Brønsted-Lowry View of Reaction of a Base with Water
Ammonia (NH 3 ) increases the hydroxide ion concentration in aqueous solution by reacting with water rather than releasing hydroxide ions directly. In fact, the Arrhenius definitions of an acid and a base focus on hydrogen ions and hydroxide ions. Are there more fundamental definitions for acids and bases?
In 1923, the Danish scientist Johannes Brønsted and the English scientist Thomas Lowry independently proposed new definitions for acids and bases. Rather than considering both hydrogen and hydroxide ions, they focused on the hydrogen ion only. A Brønsted-Lowry acid is a compound that supplies a hydrogen ion in a reaction. A Brønsted-Lowry base , conversely, is a compound that accepts a hydrogen ion in a reaction. Thus, the Brønsted-Lowry definitions of an acid and a base focus on the movement of hydrogen ions in a reaction, rather than on the production of hydrogen ions and hydroxide ions in an aqueous solution. Reactions that involve weak acids and bases are also reversible, indicated by the double arrow, and the chemical that acts as the acid in the reverse reaction is identified as the conjugate acid, while the chemical that acts as the base in the reverse reaction is the conjugate base.
The reaction of ammonia in water demonstrates the Brønsted-Lowry definitions of acid, base, conjugate acid, and conjugate base. Ammonia and water molecules are reactants, while the ammonium ion and the hydroxide ion are products:
What has happened in this reaction is that the original water molecule has donated a hydrogen ion to the original ammonia molecule, which in turn has accepted the hydrogen ion. We can illustrate this as follows:
Because the water molecule donates a hydrogen ion to the ammonia, it is the Brønsted-Lowry acid, while the ammonia molecule—which accepts the hydrogen ion—is the Brønsted-Lowry base. Thus, ammonia acts as a base in both the Arrhenius sense and the Brønsted-Lowry sense.
Brønsted-Lowry View of Reaction of an Acid with Water
Understanding how an Arrhenius acid like hydrochloric acid is still an acid in the Brønsted-Lowry sense requires understanding what really happens when HCl dissolves in water. Recall that the hydrogen atom is a single proton surrounded by a single electron. The hydrogen ion is formed by removing the electron, leaving a bare proton. Are there really bare protons floating around in aqueous solution? No. What really happens is that the H + ion attaches itself to H2O to make H3O + , which is called the hydronium ion. For most purposes, H + and H3O + represent the same species, but writing H3O + instead of H + recognizes that there are no bare protons floating around in solution. Rather, these protons are actually attached to solvent molecules
With this in mind, how is HCl defined as an acid in the Brønsted-Lowry sense? Consider what happens when HCl dissolves in H2O:
HCl is a strong electrolyte/strong acid, so there is no reverse arrow. Lewis electron dot diagrams show the movement of a proton:
Now we see that a hydrogen ion is transferred from the HCl molecule to the H2O molecule to make chloride ions and hydronium ions. As the hydrogen ion donor, HCl acts as a Brønsted-Lowry acid; as a hydrogen ion acceptor, H2O is a Brønsted-Lowry base. So HCl is an acid not just in the Arrhenius sense but also in the Brønsted-Lowry sense. Moreover, by the Brønsted-Lowry definitions, H2O is a base in the formation of aqueous HCl. So the Brønsted-Lowry definitions of an acid and a base classify the dissolving of HCl in water as a reaction between an acid and a base—although the Arrhenius definition would not have labeled H2O a base in this circumstance.
In the Brønsted-Lowry sense, water can act as an acid or a base based on its actions in a particular reaction. The H + ion always acts as an acid or a conjugate acid. The OH – ion always acts as a base or conjugate base.
Aniline (C6H5NH2) is slightly soluble in water. It has a nitrogen atom that can accept a hydrogen ion from a water molecule just like the nitrogen atom in ammonia does. Write the chemical equation for this reaction and identify the Brønsted-Lowry acid and base.
C6H5NH2 and H2O are the reactants. When C6H5NH2 accepts a proton from H2O, it gains an extra H and a positive charge and leaves an OH − ion behind. The reaction is as follows:
Because C6H5NH2 accepts a proton, it is the Brønsted-Lowry base. The H2O molecule, because it donates a proton, is the Brønsted-Lowry acid. In the reverse reaction, C6H5NH3 + donates an H + , so it is the conjugate acid. In the reverse reaction, OH − accepts an H + , so it is the conjugate base.
Caffeine (C8H10N4O2) is a stimulant found in coffees and teas. When dissolved in water, it can accept a proton from a water molecule. Write the chemical equation for this process and identify the Brønsted-Lowry acid, base, conjugate acid, and conjugate base.
There are many interesting applications of Brønsted-Lowry acid-base reactions in the pharmaceutical industry. For example, drugs often need to be water soluble for maximum effectiveness. However, many complex organic compounds are not soluble or are only slightly soluble in water. Fortunately, those drugs that contain proton-accepting nitrogen atoms (and there are a lot of them) can be reacted with dilute hydrochloric acid [HCl(aq)]. The nitrogen atoms—acting as Brønsted-Lowry bases—accept the hydrogen ions from the acid to make an ion, which is usually much more soluble in water. The modified drug molecules can then be isolated as chloride salts:
RN(sl aq) + H+(aq) → RNH+(aq) →Cl−(aq) RNHCl(s)
where RN represents some organic compound containing nitrogen. The label (sl aq) means “slightly aqueous,” indicating that the compound RN is only slightly soluble. Drugs that are modified in this way are called hydrochloride salts. Examples include the powerful painkiller codeine, which is commonly administered as codeine hydrochloride. Acids other than hydrochloric acid are also used. Hydrobromic acid, for example, gives hydrobromide salts. Dextromethorphan, an ingredient in many cough medicines, is dispensed as dextromethorphan hydrobromide.
