Bronsted Lowry and Lewis Acid-Base theory

Bronsted Lowry and Lewis Acid-Base theory

Edited By Shivani Poonia | Updated on Oct 07, 2024 05:09 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 Ka. The larger the value of Ka, 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, Kb which determines the extent of the completion of the reaction.

Amphiprotic Compounds:
The compounds that can act either as acids or as bases, NaSH, NaHCO3, 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−

Recommended topic video on (Bronsted Lowry and Lewis acid -base thoery )

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 H2 F+

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) AlCl3 and SiCl4

4) PH3 and 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.

In Lewis acid, many acids do not have protons.

e.g. BF3

BF3+NH3→BF3:NH3

(i) BCl3 and AlCl3 have vacant p-orbitals

(ii) AlCl3 has 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.


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