donate electrons (acting as bases). This amphoteric behavior in the Lewis sense allows for versatile chemical interactions.

11. How does the Lewis acid-base theory explain the formation of adducts?

Adducts form when a Lewis base donates an electron pair to a Lewis acid, creating a new chemical bond without displacing any atoms. This process, called adduct formation, results in a stable complex held together by a coordinate covalent bond. The Lewis theory effectively explains this phenomenon in terms of electron pair sharing.

12. How does the Lewis acid-base theory explain reactions between metals and non-metals?

The Lewis theory explains metal-nonmetal reactions as electron transfer processes. The metal acts as a Lewis base, donating electrons to the nonmetal, which acts as a Lewis acid. This electron transfer leads to the formation of ionic compounds, demonstrating how the Lewis concept extends beyond traditional acid-base reactions to explain broader chemical bonding.

13. How does the concept of Lewis acids and bases apply to organic chemistry reactions?

In organic chemistry, many reactions involve electron-rich species (Lewis bases) interacting with electron-deficient species (Lewis acids). This includes nucleophilic additions, where a nucleophile (Lewis base) attacks an electrophile (Lewis acid). The Lewis theory provides a framework for understanding these electron transfer processes in organic reactions.

14. How does the Lewis acid-base theory explain the concept of hypervalency?

The Lewis theory explains hypervalency by allowing atoms to expand their octet through electron acceptance. In hypervalent compounds, a central atom acts as a Lewis acid, accepting additional electron pairs from surrounding Lewis bases. This concept helps explain the structure and bonding in molecules that appear to violate the octet rule.

15. What is the role of electron affinity in determining Lewis acid strength?

Electron affinity plays a crucial role in Lewis acid strength. Atoms or molecules with high electron affinity are typically stronger Lewis acids as they more readily accept electron pairs. This property helps predict the likelihood and strength of Lewis acid-base interactions, guiding our understanding of chemical reactivity.

16. Why is the Lewis acid-base theory particularly useful for explaining reactions in non-aqueous and gas-phase systems?

The Lewis theory is valuable for non-aqueous and gas-phase systems because it doesn't rely on proton transfer or solvent properties. It focuses on electron pair donation and acceptance, which can occur in any phase or medium. This makes it more versatile for explaining a wide range of chemical interactions beyond traditional aqueous acid-base chemistry.

17. How does the concept of Lewis acids and bases relate to molecular orbital theory?

Lewis acid-base theory aligns with molecular orbital theory by focusing on electron interactions. Lewis acids have low-lying unoccupied molecular orbitals that can accept electrons, while Lewis bases have high-energy occupied orbitals that can donate electrons. This orbital perspective provides a deeper understanding of Lewis acid-base interactions at the molecular level.

18. What is the significance of Pearson's Hard and Soft Acid-Base (HSAB) principle in Lewis acid-base chemistry?

Pearson's HSAB principle categorizes Lewis acids and bases as hard or soft based on their polarizability. It predicts that hard acids prefer to bind to hard bases, and soft acids to soft bases. This principle helps explain and predict the stability of acid-base complexes, reaction rates, and selectivity in various chemical processes.

19. How does the concept of Lewis acids and bases apply to transition metal chemistry?

In transition metal chemistry, metal ions often act as Lewis acids, accepting electron pairs from ligands (Lewis bases). This interaction forms coordination compounds. The Lewis acid-base concept is crucial for understanding the formation, stability, and properties of these complexes, including their colors, magnetic properties, and reactivity.

20. What is the relationship between electronegativity and Lewis acid-base behavior?

Electronegativity influences Lewis acid-base behavior. Highly electronegative elements tend to be good Lewis bases as they have lone pairs to donate. Conversely, elements with low electronegativity often make good Lewis acids as they can accept electron pairs. This relationship helps predict the likelihood and strength of Lewis acid-base interactions.

