Definition of a Conjugate Base
A conjugate base is the species formed when an acid donates a proton (H⁺) in an acid-base reaction. According to the Brønsted-Lowry theory, an acid is a proton donor, and when it loses this proton, the remaining species is its conjugate base, capable of accepting a proton back.
Key Relationship with its Acid
Every Brønsted-Lowry acid has a corresponding conjugate base, and together they form a conjugate acid-base pair. The acid and its conjugate base differ by exactly one proton (H⁺). The stronger the acid, the weaker its conjugate base, and vice-versa, illustrating an inverse relationship in strength within the pair.
A Practical Example
Consider the reaction of hydrochloric acid (HCl) with water (H₂O): HCl(aq) + H₂O(l) ⇌ H₃O⁺(aq) + Cl⁻(aq). In this reaction, HCl acts as the acid, donating a proton to water. The species that remains after HCl loses its proton is Cl⁻ (chloride ion), which is the conjugate base of HCl.
Importance in Acid-Base Chemistry
Understanding conjugate bases is crucial for predicting the direction of acid-base reactions and for calculating pH, especially in buffer solutions. The relative strengths of conjugate acid-base pairs determine the extent to which a reaction proceeds in either the forward or reverse direction, impacting chemical equilibrium.