Basicity of Acids and Dissociation of Diprotic Acids

In this JC2 webinar we want to explain basicity of acids and learn how to interprete the dissociation of diprotic acids.

1. Basicity of Acids

Basicity of acids is related to the number of protons an acid can donate.

A monoprotic acid donates 1 proton or 1 H⁺, eg strong acid HCl and weak acid CH₃COOH

A diprotic acid donates 2 protons or 2 H⁺, eg strong acid H₂SO₄ and weak acid H₂CO₃

Similarly, a monobasic base accepts 1 proton and a dibasic base accepts 2 protons.

2. Dissociation of Diprotic Acid H₂CO₃

A diprotic acid will not donate both of its protons in 1 reaction.

The dissociation will occur in 2 stages:

Notice each step now becomes the dissociation of a monoprotic acid.

The acid in the first dissociation is H₂CO₃, while the acid in the second dissociation is HCO₃^-.

This means we need to be very familiar with the concept and dissociation of monoprotic acids, and apply them to diprotic or polyprotic species.

3. Comparing Acid Strength

For a polyprotic acid, the first acid is always the strongest and the strength decreases as the dissociation proceeds.

This means that the first acid is always stronger than the second acid, which is in turn stronger than the third acid (if triprotic).

For the H₂CO₃ example given earlier, H₂CO₃ is a stronger acid than HCO₃^-.

H₂CO₃ has more protons to donate than HCO₃^-, while HCO₃^- is negatively charged so it's harder to lose a positive H⁺.

Hence H₂CO₃ is a stronger acid than HCO₃^-.

For a polyprotic base, the trend is the same.

The first base is always stronger than the second base which is in turn stronger than the third base (for triprotic bases).

Understanding how we handle polyprotic acids and bases as a series of monoprotic species will be useful for subsequent concepts discussed in Ionic Equilibria, eg determine pH of diprotic acids.

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

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