21. How does the Lewis acid-base theory explain the concept of frustrated Lewis pairs?

Frustrated Lewis pairs occur when a Lewis acid and base are sterically hindered from forming a classical adduct. Despite their inability to form a direct bond, these pairs can still exhibit unique reactivity. This concept, explained by the Lewis theory, has led to new developments in catalysis and small molecule activation.

22. How does the concept of Lewis acids and bases apply to main group element chemistry?

In main group chemistry, Lewis acid-base interactions explain various bonding patterns and reactivities. Elements on the left of the periodic table often form cations that act as Lewis acids, while those on the right form anions that act as Lewis bases. This concept helps predict and explain the formation of compounds and their chemical properties.

23. How does the Lewis acid-base theory explain the concept of backbonding in organometallic compounds?

Backbonding in organometallic compounds involves a metal (usually acting as a Lewis acid) donating electrons back to a ligand (usually a Lewis base). This occurs when the metal has filled d-orbitals that can overlap with empty orbitals on the ligand. The Lewis theory explains this as a reciprocal acid-base interaction, strengthening the metal-ligand bond.

24. How does the concept of Lewis acids and bases apply to solid-state chemistry?

In solid-state chemistry, Lewis acid-base interactions explain various phenomena. For instance, in some crystal structures, metal ions act as Lewis acids, accepting electron density from surrounding anions (Lewis bases). This concept helps explain the formation and properties of many inorganic solids, including their structure, stability, and reactivity.

25. Why is it important to consider kinetics in addition to thermodynamics when studying Lewis acid-base reactions?

While thermodynamics predicts the favorability of Lewis acid-base reactions, kinetics determines their rate and mechanism. Some reactions may be thermodynamically favorable but kinetically slow due to factors like steric hindrance or activation energy. Considering both aspects provides a complete understanding of Lewis acid-base behavior in real systems.

26. How does the Brønsted-Lowry theory explain the behavior of amphoteric substances?

The Brønsted-Lowry theory explains amphoteric substances as species that can act as both acids and bases depending on the reaction partner. They can donate protons to stronger bases and accept protons from stronger acids. This dual nature allows amphoteric substances to participate in various acid-base equilibria.

27. Why are some Lewis acids not considered Brønsted-Lowry acids?

Some Lewis acids, like BF3, don't have protons to donate, so they can't be Brønsted-Lowry acids. They can accept electron pairs but don't participate in proton transfer reactions. This distinction highlights the broader applicability of the Lewis acid-base theory to non-proton-involving reactions.

28. How does the Brønsted-Lowry theory explain acid-base behavior in non-aqueous solvents?

The Brønsted-Lowry theory extends to non-aqueous solvents by focusing on proton transfer rather than specific solvent properties. Acids still donate protons, and bases still accept them, but the solvent's properties affect the extent of proton transfer and the resulting equilibria, allowing for acid-base reactions in diverse environments.

29. What is the relationship between pH and the Brønsted-Lowry acid-base theory?

pH is directly related to the Brønsted-Lowry theory as it measures the concentration of H+ ions (protons) in solution. A low pH indicates a high concentration of H+ ions (acidic solution), while a high pH indicates a low concentration (basic solution). This aligns with the Brønsted-Lowry concept of acids as proton donors and bases as proton acceptors.

30. Why is the Brønsted-Lowry theory particularly useful for understanding buffer solutions?

The Brønsted-Lowry theory is crucial for understanding buffers because it focuses on proton transfer. Buffers consist of a weak acid and its conjugate base (or vice versa), which can donate or accept protons to maintain a relatively constant pH. This proton exchange mechanism is best explained using the Brønsted-Lowry framework.

31. What is the significance of the autoionization of water in the Brønsted-Lowry theory?

The autoionization of water (2H2O ⇌ H3O+ + OH-) is a key concept in the Brønsted-Lowry theory. It demonstrates water's amphoteric nature, acting as both an acid and a base. This process establishes the pH scale and is fundamental to understanding acid-base equilibria in aqueous solutions.

