Conjugate Acid vs Conjugate Base
In 1923, two scientists, Bronsted and Lowry presented a theory on acid base behavior. According to Bronsted – Lowry theory, an acid is a proton donor and a base is a proton acceptor. Therefore, a molecule to behave as an acid it should encounter a proton acceptor. On the other hand, a molecule to behave as a base it should encounter a proton donor. Therefore, for an acid base reaction, both proton donors and acceptors should be there. Water can act as both acid and base. When water accepts a proton, it forms a hydronium ion, and when it donates a proton, it produces a hydroxide ion.
Conjugate base is a substance formed after an acid gives up a proton to a base. This has the capability to accept a proton again; thus, it has basic characteristics. The potential proton acceptor formed from the parent acid is known as the conjugate base. When the conjugate base accepts a proton, it is reverse to the parent acid again. Many solvents can act as proton donors or acceptors; therefore, induce the acidic or basic behavior in solutes. For example, when ammonia dissolves in water, water acts as the acid and gives a proton to ammonia, thus forming an ammonium ion. Meanwhile, the water molecule is concerted to a hydroxide anion. Here, the conjugate base of the water is the hydroxide anion. And the conjugate base of the ammonium is ammonia. Following is the reaction of this.
NH3+ H2O ⇌ NH4+ + OH–
Conjugate acid is a substance made from a base. When a base accepts a proton from another molecule, it forms a conjugate acid. Conjugate acid can remove the electron and return back to the parent base. Thus, conjugate acids have acidic properties. In the above example, ammonium ion is the conjugate acid of ammonia. Likewise, when considering the backward reaction, water is the conjugate acid of hydroxide base.
The acid and its conjugate base are known as the conjugate acid base pair. And this is the same for a base and its conjugate acid.
Normally, a reaction is favored, when it produces conjugate bases or conjugate acids that are weaker than the parent acid or the base. For example, when HCl is dissociated to give a Cl– ion as the conjugate base, it is weak, therefore, does not tend to accept a proton and reverse into HCl.
The acidic strength of an acid can be predicted by looking at its conjugate base and vice versa. For example, if the conjugate base is very strong, then the acidity of the parent acid is less. Therefore, acids and bases can be compared by looking at their conjugate acids or conjugate bases. For instance, if we arrange a few acids in the increasing order of their acidic strength, then the basic strengths of their conjugate bases, increase in the reverse direction.
What is the difference between Conjugate Acid and Conjugate Base?
• Conjugate acids can donate protons, whereas conjugate bases can accept protons.
• Conjugate acids are formed from bases; conversely, conjugate bases are formed from acids.
• Conjugates acids and bases, formed in a spontaneous reaction, are much weaker than their parent molecules.