Acid-base chemistry is a fundamental concept in chemical reactions. The Brønsted-Lowry model offers a practical framework for understanding these reactions by focusing on the movement of a single particle. This approach simplifies determining which substances are the acids and which are the bases within a chemical event. Identifying these species based on the transfer of a proton is the first step toward understanding how they function.
Defining Brønsted-Lowry Acids and Bases
The Brønsted-Lowry theory defines acids and bases based on their behavior toward a proton (\(H^+\)). An acid is a proton donor, meaning it gives away a hydrogen ion during a reaction. A base is defined as a proton acceptor, requiring it to have a lone pair of electrons capable of forming a new bond with the incoming proton. This definition is more general than earlier theories, as it does not require the reaction to occur in water. The classification is purely functional, determined by whether the substance loses or gains an \(H^+\) in the specific reaction being observed.
Identifying Proton Transfer in Reactions
The most practical way to determine an acid and a base is to compare the chemical formulas of the reactants to their corresponding products. Consider the reaction: \(HCl + H_2O \rightleftharpoons H_3O^+ + Cl^-\).
In this reaction, \(HCl\) transforms into \(Cl^-\), which involves the loss of a proton (\(H^+\)). Because \(HCl\) donated a proton, it is classified as the Brønsted-Lowry acid. Simultaneously, \(H_2O\) transforms into \(H_3O^+\), indicating that the water molecule gained a proton. Since \(H_2O\) accepted the proton, it is the Brønsted-Lowry base. The core determination rule is to track the hydrogen count: the species that decreases its number of hydrogens is the acid, and the species that increases its number of hydrogens is the base.
Forming Conjugate Acid and Base Pairs
Once the acid and base on the reactant side are identified, the products they form are known as their conjugates. The substance that remains after the Brønsted-Lowry acid has donated its proton is termed the conjugate base. In the example reaction, \(HCl\) (the acid) loses a proton to become \(Cl^-\), which is its conjugate base.
The species that is formed after the Brønsted-Lowry base accepts a proton is called the conjugate acid. In our example, \(H_2O\) (the base) gains a proton to become \(H_3O^+\), which is its conjugate acid. Every Brønsted-Lowry acid-base reaction contains two such pairs. This pairing is interconnected, as a strong acid will produce a weak conjugate base, and conversely, a weak base will form a relatively strong conjugate acid.
When Substances Act as Both
Some substances possess the ability to act as either a proton donor (an acid) or a proton acceptor (a base), depending on the chemical environment. These compounds are known as amphiprotic substances. Their role is relative to the other reactant present in the system.
Water (\(H_2O\)) is the most frequently cited example. When water reacts with a stronger acid, such as \(HCl\), it accepts a proton and functions as a base. When water reacts with a stronger base, such as ammonia (\(NH_3\)), it donates a proton and functions as an acid.