Conjugate Base Overview & Examples

What is a Conjugate Base?

The conjugation definition chemistry refers to a species ability to form a link with another species. The conjugate base definition describes a chemical compound that was originally an acid that has lost its proton. The conjugate base is an ex-acid that lost its positively charged hydrogen when reacting with a proton acceptor. This chemical compound has the ability to accept a proton to, once again, transform back into an acid. The reason why these compounds are called conjugate bases is because:

• They act as bases by accepting protons.
• They have the ability to reform the lost link with their protons. The word conjugation means linking with other species.

An acid is a substance that donates at least one proton to another substance. Once it does that, it will turn into a conjugate base. Taking hydrofluoric acid {eq}HF {/eq} as example. Weak as it may be, it is still capable of losing its proton to form the conjugate base {eq}F^- {/eq}, as shown in its dissociation equation:

{eq}HF \leftrightharpoons H^+ + F^- {/eq}

Hydrofluoric acid dissociates into its respective conjugate base {eq}F^- {/eq} and to protons {eq}H^+ {/eq}. The conjugate base {eq}F^- {/eq} behave as most bases behave; by accepting protons from other atoms. The table serves as a brief overview of acids, bases, conjugate acids, and bases. Conjugate acids are the particles formed after its base accepts a proton.

It should be noted that protons {eq}H^+ {/eq} never existed freely in aqueous solutions; there is no such solution that is composed of nothing but protons. Protons are always within some electron density.

Bronsted-Lowry Acid-Base Theory

Bronsted-Lowry Acid-Base Theory defines acids and bases as:

• Compounds that give up their protons are acids.
• Compounds that accept protons are bases.

The term conjugate base comes from the Bronsted-Lowry Acid-Base Theory because they originate from acids, but they act as bases. They act as bases by having the tendency to accept protons. A conjugate acid acts like most acids do, by donating protons. Conjugate acids are originally bases that accepted protons from different molecules. Conjugate bases and conjugate acids are formed in acid-base reactions, where an actual acid reacts with an actual base. An example of an acid-base reaction is the following, the reaction of acetic acid with the base, ammonia.

Figure 1: Acetic acid reacts with ammonia

Where:

• Acetic acid {eq}CH_3COOH {/eq} is the acid. It donates its proton to the base.

• Ammonia {eq}NH_3 {/eq} is the base. It accepts a proton from the acid.

• Acetate anion {eq}CH_3COO^- {/eq} is the conjugate base. It is formed when the acetic acid loses its proton. The acetate anion is capable of accepting a proton and becoming an acid again.

• Ammonium ion {eq}NH^+_4 {/eq} is the conjugate acid. It is formed when ammonia gains an additional proton. The ammonium ion is capable of donating its new proton and reverting back to a base.

It can be concluded that in acid-base reactions, the acid transfers its proton to the base to form their respective conjugate base and conjugate acid. The general equation between acids and bases that are in equilibrium with one another is shown by the following.

Figure 2: The respective conjugate bases and conjugate acids are formed when acids and bases react

The Strength of Conjugate Bases

The strength of the conjugate base can be predicated by having knowledge about its acid. As the old saying in chemistry goes, the conjugate base of a strong acid is always weak. According to Brostond-Lowry's theory, the strength of an acid is assessed by its ability to donate its proton(s). Strong acids readily give up their protons; their attachment to their {eq}H^+ {/eq} is weak, which is why they have no quarrel in giving them up. Weak acids are the opposite. Their attachment to their protons is so strong that they do not give them up easily. The equilibrium constant, Ka, for acids is a measure of how strong an acid is and how well it dissociates into its respective ions.

• Strong acids dissociate completely into their conjugate bases and protons. Their Ka value is high.
• Weak acids dissociate partially into their conjugate bases and protons. Their Ka value is low.

The following can be deduced about the strength of conjugate bases.

• The conjugate base of a strong acid is weak. Its tendency to reaccept its lost proton is weak.
• The conjugate base of a weak acid is strong. Its tendency to reaccept its lost proton is strong.

HCl is a strong acid, its conjugate base is weak. Hydrochloric acid dissociates completely into its ions. There is no equilibrium, which is why its conjugate base, {eq}Cl^- {/eq}, does not reattach itself to a proton.

{eq}HCl_{(aq)} \rightarrow H^+_{(aq)} + Cl^-_{(aq)} {/eq}

Acetic acid, on the other hand, is a very weak acid. Its conjugate base is strong. The acetate ion {eq}CH_3COO^- {/eq} is capable of restoring its bond with its lost proton. This explains why the dissociation reactions is at equilibrium.

{eq}CH_3COOH_{(aq)} \leftrightharpoons H^+_{(aq)} + CH_3COO^-_{(aq)} {/eq}

The table documents a few of the important acids along with their respective conjugate bases. The strength of the acid decreases going down the column, while the strength of the conjugate base increases.