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Introduction to Lowry Bronsted Theory - Bronsted Acid and Base, Amphoteric Substances and Conjugate Acid-Base Pair

Introduction to Lowry Bronsted Theory - Bronsted Acid and Base, Amphoteric Substances and Conjugate Acid-Base Pair

Can you guess, NH3 a base or not ??? 

Arrhenius made the acid and base concept simple but there are some unresolved queries. See, NH3 does not contain OH-ions. Hence, according to Arrhenius, it is not a base. But we all know that NH3 is a very common base right? So, this confusion was resolved by Brönsted and Lowry who independently gave their theories for acid and base, but after some time, both theories got merged and became known as Brönsted-Lowry Concepts of Acid and Bases. It is a generalization of the Arrhenius concept. 

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Table of contents

  • Brönsted Acid
  • Brönsted Base
  • Amphoteric Substance
  • Conjugate Acid-Base pair
  • Practice Problems
  • Frequently Asked Questions-FAQ

Brönsted-Lowry concept of Acids and Bases

Brönsted Acids (Proton Donors)

Any substance that is capable of donating a hydrogen ion H+ (proton) is called an acid.

e.g.: HCl, H2SO4, CH3COOH etc

H2SO4(aq)2H+(aq)+SO42-(aq)

Brönsted bases (Proton Acceptors)

Any substance that is capable of accepting a hydrogen ion (H+) is called a base.

e.g.: H2O, NH3, Cl- etc

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Amphoteric Substance

Any substance that is capable of accepting as well as donating a proton (H+)is called an Amphoteric substance. As per the Brönsted-Lowry concept, this species can act as both acids and bases.

For example NH3 

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Conjugate Acid-Base pair

The Acid-Base pair that differs only by one proton is called a conjugate acid-base pair. 

Consider the following general reaction:

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Conjugate acid

Species formed by the gain of a proton by the base is known as the conjugate acid of that base. For example, NH4+ is the conjugate acid of the base NH3

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Conjugate base

Species formed by the loss of a proton from acid is known as the conjugate base of that acid.

For example, CH3COO-is the conjugate base of the acid, CH3COOH.



Relative strength of an acid-base pair

A strong acid/base will have a weak conjugate base/acid

Consider the given reaction. HCl is a strong acid and has a strong tendency to donate H+. Cl- is a weak conjugate base which has less tendency to accept H+ and revert to HCl.

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A weak acid/base will have a strong conjugate base/acid

Consider the given reaction. As CH3COOH is a weak acid, it has less tendency to donate H+, and as CH3COO- is a strong conjugate base, it has a strong tendency to accept H+ and revert to CH3COOH.

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Therefore, for the reaction of acetic acid and water, as CH3COOH is a weak acid, CH3COO- will behave as a strong conjugate base. Also, H2O is a weak base and therefore, H3O+ behaves as a strong conjugate acid.

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Recommended Video:  Discussion on Acid-Bases, Ostwald's Dilution Law, Water, pH Scale | JEE

Practice Problems

Q 1. Conjugate bases of HF, H2SO4and HCO3- are…..

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Answer : (C)
Species formed by the loss of a proton from acid is known as the conjugate base of that acid. Conjugate bases of HF, H2SO4and HCO3- are F-, HSO4-, CO32-

Q 2. Conjugate Acid of NH2-, NH3, HCOO- are

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Answer : (A)
Species formed by the gain of a proton from a base is known as the conjugate acid of that base. Conjugate Acid of NH2-, NH3, HCOO- are NH3, NH.4+, HCOOH.

Q 3. Find the conjugate acid/base for the species: HNO2, CN-

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Answer : (D)
We can form the conjugate base of HNO2 because it can lose H+. We can form conjugate acid of CN-because it can gain H+. Hence, the conjugate acid/base for HNO2, CN- is NO2-, HCN.

Q 4. Find the conjugate acid/base for HClO4

A. ClO4-
B. ClO42-
C. ClO32-
D. H2ClO4+

Answer : (A)
We can form the conjugate base of HClO4 because it can lose H+. Hence, the conjugate acid/base for HClO4 is ClO4-.

Frequently Asked Questions – FAQs

Q 1. Every bronsted acid is an Arrhenius acid but every bronsted base is not an Arrhenius base. Comment.

As per definition, Arrhenius acid and bronsted acid are those which can donate H+ions but in the case of bases, their definitions are different. Arrhenius bases are those which can donate OH-ions and bronsted bases are those which can accept H+ions. For example: NH3 is bronsted base because it can accept H+ions to give NH4+ but NH3 is not an Arrhenius base because it can not donate OH-ions.

Q 2. Why was bronsted lowry's concept supercilious than Arrhenius's theory?

Bronsted and lowry include such compounds in the category of the base which are excluded by Arrhenius. For example, Br-is not a base as per Arhhenius theory because it can not donate OH- ion. But its actual nature is basic which is defined by bronsted lowry's concept. According to this concept, Br- can accept a proton, hence is a base.

Q 3. Why does every strong acid have a weak conjugate base?

Let's take the example of strong acid HCl. Being a strong acid, HCl can easily dissociate its H+. After dissociation, the species i.e.; conjugate base Cl- become stable or weak because Cl- is not so eager to accept the H+ ions. So, it's pretty simple that Every strong acid has a weak conjugate base.

Related Topic Links

Acids, Bases and Salts

Arrhenius theory

Lewis Acid-Base Concept

Electrolytes and Non-Electrolytes



 

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