The Bronsted-Lowry Concept of Acids and BasesThe early definitions of acids and bases were strictly experimental. An acid was a substance whose water solution: (1) turns blue litmus red, (2) neutralizes bases, (3) reacts with active metals with evolution of hydrogen, and (4) tastes sour. On the other hand, a base was a substance which is water solution: (1) turns red litmus blue, (2) neutralizes acids, (3) tastes bitter, and (4) feels soapy.
Limitations of the Arrhenius ConceptThe brilliant Swedish chemical pioneer, Svante Arrhenius, whose ideas were far ahead of his time as they are behind ours, proposed in 1887 that the characteristic properties of acids in water solution are the properties of the hydrogen ion, H
+, which hydrolyzes (reacts with water) in water to form the hydronium ion, H
3O
+ as seen in the table in
Post 445267 (missing)
(Aurelius: "Conjugate Acid-Bases and Their Relative Strengths", Chemistry Discourse)and those of bases, the properties of hydroxide ion, OH
-. An acid was, therefore, a substance whose water solution contains an excess of , H
+ ions and a base, of OH
- ions.
It is apparent that the Arrhenius concept, applying as it does to aqueous solutions only, is exceedingly limited. His idea that the characteristic properties of the following bases, NaOH- sodium hydroxide, Ca(OH)
2- calcium hydroxide, Mg(OH)
2- magnesium hydroxide and NH
4OH- ammonium hydroxide
* are actually those of the hydroxide ion are correct. He assumed, however, that such ionic bases as sodium and potassium hydroxide were ionized to produce hydroxide ion only when dissolved in water. Today, since we know that ionic compounds exist as ions even in the crystalline state, this view is no longer tenable.
*Ammonium Hydroxide is a theoretical species believed to only exist in aqueous solution as a result of its reactions indicating such a species. It is formed by dissolving Ammonia gas, NH
3 in water in a hydrolysis reaction analogous to that of the formation of carbonic acid as shown in
Post 445267 (missing)
(Aurelius: "Conjugate Acid-Bases and Their Relative Strengths", Chemistry Discourse).
Arrhenius assumed further that the excess hydrogen ions in a water solution (aqueous) of an acid were formed by a simple equilibrium ionization of the acid as it dissolved in the water. Thus, for HCl, hydrogen chloride, Arrhenius postulated that the reaction consisted of a dissociation of some of the molecules into positive hydrogen ions and negative chloride ions, which existed in equilibrium with undissociated hydrogen chloride molecules.
HCl + H2 gives (in equilibrium) H+ + Cl- In the light of more recent knowledge, however, this interpretation is suspect on two counts. First, a hydrogen ion, being nothing more than a proton, is unique among cations (positive ions); it contains no electrons at all and its effective radius is only about 10
-13cm compared to 10
-8 for several other simple cations. It is certain that a proton, with its enormously high ratio of charge to radius, could not exist unhydrated (having gone though hydrolysis with water) in a water solution, completely surrounded by water molecules, whose oxygen atoms, of course, bear a partial negative charge, as well as two unshared pairs of electrons.
In fact, approximate calculations show that the union of a proton with a water molecule would be so exothermic (thermodynamically favorable, giving off heat) to the extent of about 300,000 calories per mole (a standard chemical equivalent used for calculations involving more than one compound or ion) and that the fraction of protons which would remain unhydrated in water solution would be roughly 10
-190. This is equivalent to saying that free hydrogen (unhydrated) ions (or protons) simply do not exist in aqueous solution. The same conclusion holds for any other solvent whose molecules have unshared pairs of electrons, and these are the only solvents which normally give conducting solutions (electricity) even with the strongest acids dissolved in them. It appears certain, therefore, that although the bare proton can be produced in a discharge tube or in nuclear reactions, and can exist in gaseous solutions at very low pressures, it cannot be responsible for the properties of acids in solutions- especially aqueous solutions.
Secondly, the Arrhenius postulation that acids are ionized to form proton flies in the face of all we now know about atomic an molecular structure. The filled first shell, with its single orbital, holds two electrons (electronic configuration of helium). This is the stable electronic configuration for hydrogen.
Hence, there could be no possible driving force in chemical reaction that would induce a hydrogen atom in hydrogen chloride to part with its share in the electron pair simply to form a bare proton. The simple ion which hydrogen tends to form through chemical reactions is not proton, H
+, but the hydride ion, H
-.
For the sake of convenience, knowledgeable chemists today sometimes represent the ionization of acids as forming simple H
+, but as with the taking of liberties in the use of good grammar or in the exhibition of good manners, such a practice is suspect unless we are certain that the offender really knows better.
The Ionization of Covalent Acids and BasesIf the hydrogen ion or proton cannot exist unhydrated in water solution, we can most logically represent the ionization of hydrogen chloride in water as involving actual reaction with water molecules:
HCl + H2O to give H3O+ + Cl-The H
3O
+ ion, because of resemblance to the ammonium ion (NH
4+) is called the hydronium ion. Because water molecules are themselves associated (form weak bonds with each other, called hydrogen bonds), it appears certain that each proton is associated with a variable number of water molecules. Actually, the hydronium ions in solution are probably (H
5O
2+), (H
7O
3+), (H
9O
4+), etc. The average extent of hydration being dependent upon the concentration of the acid and the temperature to provide for the most stable configuration of atoms and molecules. The formula H
3O
+, however, is customarily used, as it is the simplest formula that denotes a hydrated hydrogen cation. In fact, the existence of this ion was proved conclusively in 1957 by means of infrared spectroscopy.
On this basis, the ionization in water of any monoprotic (only one hydrogen that can be considered acidic in the molecule) acid, expressed as HA, can be represented by the general equation:
HA + H2O to give H3O+ + A-After review of the material presented, we can now conclude that the properties listed for acids earlier are actually characteristics of the hydronium ion, not acids.
There is also a similar reaction of water with bases (hydrolysis) to cause the formation of the hydroxide ion from bases that are not ionic, rather covalent. The ionic bases merely release hydroxide ions from themselves into the water solution rather than react with water to cause the formation of the hydroxide ion. Covalent compounds cannot do this in such a direct manner. The most common example, and probably the simplest is the use of ammonia in water to form ammonium ions and hydroxide ions. The equilibrium reaction is shown below.
NH3 + H2O to give NH4+ + OH-Thus, a general reaction formula for the hydrolysis of a covalent compound becomes:
B + H2O to give BH+ + OH-