Acids and bases are fundamental chemical compounds encountered frequently in daily life, from the foods we consume to the cleaning products we use. These substances play important roles in various natural processes, including digestion in biological systems and ecosystems. Understanding what precisely classifies a compound as an acid or a base provides insight into their distinct behaviors and reactions, and helps unravel the principles governing their chemical properties.
The Arrhenius Definition
The earliest formal definition of acids and bases, proposed by Svante Arrhenius in the late 19th century, focuses on how these substances behave when dissolved in water. According to this theory, an Arrhenius acid is a compound that increases the concentration of hydrogen ions (H+) in an aqueous solution. These hydrogen ions (H+) often combine with water to form hydronium ions (H3O+). For instance, hydrochloric acid (HCl) produces hydronium ions in water.
Conversely, an Arrhenius base is a substance that increases the concentration of hydroxide ions (OH-) when dissolved in water. Sodium hydroxide (NaOH), a common base, releases hydroxide ions in water. This definition has limitations because it exclusively applies to reactions occurring in aqueous solutions. It cannot explain why substances like ammonia (NH3) exhibit basic properties, as it does not contain hydroxide ions.
The Brønsted-Lowry Definition
Expanding upon the Arrhenius concept, the Brønsted-Lowry theory offers a more comprehensive definition of acids and bases, applicable beyond aqueous environments. Developed independently by Johannes Brønsted and Thomas Lowry in 1923, this theory defines an acid as a “proton donor” and a base as a “proton acceptor.” A proton, in this context, refers to a hydrogen ion (H+), which is essentially a hydrogen atom that has lost its single electron. This definition highlights the transfer of a proton during an acid-base reaction.
In a Brønsted-Lowry reaction, an acid donates a proton to a base. For example, when hydrochloric acid (HCl) reacts with water (H2O), HCl acts as a proton donor, making it an acid, while water acts as a proton acceptor (a base). This interaction forms a conjugate acid-base pair, where the acid becomes its conjugate base (e.g., Cl- from HCl), and the base becomes its conjugate acid (e.g., H3O+ from H2O). Water is notable for its amphoteric nature, meaning it can function as both a proton donor or a proton acceptor depending on the chemical environment.
The Lewis Definition
The most expansive definition of acids and bases, proposed by Gilbert Lewis in 1923, extends beyond the transfer of protons to encompass electron pair interactions. A Lewis acid is defined as an “electron pair acceptor,” while a Lewis base is an “electron pair donor.” This definition is powerful as it includes reactions that do not involve hydrogen ions or proton transfer, broadening the scope of what can be considered an acid or a base. For example, boron trifluoride (BF3) acts as a Lewis acid because it can accept a pair of electrons due to its incomplete octet.
Ammonia (NH3), a limitation for the Arrhenius definition, functions as a Lewis base because its nitrogen atom possesses a lone pair of electrons available for donation. When BF3 reacts with NH3, the ammonia donates its electron pair to the boron trifluoride, forming a covalent bond. This electron pair donation and acceptance forms a Lewis acid-base adduct. The Lewis theory is valuable in organic chemistry and coordination chemistry, where many reactions involve the formation of bonds through the sharing of electron pairs rather than proton transfer.
The pH Scale
The pH scale serves as a standardized measure of the acidity or basicity of an aqueous solution, quantifying hydrogen ion concentration. This scale is logarithmic, meaning each whole number change in pH represents a tenfold change in hydrogen ion concentration. For example, a solution with a pH of 4 is ten times more acidic than a solution with a pH of 5.
The pH scale typically ranges from 0 to 14. Solutions with a pH less than 7 are considered acidic, indicating a higher concentration of hydrogen ions. Lemon juice (pH ~2) is a strong acid; coffee (pH ~5) is mildly acidic. A pH of exactly 7 signifies a neutral solution, such as pure water.
Solutions with a pH greater than 7 are basic or alkaline, meaning they have a lower concentration of hydrogen ions and a higher concentration of hydroxide ions. Common household bleach (pH ~13) is a strong base. Maintaining specific pH levels is important in biological systems; for instance, human blood pH is tightly regulated around 7.4, with deviations causing health issues.