Acids and bases are fundamental concepts in chemistry, often first introduced through the pH scale, which measures the acidity or basicity of a solution. This scale ranges from 0 to 14, with a value of 7 being neutral, values below 7 indicating increasing acidity, and values above 7 indicating increasing basicity. However, the pH concept alone is limited because it primarily applies to substances dissolved in water.
A more flexible and comprehensive way to understand acid-base chemistry is provided by the Brønsted-Lowry theory. This definition moves beyond the requirement of water as a solvent and focuses entirely on the mechanism of the reaction itself. By observing the movement of a single particle, this theory allows chemists to classify substances as acids or bases in a much broader range of chemical environments.
The Brønsted-Lowry Definition: Proton Transfer
The Brønsted-Lowry definition centers on the transfer of a proton, which is simply a hydrogen ion (H⁺). A hydrogen atom consists of one proton and one electron. Since a hydrogen atom loses its electron to become an ion, only the proton remains. For this reason, the term “proton” is used interchangeably with “hydrogen ion” in this context.
A Brønsted-Lowry acid is defined as any substance that can donate a proton (H⁺). Conversely, a Brønsted-Lowry base is defined as any substance that can accept a proton.
The base must have a lone pair of electrons available to form a bond with the incoming proton. This definition means that a substance’s classification is not based on its chemical structure alone, but on its specific action within a given reaction. The entire acid-base reaction is understood as the movement of one proton from the acid to the base.
Identifying Acids and Bases in a Chemical Reaction
To determine if a substance is a Brønsted-Lowry acid or base, you must examine the chemical equation to track the movement of the proton (H⁺). The easiest method is to count the hydrogen atoms on each reactant and compare that number to the corresponding product. If a substance loses a hydrogen atom in the reaction, it acted as the acid; if it gains a hydrogen atom, it acted as the base.
Consider the reaction of hydrogen chloride (HCl) with water (H₂O), which forms the hydronium ion (H₃O⁺) and the chloride ion (Cl⁻). The HCl reactant starts with one hydrogen and ends as Cl⁻, which has no hydrogens, meaning HCl donated a proton and is the acid. The H₂O reactant starts with two hydrogens and ends as H₃O⁺, which has three hydrogens, meaning H₂O accepted the proton and is the base.
A second example is the reaction of ammonia (NH₃) with water, which forms the ammonium ion (NH₄⁺) and the hydroxide ion (OH⁻). Here, NH₃ gains a proton to become NH₄⁺, making it the base. In this same reaction, H₂O loses a proton to become OH⁻, meaning water is acting as the acid.
Understanding Conjugate Acid-Base Pairs
The proton transfer does not occur in isolation; it creates two new species that are chemically related to the original reactants. When an acid donates its proton, the remaining particle is called its conjugate base. This new particle is now capable of accepting a proton to reverse the reaction.
Similarly, when a base accepts a proton, the resulting particle is called its conjugate acid. This new species has the ability to donate a proton to reverse the reaction. Every Brønsted-Lowry reaction always involves two such pairs, linked by the gain or loss of a single proton.
Returning to the HCl and H₂O reaction, the HCl acid becomes the Cl⁻ conjugate base after donating its proton. The H₂O base becomes the H₃O⁺ conjugate acid after accepting the proton. In the ammonia example, the NH₃ base forms the NH₄⁺ conjugate acid, and the H₂O acid forms the OH⁻ conjugate base. The relationship is always defined by a difference of exactly one H⁺ between the acid and its conjugate base.
Amphoteric Substances
Some substances possess the ability to function as either a Brønsted-Lowry acid or a Brønsted-Lowry base, depending on the chemical environment. These molecules are known as amphoteric substances (or amphiprotic substances) because they can both accept and donate a proton. Water (H₂O) is the most common example of this dual nature.
As demonstrated earlier, water acts as a base when it reacts with a substance like HCl, accepting a proton to form H₃O⁺. However, when water reacts with a base like NH₃, it acts as the acid, donating a proton to form OH⁻.
The classification of an amphoteric substance is entirely context-dependent, determined by the reactivity of the other substance present. If it reacts with a stronger acid, it acts as a base; if it reacts with a stronger base, it acts as an acid. This flexibility allows water to maintain acid-base balance in many chemical and biological systems.