↵ Learning Objectives Identify a Brønsted-Lowry acid and a Brønsted-Lowry base. Identify conjugate acid-base pairs in an acid-base reaction. The Arrhenius definition of acid and base is limited to aqueous (that is, water) solutions. Although this is useful because water is a common solvent, it is limited to the relationship between the H + ion and the OH − ion. What would be useful is a general definition more applicable to other chemical reactions and, importantly, independent of H 2 O. In 1923, Danish chemist Johannes Brønsted and English chemist Thomas Lowry independently proposed new definitions for acids and bases, ones that focus on proton transfer. A Brønsted-Lowry acid is any species that can donate a proton (H + ) to another molecule. A Brønsted-Lowry base is any species that can accept a proton from another molecule. In short, a Brønsted-Lowry acid is a proton donor (PD), while a Brønsted-Lowry base is a proton acceptor (PA). It is easy to see that the Brønsted-Lowry definition covers the Arrhenius definition of acids and bases. Consider the prototypical Arrhenius acid-base reaction: \[\underset{acid}{H^{+}(aq)}+\underset{base}{OH^{-}(aq)}\rightarrow H_{2}O\, (l)\nonumber \] Acid species and base species are marked. The proton, however, is (by definition) a proton donor (labeled PD), while the OH − ion is acting as the proton acceptor (labeled PA): \[\underset{PD}{H^{+}(aq)}+\underset{PA}{OH^{-}(aq)}\rightarrow H_{2}O\, (l)\nonumber \] The proton donor is a Brønsted-Lowry acid, and the proton acceptor is the Brønsted-Lowry base: \[\underset{BL\, acid}{H^{+}(aq)}+\underset{BL\, base}{OH^{-}(aq)}\rightarrow H_{2}O\, (l)\nonumber \] Thus H + is an acid by both definitions, and OH − is a base by both definitions. Ammonia (NH 3 ) is a base even though it does not contain OH − ions in its formula. Instead, it generates OH − ions as the product of a proton-transfer reaction with H 2 O molecules; NH 3 acts like a Brønsted-Lowry base, and H 2 O acts like a Brønsted-Lowry acid: A reaction with water is called hydrolysis ; we say that NH 3 hydrolyzes to make NH 4 + ions and OH − ions. Even the dissolving of an Arrhenius acid in water can be considered a Brønsted-Lowry acid-base reaction. Consider the process of dissolving HCl(g) in water to make an aqueous solution of hydrochloric acid. The process can be written as follows: \[\ce{HCl(g) + H2O(ℓ) → H3O+(aq) + Cl^{-}(aq)} \nonumber \nonumber \] HCl(g) is the proton donor and therefore a Brønsted-Lowry acid, while H 2 O is the proton acceptor and a Brønsted-Lowry base. These two examples show that H 2 O can act as both a proton donor and a proton acceptor, depending on what other substance is in the chemical reaction. A substance that can act as a proton donor or a proton acceptor is called amphiprotic . Water is probably the most common amphiprotic substance we will encounter, but other substances are also amphiprotic. Example \(\PageIndex{1}\) Identify the Brønsted-Lowry acid and the Brønsted-Lowry base in this chemical equation. \[\ce{C6H5OH + NH2^{-} -> C6H5O^{-} + NH3} \nonumber \nonumber \] Solution The C 6 H 5 OH molecule is losing an H + ; it is the proton donor and the Brønsted-Lowry acid. The NH 2 − ion (called the amide ion) is accepting the H + ion to become NH 3 , so it is the Brønsted-Lowry base. Exercise \(\PageIndex{1}\): Aluminum Ions in Solution Identify the Brønsted-Lowry acid and the Brønsted-Lowry base in this chemical equation. \[\ce{Al(H2O)6^{3+} + H2O -> Al(H2O)5(OH)^{2+} + H3O^{+}} \nonumber \nonumber \] Answer Brønsted-Lowry acid: Al(H 2 O) 6 3+ ; Brønsted-Lowry base: H 2 O In the reaction between NH 3 and H 2 O, the chemical reaction does not go to completion; rather, the reverse process occurs as well, and eventually the two processes cancel out any additional change. At this point, we say the chemical reaction is at equilibrium . Both processes still occur, but any net change by one