Understanding acids and bases forms a fundamental part of how substances interact. The Brønsted-Lowry theory offers a widely accepted framework centered on the transfer of a proton. This theory provides a clear way to identify acids and bases in a reaction, clarifying many common chemical processes involving proton movement.
Understanding Bronsted-Lowry Acids and Bases
The Brønsted-Lowry theory, proposed independently by Johannes Brønsted and Thomas Lowry in 1923, defines acids and bases based on their behavior in a chemical reaction. A Brønsted-Lowry acid is any species that donates a proton (H⁺ ion) to another molecule. Conversely, a Brønsted-Lowry base is any species that accepts a proton. Bases typically have at least one lone pair of electrons available to form a new bond with the incoming proton. Water, for instance, can act as both an acid and a base, making it an amphoteric substance.
Identifying Acids in Reactions
To identify a Brønsted-Lowry acid, observe which reactant loses a proton during the process. This proton loss is the defining characteristic of a Brønsted-Lowry acid.
Consider the reaction where hydrochloric acid (HCl) dissolves in water: HCl(g) + H₂O(l) → H₃O⁺(aq) + Cl⁻(aq). In this reaction, HCl donates a proton to the water molecule. After donating its proton, HCl becomes the chloride ion (Cl⁻). Therefore, HCl acts as the Brønsted-Lowry acid.
Identifying Bases in Reactions
Similarly, identifying a Brønsted-Lowry base involves tracking which reactant gains a proton. This proton acceptance is fundamental to the Brønsted-Lowry definition of a base.
An example is the reaction of ammonia (NH₃) with water: NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq). Here, ammonia accepts a proton from the water molecule. As NH₃ gains a proton, it transforms into the ammonium ion (NH₄⁺). Consequently, ammonia functions as the Brønsted-Lowry base.
Conjugate Acid-Base Pairs
In any Brønsted-Lowry acid-base reaction, proton transfer creates two pairs of related chemical species: conjugate acid-base pairs. When an acid donates its proton, the remaining species is its conjugate base, capable of accepting a proton in the reverse reaction. Conversely, when a base accepts a proton, the newly formed species is its conjugate acid, which can then donate a proton in the reverse reaction. Each pair consists of an acid and a base that differ by only one proton (H⁺). For example, in the reaction NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, NH₃ and NH₄⁺ form one conjugate pair, while H₂O and OH⁻ form another.
Step-by-Step Examples
To determine Brønsted-Lowry acids, bases, and their conjugates, follow a systematic approach by examining proton transfer within a reaction.
Consider the reaction: CH₃COOH(aq) + H₂O(l) ⇌ CH₃COO⁻(aq) + H₃O⁺(aq).
Example 1: Acetic Acid and Water
- Identify related species: Pair reactants with products differing by one proton. CH₃COOH pairs with CH₃COO⁻, and H₂O pairs with H₃O⁺.
- Track proton transfer: CH₃COOH loses a proton to become CH₃COO⁻. H₂O gains a proton to become H₃O⁺.
- Label acid and base: CH₃COOH is the Brønsted-Lowry acid (proton donor). H₂O is the Brønsted-Lowry base (proton acceptor).
- Label conjugate pairs: CH₃COO⁻ is the conjugate base of CH₃COOH. H₃O⁺ is the conjugate acid of H₂O.
Another example: H₂S(aq) + NH₃(aq) ⇌ HS⁻(aq) + NH₄⁺(aq).
Example 2: Hydrogen Sulfide and Ammonia
- Identify related species: H₂S pairs with HS⁻, and NH₃ pairs with NH₄⁺.
- Track proton transfer: H₂S loses a proton to become HS⁻. NH₃ gains a proton to become NH₄⁺.
- Label acid and base: H₂S is the Brønsted-Lowry acid (proton donor). NH₃ is the Brønsted-Lowry base (proton acceptor).
- Label conjugate pairs: HS⁻ is the conjugate base of H₂S. NH₄⁺ is the conjugate acid of NH₃.
Following these steps allows for clear identification of all Brønsted-Lowry species in any given acid-base reaction.