Revision Notes - Acid-Base Equilibria

Lowry-Bronsted theory of acids and bases 

An acid is a proton donor.

A base is a proton acceptor.

e.g. neutralisation between nitric acid and magnesium hydroxide . 

2HNO₃(aq) + Mg(OH)₂(s) → 2H₂O(l) + Mg(NO₃)₂(aq)

Conjugate acid = the species formed after a base accepts a proton (e.g. water).

Conjugate base = the species formed after an acid donates a proton (e.g. magnesium nitrate).

Monoprotic and diprotic acids

A monoprotic acid = when one mole of acid gives up one mole of hydrogen ions. 

e.g. 3 moles of HNO₃ = 3 moles of H+

0.2 moles of HCl = 0.2 moles of H+ 

A diprotic acid = when one mole of acid gives two moles of hydrogen ions.

e.g. 4 moles of  H₂SO₄= 8 moles of H+

Strong acids Vs weak acids

A strong acid will fully dissociate in water. 

A weak acid will partially dissociate in water.

As a result, a strong and weak acid may have the same concentrations but different pH values: a strong acid will have more free H+ ions and so a lower pH value. 

DO NOT MIX THE TERMINOLOGY FOR STRONG/WEAK WITH CONCENTRATED/DILUTE. 

If something is concentrated, it means a large volume of acid has been dissolved in a set volume of water. If something is dilute, it means a small volume of acid has been dissolved in a set volume of water. This does not link to the amount of dissociation. 

Acid dissociation constant 

When an acid dissociates:

So;

Therefore, the more dissociated the acid is, the higher the value of Ka. 

Strong acids will have a higher value of Ka.

Weak acids will have a lower value of Ka. 

Ionic product of water

There is an equillibrium process in water:

The reaction is mainly to the left hand side, as almost all water exists as water molecules. 

Since the amount of water that dissociates is so small, the concentration of water can be considered constant. Therefore, the Kw expression is:

In pure water, Kw has a value of 1.00 x 10^-14 mol²dm^-6 at 298K. 

In pure water the hydroxide concentration is equal to the hydrogen ion concentration. As a result:

[H⁺] [OH^-] = 1.00 x 10^-14

[H⁺]² = 1.00 x 10^-14

[H⁺] = 1.00 x 10^-7 mol dm^-3 

Calculations involving strong acids

Since strong acids fully dissociate, the concentration of the H+ ions released is equal to the concentration of the acid.

TWO KEY EQUATIONS:

pH = - log [H⁺]

[H⁺] = 10^-pH 

Calculations involving weak acids

Two assumptions must be made: 

1. [H⁺] = [X-] 

2. Ka = [H⁺]²…