process is countered by the same net change of the other process; it is a dynamic , rather than a static , equilibrium. Because both reactions are occurring, it makes sense to use a double arrow instead of a single arrow: What do you notice about the reverse reaction? The NH 4 + ion is donating a proton to the OH − ion, which is accepting it. This means that the NH 4 + ion is acting as the proton donor, or Brønsted-Lowry acid, while the OH − ion, the proton acceptor, is acting as a Brønsted-Lowry base. The reverse reaction is also a Brønsted-Lowry acid base reaction: BL bases. NH4+ and H2O are the BL acids. lt-chem-64081" style="width: 750px; height: 173px;" width="750px" height="173px" src="/@api/deki/files/92035/3765cabac9591a0fb3dd5878f56075e2.jpg" data-quail-id="174"> This means that both reactions are acid-base reactions by the Brønsted-Lowry definition. If you consider the species in this chemical reaction, two sets of similar species exist on both sides. Within each set, the two species differ by a proton in their formulas, and one member of the set is a Brønsted-Lowry acid, while the other member is a Brønsted-Lowry base. These sets are marked here: The two sets—NH 3 /NH 4 + and H 2 O/OH − —are called conjugate acid-base pairs . We say that NH 4 + is the conjugate acid of NH 3 , OH − is the conjugate base of H 2 O, and so forth. Every Brønsted-Lowry acid-base reaction can be labeled with two conjugate acid-base pairs. Example \(\PageIndex{2}\) Identify the conjugate acid-base pairs in this equilibrium. \[(CH_{3})_{3}N+H_{2}O\rightleftharpoons (CH_{3})_{3}NH^{+}+OH^{-} \nonumber \nonumber \] Solution One pair is H 2 O and OH − , where H 2 O has one more H + and is the conjugate acid, while OH − has one less H + and is the conjugate base. The other pair consists of (CH 3 ) 3 N and (CH 3 ) 3 NH + , where (CH 3 ) 3 NH + is the conjugate acid (it has an additional proton) and (CH 3 ) 3 N is the conjugate base. Exercise \(\PageIndex{2}\) Identify the conjugate acid-base pairs in this equilibrium. \[NH_{2}^{-}+H_{2}O\rightleftharpoons NH_{3}+OH^{-} \nonumber \nonumber \] Answer H 2 O (acid) and OH − (base); NH 2 − (base) and NH 3 (acid) Chemistry is Everywhere: Household Acids and Bases Many household products are acids or bases. For example, the owner of a swimming pool may use muriatic acid to clean the pool. Muriatic acid is another name for HCl(aq). In Section 4.6, vinegar was mentioned as a dilute solution of acetic acid [HC 2 H 3 O 2 (aq)]. In a medicine chest, one may find a bottle of vitamin C tablets; the chemical name of vitamin C is ascorbic acid (HC 6 H 7 O 6 ). One of the more familiar household bases is NH 3 , which is found in numerous cleaning products. NH 3 is a base because it increases the OH − ion concentration by reacting with H 2 O: NH 3 (aq) + H 2 O(ℓ) → NH 4 + (aq) + OH − (aq) Many soaps are also slightly basic because they contain compounds that act as Brønsted-Lowry bases, accepting protons from H 2 O and forming excess OH − ions. This is one explanation for why soap solutions are slippery. Perhaps the most dangerous household chemical is the lye-based drain cleaner. Lye is a common name for NaOH, although it is also used as a synonym for KOH . Lye is an extremely caustic chemical that can react with grease, hair, food particles, and other substances that may build up and clog a water pipe. Unfortunately, lye can also attack body tissues and other substances in our bodies. Thus when we use lye-based drain cleaners, we must be very careful not to touch any of the solid drain cleaner or spill the water it was poured into. Safer, non-lye drain cleaners (like the one in the accompanying figure) use peroxide compounds to react on the materials in the clog and clear the drain. Key Takeaways A Brønsted-Lowry acid is a proton donor; a Brønsted-Lowry base is a proton acceptor. Acid-base reactions include two sets of conjugate acid-base pairs.