Acid-Base Concepts
Acids and bases are important in numerous chemical processes that occur around us, from industrial processes to biological ones, from reactions in the laboratory to those in our environment. There are several methods of defining acids and bases. While these definitions don't contradict each other, they do vary in how inclusive they are. The word acid comes from the Latin word acere, which means "sour." All acids taste sour. Well known from ancient times were vinegar, sour milk and lemon juice. Aspirin tastes sour if you don't swallow it fast enough. Its scientific name is acetosallicylic acid! Other languages derive their word for acid from the meaning of sour. So, in France, we have acide. In Germany, we have säure from saure and in Russia, kislota from kisly. The word "base" has a more complex history and its name is not related to taste. All bases taste bitter. Mustard tastes bitter. Many medicines, cough syrup is one, taste bitter. This is the reason cough syrups are advertised as having a "great grape taste."
The Arrhenius Concept of Acids and Bases
One of the first definitions of acids and bases was suggested by Arrhenius. The Arrhenius definition emphasizes the H+(aq) and OH-(aq) ions in water. According to Arrhenius, acids are substances that when dissolved in water increase the concentration of H+ ions. Likewise bases are substances that when dissolved in water increase the concentration of OH- ions. Because of the equilibrium in water, increasing the concentration of one of these ions decreases the concentration of the other. For Simplicity, chemists often use the notation H+(aq) for the H3O+(aq) ion, and call it the hydrogen ion. Please keep in mind that it is really the hydronium ion, H3O+(aq), that exists in water and not the hydrogen ion, H+(aq). The special role of the H3O+ and the OH- ions in aqueous solutions comes from the following equilibrium.
2 H2O(l) <==> H3O+(aq) + OH-(aq) or less correctly H2O(l) <==> H+(aq) + OH-(aq)
In Arrhenius's theory a strong acid is a substance that is 100% ionized in water to give H3O+(aq) and an anion. An example is nitric acid, HNO3.
HNO3(aq) + H2O(l) ---> H3O+(aq) + NO3-(aq)
A strong base is a substance that is 100% ionized in water to give OH-(aq) and a cation. An example is sodium hydroxide, NaOH.
H2O NaOH(s) ------------> Na+(aq) + OH-(aq)
The following is a list of the strong acids and strong bases ( memorize it)
| Strong Acids | Strong Bases | |
| HClO4 | LiOH | |
| H2SO4 | NaOH | |
| HI | KOH | |
| HBr | Ca(OH)2 | |
| HCl | Sr(OH)2 | |
| HNO3 | Ba(OH)2 |
The Bronsted-Lowry Concept of Acids and Bases
In these terms, when HCl dissolves in water it acts as a Bronsted-Lowry acid ( it donates a proton to H2O which becomes H3O+), and H2O acts as a Bronsted-Lowry base ( it accepts a proton from HCl). Any substance which is an acid according to the Arrhenius concept will also be an acid according to Bronsted-Lowry. The same can be said for bases. Note however that some Bronsted-Lowry acids and bases will not be defined as such under the Arrhenius concept. For example the following gas phase reaction would not be considered an acid-base reaction according to the Arrhenius concept because it does not take place in water, but is an acid-base reaction according to the Bronsted-Lowry concept in which a proton is transferred from HCl to NH3.
HCl(g) + NH3(g) ---> NH4Cl(s)
Many substances can act as an acid in one reaction and as a base in another. For example water is a Bronsted-Lowry base in its reaction with HCl and an acid in its reaction with NH3. A substance that can act as an acid or a base is called amphoteric.
In any acid-base equilibrium both the forward reaction and the reverse reaction involve proton transfers. For example consider the generic acid HA with water.
HA(aq) + H2O(aq) <==> A-(aq) + H3O+(aq)
In the forward reaction (to the right) HA donates a proton to H2O. Therefore HA is the Bronsted-Lowry acid, and H2O is the Bronsted-Lowry base. In the reverse reaction the H3O+ ion donates a proton to the A- ion; H3O+ is the acid and A- is the base. Note that when the acid HA donates a proton, it leaves behind a substance, A- which acts as a base. Likewise, when H2O acts as a base, it generates H3O+, which can act as an acid.
An acid, and a base such as HA and A- that differ only in the presence or absence of a proton are called a conjugate acid-base pair. Every acid has a conjugate base, formed by the removal of a proton from the acid. Similarly, every base has associated with it a conjugate acid, formed by the addition of a proton to the base.
The following is a short list of some conjugate acid-base pairs
| ACID | BASE | |
| HCl | Cl- | |
| H2SO4 | HSO4- | |
| HI | I- | |
| H2O | OH- | |
| H3O+ | H2O | |
| OH- | O2- |
Remove a proton from the acid to get its conjugate base, add a proton to a base to obtain its conjugate acid.
One final point on Bronsted-Lowry acids and bases. The stronger an acid is, the weaker its conjugate base will be.
The Lewis Concept of Acids and Bases
Many reactions have the characteristics of acid-base reactions but are not classified as such by the Arrhenius concept or the Bronsted-Lowry concept. An example is the reaction of boron trifluoride with ammonia.
BF3(g) + NH3(g) ---> NH3BF3(s)
According to the Lewis concept, a Lewis acid is a species that can form a covalent bond by accepting an electron pair from another substance; a Lewis base is a species that can form a covalent bond by donating an electron pair from another substance. Thus in the reaction above BF3 accepts an electron pair from NH3 and is a Lewis acid and of course ammonia is the Lewis base.