8.3: Brønsted-Lowry Acids and Bases

The Arrhenius definition of acid and base is limited to aqueous solutions (solutions where the solvent is water). 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 H₂O.

Proton Transfer

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: the transfer of a proton, H⁺, from one compound to another. A Brønsted-Lowry acid is any species that can donate a proton 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:

[latex]\underset{acid}{H^{+}(aq)}+\underset{base}{OH^{-}(aq)}\rightarrow H_{2}O\, (l)\nonumber[/latex]

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):

[latex]\underset{PD}{H^{+}(aq)}+\underset{PA}{OH^{-}(aq)}\rightarrow H_{2}O\, (l)\nonumber[/latex]

The proton donor is a Brønsted-Lowry acid, and the proton acceptor is the Brønsted-Lowry base:

[latex]\underset{BL\, acid}{H^{+}(aq)}+\underset{BL\, base}{OH^{-}(aq)}\rightarrow H_{2}O\, (l)\nonumber[/latex]

Thus H⁺ is an acid by both definitions, and OH⁻ is a base by both definitions.

Ammonia (NH₃) 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 H₂O molecules; NH₃ acts as a Brønsted-Lowry base, and H₂O acts as a Brønsted-Lowry acid. In the following equation, the blue H is being transferred from water to ammonia:

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:

HCl (g) + H₂O (ℓ) → H₃O⁺ (aq) + Cl⁻ (aq)

HCl (g) is the proton donor and therefore a Brønsted-Lowry acid, while H₂O is the proton acceptor and a Brønsted-Lowry base. These two examples show that H₂O 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.

Chemical Equilibrium and Conjugate Acid-Base Pairs

In the reaction between NH₃ and H₂O,

the chemical reaction does not go to completion (i.e. the reactants do not get completely converted to products). Rather, the reverse process occurs as well – the products of the reaction react together and produce the original reactants.  Eventually the two processes – the forward reaction, from left to right, and the reverse reaction, from right to left – result in no further net change in the amounts of all substances. At this point, we say the chemical reaction is at equilibrium. Both processes still occur, but because 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 NH₄⁺ ion is donating a proton to the OH⁻ ion, which is accepting it. This means that the NH₄⁺ 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 (ammonium ion and hydroxide ion reacting to form ammonia and water) is also a Brønsted-Lowry acid base reaction. This means that both reactions, forward and reverse, 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—NH₃/NH₄⁺ and H₂O/OH⁻—are called conjugate acid-base pairs. We say that NH₄⁺ is the conjugate acid of NH₃, OH⁻ is the conjugate base of H₂O, and so forth. Every Brønsted-Lowry acid-base reaction can be labeled with two conjugate acid-base pairs.

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). Vinegar is a dilute solution of acetic acid [HC₂H₃O₂ (aq)]. In a medicine chest, one may find a bottle of vitamin C tablets; the chemical name of vitamin C is ascorbic acid (HC₆H₇O₆).

One of the more familiar household bases is NH₃, which is found in numerous cleaning products. NH₃ is a base because it increases the OH⁻ ion concentration by reacting with H₂O:

NH₃ (aq) + H₂O (ℓ) → NH₄⁺ (aq) + OH⁻ (aq)

Many soaps are also slightly basic because they contain compounds that act as Brønsted-Lowry bases, accepting protons from H₂O 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.