Bronsted Acids and Bronsted Bases

According to Bronsted-Lowry theory of acids and bases,

- an acid is a proton, H₃O⁺ or H⁺ donor

- a base is a proton, H₃O⁺ or H⁺ acceptor

The strength of an acid or base is related to the extent of dissociation in solution:

- a strong acid or base is fully dissociated

- a weak acid or base is partially dissociated

So therefore we will have 4 possible types of acids and bases:

1. Weak Acid

2. Strong Acid

3. Weak Base

4. Strong Base

Also, each of these acids and bases will dissociate in solution to give different species.

1. Weak Acid Dissociation eg CH₃COOH

CH₃COOH is a weak acid and dissociates partially in solution (as indicated with reversible arrow) to form H⁺ and CH₃COO^- ions.

Since this is a reversible process, CH₃COO^- can accept H⁺ to form back CH₃COOH.

Therefore the nature of CH₃COO^- is basic and we call CH₃COO^- the conjugate base of CH₃COOH.

2. Strong Acid Dissociation eg HCl

HCl is a strong acid and will dissociate fully (as indicated with full arrow) in solution to form H⁺ and Cl^- ions.

Since this is an irreversible process, Cl^- has no tendency to accept H⁺ and form back HCl.

Therefore the nature of Cl^- is neutral.

3. Weak base dissociation eg NH₃

NH₃ is a weak base and dissociates partially in solution (as indicated with reversible arrow) to form OH^- and NH₄⁺ ions.

Since this is a reversible process, NH₄⁺ can donate H⁺ to form back NH₃.

Therefore the nature of NH₄⁺ is acidic and we call NH₄⁺ the conjugate acid of NH₃.

4. Strong base dissociation eg NaOH

NaOH is a strong base and will dissociate fully (as indicated with full arrow) in solution to form OH^- and Na⁺ ions.

Since this is an irreversible process, Na⁺ has no tendency to donate H⁺ and form back NaOH.

Therefore the nature of Na⁺ is neutral.

So in summary:

- weak acid will dissociate to give conjugate base

- weak base will dissociate to give conjugate acid

- strong acid will dissociate to give a neutral ion

- strong base will dissociate to give a neutral ion

These concepts are fundamental and very important in Ionic Equilibria.

Check out this video lesson to learn more about acids and bases and their dissociation in solution!

Topic: Ionic Equilibria, Physical Chemistry, JC, H2, A Level Chemistry, Singapore

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