Conjugate
Definition and meaning of Conjugate in chemistry.
Conjugate describes either species in a conjugate acid-base pair: two chemical species that differ from each other by exactly one proton (H+), as defined by the Brønsted-Lowry theory of acids and bases.
In more detail
When a Brønsted-Lowry acid donates a proton, it becomes its conjugate base; when a base accepts a proton, it becomes its conjugate acid. Every acid-base reaction produces a new conjugate pair, and the strength of an acid or base is inversely related to the strength of its conjugate (a strong acid has a very weak conjugate base, and vice versa). This relationship underlies buffer chemistry, since a buffer solution relies on a weak acid and its conjugate base (or a weak base and its conjugate acid) coexisting to resist pH change.
Key facts
| Field | General Chemistry |
|---|---|
| Defined by | Brønsted-Lowry acid-base theory |
| Relationship | Conjugate pair differs by one H+ |
| Common pair | NH4+ / NH3 |
When acetic acid (CH3COOH) donates a proton to water, it forms its conjugate base, the acetate ion (CH3COO-), while water accepts the proton to become its conjugate acid, the hydronium ion (H3O+).
Frequently asked questions
How do you find the conjugate base of an acid?
Remove one H+ (and reduce the overall charge by one) from the acid's formula. For example, removing H+ from HCl gives its conjugate base, Cl-.
Can a species act as both a conjugate acid and a conjugate base?
Yes; amphiprotic species like HCO3- or H2O can act as either, since they can both donate and accept a proton depending on the reaction partner.