What is a Conjugate Base?
A conjugate base, according to the Bronsted-Lowry theory of acids and bases, is the species in a reaction that donates a proton during the chemical reaction. In other words, the conjugate base is what is left over after the acid has donated a proton to the base.
The proton exchange between the acid and the base, results in the formation of a new acid (base that receives the proton) and a new base (acid that donates the proton). These new products are called the conjugate acid and base. Because the reactions that occur between these products can be repeated, or used to fuel other reactions, it is possible to reverse the equation and maintain the same outcomes.
A Brief History of Bases and Conjugate Bases
Bases and acids have been integral to chemistry and other sciences since man first began to study how elements interacted with one another, but they were not properly defined until the 19th and 20th centuries. During this time period, several key discoveries concerning bases and acids were discovered.
The first was made during 1884 when Svante Arrhenius was able to prove that bases could easily be determined by observing how an element reacted to water. He was able to prove that all bases released negative hydroxide ions (OH–) when they came in contact with water. While this was essential in helping to determine exactly what a base was, however, it didn’t allow scientists to determine what product(s) served as a base in reactions that didn’t use water.
This would be fixed in 1923 when Johannes Nicolaus Bronsted and Thomas Martin Lowry were both able to prove that acids and bases behave the same way in all types of reactions – regardless of whether water was involved in the reaction. They were able to create the Bronsted-Lowry rule of acids and bases, which stated that in chemical reactions between acids and bases, the acid was the species that donated a proton and the base was the species that accepted a proton in the reaction.
While working to create this rule, Bronsted and Lowry also discovered an interesting connection between the original acids and bases and the products that were formed as a result of their reactions. This led them to create a definition for conjugate acids and bases – the products that were created as a result of the proton donation and acceptance between the original acids and bases.
How are Conjugate Bases Determined?
In the Bronsted-Lowry definition of bases and acids, it is stated that these products can be determined by examining which of the products donates a proton in a chemical reaction and which of the products accepts the proton during the reaction. The base will always accept the proton while the acid will always donate the proton. The conjugate base can be determined by looking at the results of the chemical reaction.
In the above reaction, the acid (CH3COOH) donates a hydrogen ion (or proton) to the base (PO43-). This donation of a proton to the base takes away the neutral charge from the acid and leaves a negative charge – which creates a new base (CH3COO–). This creation of a new base is called the conjugate base.
It is also worthwhile to note that this equation could easily be reversed. Examine the following outcome below:
In this equation, the conjugate base (CH3COO–) now serves as the original base and the conjugate acid (HPO42-) serves as the original acid. When they react together, they produce a new conjugate acid and a new conjugate base.