What is an Acid?
In chemistry, acid is a substance that has a hydrogen ion or gives a hydrogen ion (H+) when dissolved in water.
Different scientists have tried to explain acid. Lewis states that an acid is a substance that accepts a pair of electrons to form a new bond. In contrast, Arrhenius says that an acid is a substance that increases the concentration of hydronium ions (H+) when added to the water. At the same time, Bronsted Lowry defined an acid as a proton donor.
This section will discuss lewis acid and bronsted acids in detail.
Table of Contents
- What is a Lewis Acid?
- What is a Bronsted Acid?
- Lewis Acid vs Bronsted Acid
- Frequently Asked Questions – FAQs
What is a Lewis Acid?
According to Lewis, an acid is a substance having a vacant orbital capable of accepting electron pairs to form a Lewis adduct.
- Lewis Acids are electrophilic.
- Lewis said that acid must not contain a hydrogen ion (H+) or hydroxide ion (OH–) to be classified as an acid.
- All cations are regarded as lewis acid since they can accept electrons. Examples: Ti3+, Cu3+, Mn2+ etc.
- An atom, molecule, or ion with an incomplete valence shell of electrons is regarded as a lewis acid. Examples BF3, AlCl3.
- Molecules having more than 8 electrons in the central atom are also regarded as lewis acid since they can accept electrons. Examples: SiBr4, SiF4
- Molecules with multiple bonds between two atoms of different electronegativities are also regarded as lewis acid. Examples: CO2, SO2.
What is a Bronsted Acid?
According to Bronsted, an acid is a substance capable of donating proton or H+ to another compound and forming a conjugate base.
- Bronsted regarded acid as a proton donor.
Consider a reaction,
HCl(aq)+NH3(aq) → NH4+(aq)+Cl−(aq)
Here, hydrochloric acid (HCl) donates its proton (H+) to ammonia (NH3), forming a negatively charged chloride ion (Cl–) and a positively charged ammonium ion (NH4+).
Thus, hydrochloric acid (HCl) is a bronsted-lowry acid, and chloride ion (Cl–) is the conjugate base of the hydrochloric acid (HCl). While ammonia (NH3) is a bronsted-lowry base, and ammonium ion (NH4+) is the conjugate acid of ammonia (NH3).
Lewis Acid vs Bronsted Acid
| S No. | Lewis Acid | Bronsted Acid |
|---|---|---|
| 1. | According to Lewis, an acid is a substance having a vacant orbital capable of accepting electron pairs to form a Lewis adduct. | According to Bronsted, an acid is a substance capable of donating proton or H+ to another compound and forming a conjugate base. |
| 2. | Lewis called acids electrophilic. | Bronsted called acid as a proton donor. |
| 3. | Lewis classified cations as a lewis acid. | Bronsted didn’t explain the acidity of cations. |
| 4. | Lewis said that acid must not contain a hydrogen ion (H+) or hydroxide ion (OH–) to be classified as an acid. | Bronsted said the acid must contain a hydrogen ion (H+) or hydroxide ion (OH–) to be classified as an acid. |
| 5. | All bronsted acids are lewis acid. | All lewis acids are not bronsted acid. |
Frequently Asked Questions on Lewis Acid vs Bronsted Acid
What is a lewis acid?
According to Lewis, an acid is a substance having a vacant orbital capable of accepting electron pairs to form a Lewis adduct.
- Lewis said that acid must not contain a hydrogen ion (H+) or hydroxide ion (OH–) to be classified as an acid.
Is hydrochloric acid a Lewis acid?
No hydrochloric acid is not a lewis acid due to its inability of accepting electron pairs. However, hydrochloric acid dissociates into a hydrogen ion (H+) (a criterion of lewis acid).
Due to its inability to accept electron pairs, it is often referred to as a classical acid rather than a Lewis acid.
What is a Bronsted acid?
According to Bronsted, an acid is a substance capable of donating proton or H+ to another compound and forming a conjugate base.
- Bronsted regarded acid as a proton donor.
Are Lewis acid and Bronsted acid the same?
No, Lewis acid and bronsted acid are different. Bronsted said that an acid is a substance capable of donating proton or H+ to another compound and forming a conjugate base. In contrast, Lewis said that acid has a vacant orbital capable of accepting electron pairs to form a Lewis adduct.
Are cations Bronsted acid?
No, cations are not classified as a bronsted acid. Bronsted said that acid must contain a Hydrogen ion (H+) or hydroxide ion (OH–) to be classified as an acid.
Comments