• Explain The Concept Of A Conjugate Acid-Base Pair

Topic 11 - Yassine Assim
Question:
Explain the concept of a conjugate acid-base pair. Using equilibrium law and K, explain the relationship between the strength of an acid/base and its conjugate.
Conjugate Acid-Base Pair:
Conjugate acids and bases were presented by Bronsted-Lowry, who was a chemist who proposed the theory that both acids and bases required the transfer of a proton in order for there to be a reaction. An acid will donate a proton (proton donor), forming what is considered to be a conjugate base. When the base accepts a proton
(proton acceptor), the conjugate acid is formed.
In the example below, we have Ammonia and Water forming Ammonium and a
Hydroxide ion. In the forward reaction, H2O loses a proton, forming OH-. This becomes
In the reverse reaction, NH4+, the conjugate acid, loses a proton, forming our base NH3.
Acid-Base Strength and Conjugate:
The relative strength of acids is often described in terms of an acid-dissociation equilibrium constant, Ka.
Generic Equation:
HA(aq) + H20(l) ↔ H3O+(aq) + A-(aq)
Therefore, Ka = [H3O+][A-]/HA
When a strong acid dissolved in water, the acids reacts extensively with water to form H3O+ and A- ions. The product of the concentrations for both these ions is larger than the concentration of HA molecules, therefore Ka for a strong acid is greater than 1.
Example:
The Ka of Hydrochloric acid is 1x10^6
HCL + H2O ↔ H3O+ + Cl- …
Ka can be used to distinguish between strong and weak acids.
- Strong Acids: Ka > 1
- Weak Acids: Ka < 1
Strong acids have a weak conjugate base:
E.g. HCl(g) + H2O(l) ↔ H3O+(aq) + Cl- (aq)
- HCl is our strong acid and Cl- is our weak