This article guides you through identifying a Brønsted-Lowry acid, a classification central to understanding chemical reactions. The Brønsted-Lowry theory defines acids and bases based on the transfer of a proton. This framework allows chemists to categorize substances and predict their behavior.
The Fundamental Definition of a Bronsted-Lowry Acid
A Brønsted-Lowry acid is defined as any chemical species that functions as a proton donor during a reaction. In chemistry, a “proton” refers specifically to a hydrogen ion (H+), which is a hydrogen atom stripped of its single electron. To be classified as a Brønsted-Lowry acid, a substance must possess a hydrogen atom capable of being given away to another molecule.
Conversely, the Brønsted-Lowry base is a proton acceptor. An acid-base reaction is fundamentally a proton-transfer event where the acid loses the H+ and the base gains it. This definition expands acid-base chemistry beyond reactions that only occur in water, a limitation of earlier theories.
Structural Clues for Potential Proton Donors
To anticipate whether a molecule can act as a Brønsted-Lowry acid, one must examine its structural features. A molecule must contain at least one hydrogen atom to be a proton donor. The acidity of that hydrogen atom is determined by the electronegativity of the atom it is bonded to.
For the hydrogen to be easily detachable as an H+ ion, it must be bonded to a highly electronegative atom, such as oxygen, nitrogen, or a halogen (like chlorine or fluorine). This difference creates a polar bond, pulling electron density away from the hydrogen atom and making it susceptible to removal by a base. Common functional groups containing acidic hydrogens include the hydroxyl group (–OH), found in alcohols and water, and the carboxyl group (–COOH), characteristic of organic acids.
The carboxyl group is a much stronger acid than a simple alcohol because the resulting conjugate base, the carboxylate ion, is stabilized through resonance. Resonance allows the negative charge left behind after proton donation to be shared across multiple atoms, distributing the charge and lowering the ion’s energy. This structural stabilization makes the proton easier to release, indicating the molecule is an effective Brønsted-Lowry acid.
Observing the Proton Transfer Reaction
The most certain way to identify a Brønsted-Lowry acid is by observing the chemical transformation in a balanced equation. The acid is the reactant species that has one fewer proton (H+) on the product side. For example, in the reaction between hydrochloric acid and water, HCl donates a proton to H2O to become Cl-.
This transfer creates a conjugate acid-base pair, which are two species that differ only by the presence of a single proton. When the acid (HCl) donates its proton, the resulting species (Cl-) is called its conjugate base. The water molecule (H2O), acting as the base by accepting the proton, forms its conjugate acid (H3O+). All Brønsted-Lowry reactions contain two such conjugate pairs. By comparing the chemical formulas of the reactants and products, the acid is identified as the species that lost the H+.
How to Identify Amphiprotic Substances
A complication arises with amphiprotic substances, which are molecules or ions capable of acting as either a Brønsted-Lowry acid or a Brønsted-Lowry base. These substances must possess a removable hydrogen atom to act as an acid, and a lone pair of electrons to accept a proton to act as a base. Water (H2O) is the most common example, capable of donating a proton to form OH- or accepting a proton to form H3O+.
Other examples include the hydrogen carbonate ion (HCO3-) and the hydrogen sulfate ion (HSO4-). To determine if an amphiprotic substance is acting as an acid in a specific reaction, one must look at its reaction partner. If water reacts with a stronger base, such as ammonia (NH3), it will donate a proton and function as the acid. Conversely, if water reacts with a stronger acid, it will accept a proton and act as the base.