3.1.10 & 3.1.11 – Acid& base dissociation constants (HL)
📌 Strength of acids and bases
- Since the strength of acids and bases is based on the extent of dissociation, there are acid and base dissociation constants that can be used to measure the relative strength of an acid or base
- For weak acids/bases the dissociation is represented as a reversible reaction and the value of the constant will determine the relative strength.
Generic equation for acid dissociation: HA (aq) + H2O(l) ⇌ H+(aq) + A–(aq)
FORMULA FOR ACID CONSTANT : Ka = [H+][A–] / [HA]
We do not include the [H2O] as we know that the K of water is constant
- Ka is a fixed value for each specific acid at each specific temperature
- The higher the value of Ka, the stronger the acid is
- Using this same logic, we can ask get the following formula for a generic base ionisation
Generic equation for acid dissociation: B (aq) + H2O(l) ⇌ BH+(aq) + OH–(aq)
FORMULA FOR ACID CONSTANT : Kb = [OH–][BH+] / [B]
We do not include the [H2O] as we know that the K of water is constant
- The greater the value of Kb, the stronger the base is
📌 Calculations
Generic equation for acid dissociation: HA (aq) + H2O(l) ⇌ H+(aq) + A–(aq)
FORMULA FOR ACID CONSTANT : Ka = [H+][A–] / [HA]
We do not include the [H2O] as we know that the K of water is constant
- Ka is a fixed value for each specific acid at each specific temperature
- The higher the value of Ka, the stronger the acid is
- Using this same logic, we can ask get the following formula for a generic base ionisation
Generic equation for acid dissociation: B (aq) + H2O(l) ⇌ BH+(aq) + OH–(aq)
FORMULA FOR ACID CONSTANT : Kb = [OH–][BH+] / [B]
We do not include the [H2O] as we know that the K of water is constant
- The greater the value of Kb, the stronger the base is