Acids are chemical compounds that undergo a specific reaction called ionization or dissociation when dissolved in water. The Brønsted-Lowry model defines an acid as a proton donor—a molecule that gives away a positively charged hydrogen ion (\(\text{H}^+\)). In an aqueous solution, the water molecule acts as the proton acceptor, initiating the chemical transformation that produces the solution’s two main products.
The Mechanism of Hydronium Ion Formation
The primary product formed when an acid dissolves in water is the hydronium ion, which has the chemical formula \(\text{H}_3\text{O}^+\). The generation of this ion begins when the acid molecule donates its proton, or hydrogen ion (\(\text{H}^+\)), to a water molecule (\(\text{H}_2\text{O}\)). This proton transfer is the core of the acid-base reaction, where the acid acts as the donor and the water acts as a base, or acceptor.
A bare proton is highly unstable and reactive in an aqueous environment. It cannot exist freely in water and is immediately attracted to the partially negative oxygen atom of a water molecule. The water molecule uses its lone pairs of electrons to form a new covalent bond with the incoming proton, creating the hydronium ion (\(\text{H}_3\text{O}^+\)). The resulting hydronium ion carries the positive charge that the original proton possessed, and its concentration in the solution is what chemists use to measure the solution’s acidity, or pH.
The Identity of the Conjugate Base
The second product of the acid-water reaction is the conjugate base, which is formed the instant the acid donates its proton. The conjugate base is simply the remainder of the original acid molecule after it has given up its \(\text{H}^+\) ion. Since the acid loses a positively charged particle, the resulting conjugate base is always an anion, meaning it carries a negative charge.
This negatively charged ion is named a “conjugate base” because it now has the potential to accept a proton, essentially reversing the reaction to reform the original acid. For example, when hydrochloric acid (\(\text{HCl}\)) transfers its proton to water, the remaining product is the chloride ion (\(\text{Cl}^-\)). The chloride ion is thus the conjugate base of hydrochloric acid.
The conjugate base maintains the electrical neutrality of the solution. Its negative charge balances the positive charge introduced by the newly formed hydronium ions.
The Difference Between Strong and Weak Acids
The distinction between strong and weak acids is based on the degree of ionization that occurs in the solution. A strong acid completely dissociates in water, meaning every acid molecule transfers its proton to a water molecule. This complete reaction results in a high concentration of hydronium ions (\(\text{H}_3\text{O}^+\)) and conjugate base anions. Hydrochloric acid (\(\text{HCl}\)) is a common example that ionizes fully to produce \(\text{H}_3\text{O}^+\) and \(\text{Cl}^-\).
In contrast, a weak acid only ionizes partially in water, with only a small fraction of its molecules donating a proton. This partial reaction establishes a chemical equilibrium where most acid molecules remain undissociated. Acetic acid (\(\text{CH}_3\text{COOH}\)), the acid found in vinegar, is a weak acid that only slightly dissociates.
The degree of ionization directly controls the acidity of the solution. Strong acids generate a far greater quantity of \(\text{H}_3\text{O}^+\) ions, resulting in a much lower pH than a weak acid of the same concentration. While both types of acids produce the hydronium ion and a conjugate base, they differ dramatically in the quantity of those products.