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Introduction: Brønsted–Lowry acid–base theory
A conjugate acid is a species formed by the reception of a proton (H+), by a base
(the base with a hydrogen ion added to it.)
A conjugate base is what is left after an acid has donated a proton.
(a species formed by the removal of a proton from an acid.)
Acid + Base ⇔ Conjugate Base + Conjugate Acid
In an acid-base reaction, an acid plus a base reacts to form a conjugate base plus a conjugate acid:
Conjugates are formed when an acid loses a hydrogen proton or a base gains a hydrogen proton.
Acetic acid example
Now let’s look at some details. Adapted from “Saskatchewan Evergreen Curriculum for Chemistry 30.” Let’s look at ammonia:
ammonia reacts with water to form ammonium
Consider how NH3 changes to: NH4+ NH3 → NH4+
The formulas differ by a single hydrogen; NH3 gains an H+ to become NH4+
Consider how H2O (or HOH) changes to OH–: HOH → OH–
Again the formulas differ only by a single hydrogen; H2O lost a H+ forming OH–
Now consider these two changes as reversible reactions. What if the reaction proceeds in the opposite direction:
NH4+ can change back to NH3: NH4+ → NH3
OH– can change back into H2O: OH– → HOH
Putting these observations together we see that
* ammonia acts as a base because it can combine with a hydrogen ion. It’s partner ammonium is now an acid, for it has a hydrogen ion that it can give up; once it does it is converted back into ammonia.
* water acts as an acid because it gives away a hydrogen ion to ammonia. Once it has lost the hydrogen ion and becomes hydroxide, the hydroxide in turn can act as a base and accept a hydrogen ion from ammonium.
What we have here are conjugate acid-base pairs
Conjugate acid-base pairs differ from each other by the presence or absence of a single hydrogen ion (proton).
Every acid has a conjugate base, and every base has a conjugate acid.
The conjugates will always be listed on the product side of the reaction.
1.1 What are acids and bases?
1.2 Arrhenius Theory of Acids & Bases
1.3 Ionization & Dissociation
1.4 Brønsted-Lowry Theory of Acids & Bases