32. Why are some strong Brønsted-Lowry acids weak Lewis acids, and vice versa?

The strength of Brønsted-Lowry acids depends on proton donation, while Lewis acid strength relates to electron pair acceptance. Some strong Brønsted-Lowry acids may be weak Lewis acids if they can't effectively accept electron pairs after donating protons. Conversely, some strong Lewis acids may not donate protons at all, making them weak or non-existent Brønsted-Lowry acids.

33. How does the Brønsted-Lowry theory explain the concept of leveling effect in solvents?

The leveling effect occurs when a solvent limits the strength of an acid or base. In the Brønsted-Lowry context, a very strong acid in water can only donate protons to form H3O+, as water acts as the base. This "levels" the acid strength to that of the hydronium ion, demonstrating how solvent properties can affect acid-base behavior.

34. How does the Brønsted-Lowry theory explain the concept of neutralization?

In the Brønsted-Lowry framework, neutralization occurs when an acid (proton donor) reacts with a base (proton acceptor) to form water and a salt. The proton transfer from the acid to the base results in the formation of their respective conjugate base and acid. This process explains why the resulting solution is often neutral or close to neutral pH.

35. How does the Brønsted-Lowry theory explain the concept of hydrolysis in salt solutions?

The Brønsted-Lowry theory explains hydrolysis as the reaction of salt ions with water, acting as either an acid or a base. When the anion of a weak acid or the cation of a weak base interacts with water, it can accept or donate protons, respectively. This process results in the solution becoming basic or acidic, deviating from neutrality.

36. Why is it important to consider both Brønsted-Lowry and Lewis theories in understanding complex acid-base reactions?

Considering both theories provides a more comprehensive understanding of acid-base chemistry. While the Brønsted-Lowry theory is excellent for explaining proton transfer reactions, the Lewis theory covers a broader range of interactions, including those without protons. Together, they offer a more complete picture of chemical reactivity and bonding.

37. How does the Brønsted-Lowry theory explain the concept of acid-base titrations?

In the Brønsted-Lowry framework, acid-base titrations involve the gradual addition of a base (proton acceptor) to an acid (proton donor) or vice versa. The endpoint is reached when proton transfer is complete, neutralizing the solution. This theory helps explain the changes in pH during titration and the selection of appropriate indicators.

38. Why can some compounds act as acids in one reaction but bases in another according to the Brønsted-Lowry theory?

This behavior, known as amphoterism, occurs because the acid-base nature in the Brønsted-Lowry theory is relative to the reaction partner. A compound can donate protons to a stronger base (acting as an acid) but accept protons from a stronger acid (acting as a base). This relativity is key to understanding complex acid-base equilibria.

39. What is the significance of the acid dissociation constant (Ka) in the Brønsted-Lowry theory?

The acid dissociation constant (Ka) quantifies the strength of a Brønsted-Lowry acid by measuring its tendency to donate protons in solution. A higher Ka indicates a stronger acid that dissociates more readily. This constant is crucial for predicting acid-base behavior, calculating pH, and understanding equilibrium in acid-base reactions.

40. Why is water considered a better base than alcohol in the Brønsted-Lowry theory?

Water is generally a better Brønsted-Lowry base than alcohol because it's more willing to accept protons. The oxygen in water has a higher electron density compared to alcohol, making it more attractive to protons. This difference in basicity affects the equilibrium constants of acid-base reactions and the relative strengths of conjugate acids.

41. What is the role of solvent in determining the strength of Brønsted-Lowry acids and bases?

Solvents can significantly affect the strength of Brønsted-Lowry acids and bases by influencing proton transfer. Polar protic solvents like water can solvate ions, facilitating acid dissociation. The solvent's ability to accept or donate protons (its own acid-base character) also affects the relative strengths of dissolved acids and bases.

42. How does the Brønsted-Lowry theory explain the concept of buffer capacity?

Buffer capacity in the