Which of the following species is the conjugate acid of water, H₂O? A) O₂, B) OH‒, C) O₂‒, D) H₃O⁺, E) H₂.
Firstly, we need to define the term species and conjugate acid. Chemical species are particles that are identical to each other. The word is used to describe atoms, molecules, ions, fragments, and radicals as examples. Sometimes, we use the word chemical entity instead. All of the possible answers here are chemical species.
Conjugate acid is a phrase that comes from acid–base theory. The Bronsted–Lowery theory of acids and bases states that an acid is a hydrogen ion, or proton, donor. A base is a hydrogen ion, or proton, acceptor. A hydrogen ion, H⁺, is often referred to as a proton. It’s when the hydrogen atom loses its only electron, a single proton is all that is left behind.
So, acids are hydrogen ion donors when they are mixed with water. You never get H⁺ ions by themselves in water, though. They’re always combined with water to form the hydroxonium ion or hydronium ion, H₃O⁺.
In the example reaction shown here involving a strong acid, HA, the acid is releasing a hydrogen ion. The H–A bond is breaking in this process. Water accepts the hydrogen ion using one of its lone pairs to form a covalent bond. Here, it’s behaving as a base. The hydronium ion, H₃O⁺, is formed as a result. We call this the conjugate acid. The negative ion released from the ionization of the acid, A⁻ in this case, is referred to as the conjugate base.
In strong acid solutions, the acid is so good at giving away hydrogen ions that the ionization is considered to be virtually 100 percent complete. The reaction is a one-way process. If the process were to be reversed, the hydronium ion would release a hydrogen ion, so it’s referred to as a conjugate acid. In this case, the A⁻ ion will accept the hydrogen ion from the hydronium ion. So, it is referred to as a conjugate base.
In weak acid solutions and in pure water shown here, a similar process takes place but the ionization is not 100 percent complete. The reaction is reversible and an equilibrium is established. The equilibrium lies far to the left-hand side at room temperature for pure water. In this self-ionization reaction involving pure water, one water molecule is releasing hydrogen ion, and it’s behaving as an acid. The other water molecule is accepting the hydrogen ion, and it’s is behaving as a base.
If the reaction were running reverse, the hydronium ion would release a proton, behaving as an acid. And the hydroxide ion would accept the proton, behaving as a base. The hydronium ion, H₃O⁺, is therefore given the term conjugate acid. It is the conjugate pair of the H₂O molecule that behaved as a base.
So, the conjugate acid of water is H₃O⁺. There is no redox chemistry in the acid–base behavior or self-ionization of water. This eliminates answers A and E where elements are seen as possible products. The correct answer is D.