MODELS ILLUSTRATING THE LEWIS THEORY OF ACIDS AND BASES FRED Y. HERRON University of Pittsburgh, Pittsburgh, Pennsylvania
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S I N C E the Lewis concept of acidity and basicity is being used more and + more in the fields of analytical and organic chemistry to account for %ti *+ certain experimental observations + I which cannot be explained satisfactorily by other theories, i t seems desirable to introduce into general chemistry courses a t the college level at least the rudiments of this concept. Yet few of the more widely used general chemistry texts even so much as mention the Electron s~mbola: Lewis theory. H -X The purpose of this article is to describe a method of presenting the elements of this theory by means of cardboard models. Actually these models are nothing more than the commonly used electronic formulas in resent ions or molecules. In joining the discs, the which the nucleus of the atom and all of its electrons appropriate bolts are removed, the discs are lapped so except those of the valence shell are represented by the that the holes coincide and the bolts are inserted in the symbol of the element and the valence electrons by dots. proper holes. Where it is necessary to indicate a I t is sometimesdesirable to trace the electron changes in double bond such as that between sulfur and one of the a reaction and, for this purpose, the valence electrons of oxygens of sulfur trioxide in Figure 4, two pairs of holes the different atoms involved are often represented by are provided to accommodate the four bolts needed and distinguishing symbols for each variety of atom rather the discs consequently overlap to a greater extent. tban by dots. In this way, the transfer of an electron When, as in this case, the double bond is broken in the from one atom to another is clearly indicated and dif- reaction, two of the bolts are moved over to the vacant ferentiation between covalent and co-ordinate covalent holes provided for them on the oxygen disc and the oxygen and sulfur discs are reconnected in the usual bonds is clearly accomplished. Themodels used are circular discs of white cardboard manner by the remaining pair of bolts. with the symbol of the element printed in large letters Covalent and co-ordinate covalent bonds are easily near the center and the valence electrons are represented distinguishable here for the former will be represented by large spots near the circumference. Instead of using by two bolt heads of different colors (different symbols different symbols to distinguish between the valence in this article) and the latter by two bolt heads of the electrons of different atoms as described above, the same color. Where an ion has been formed by the loss symbol and the dots representing the valence electrons of one or more electrons from an atom, the former posiare printed in a differentcolor for each element. How- tion of each lost electron is indicated by thevacant hole ever, due to the fact that color printing is impossible in the disc (an open circle in this article). Ions formed here, differentiating symbols for electrons will be used by gain of electrons will be distinguished by electron in the illustrations accompanying this article. spots of more than one color on the disc. Admittedly, In order to provide a means for joining the discs to the practice of differentiating in such ways between the represent ions or molecules held together by covalent electrons belonging to different atoms is not strictly or co-ordinate covalent bonds, certain of the electron proper since all electrons are alike and it may lead to misspots are replaced by punched holes in which are inserted understanding. However, this is commonly done and short large-headed bolts with their heads painted in the disadvantages arc probably outweighed by the the appropriate color. On other discs, holes are punched advantages Figure 1 is a representation of the reaction involved in in positions not occupied by electrons in the atom of the free element but which will be occupied by shared or the neutralization of hydrochloric acid by sodium donated electrons when these discs are joined to rep- hydroxide according to the Arrhenius theory as com-
=
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JOURNAL OF CHEMICAL EDUCATION
cc-ordinate pair
+ =
NaOH Electron symbols : H- X 0-
cl
-*
caordinafe pair \
Na-A
monly usedfor many years. The acid and the hydroxide are assumed to break up in solution into their ions as indicated; the electron originally belonging to the hydrogen of the acid being retained by the chlorine and that originally possessed by the sodium being retained by the hydroxyl group, thus producing charged ions. Then the hydrogen ion and the hydroxyl ion unite to form practically un-ionized water, thus causing the whole reaction to go to completion. Since the hydrogen ion has no electron either to give to or to share with the hydroxyl ion, it can unite with the latter only by accepting a share in one of the available pairs of electrons belonging to the oxygen of the hydroxyl ion. Thus we have the co-ordinate covalent bond between the hydrogen and the oxygen of the water as indicated in the figure. The sodium and chloride ions are then assumed to be attracted to each other by reason of their opposite charges. Figure 2 represents the same reaction according to the more modern Bronsted theory. Here, water is said to be a base because it accepts proton from the acid or proton donor, hydrogen chloride. Since the proton has no electrons to share or give, it can unite with the water only by accepting a share in one of the available pairs of electrons possessed by the oxygen of the water to form hydronium ion as indicated in the figure. The hydroxyl ion of the sodium hydroxide, being a stronger base than water, can, when the solutions of hydrogen cbloride and sodium hydroxide are mixed, take the proton away from the hydronium ion and, again, the proton unites with the hydroxyl ion by accepting a share in a pair of electrons belonging to the oxygen of that ion. Solutions of hydrogen chloride and ammonia will react to form the salt-like substance ammonium chloride,
the hydronium ion produced by the action of the hydrogen chloride on water combining with the hydroxyl ion produced by the interaction of ammonia and water to produce water and leaving the two ions NHl+ and CI-. These two substances will also react in the gaseous state to form ammonium chloride as indicated in Figure 3. Here, the ammonia molecule, being a stronger base than chloride ion, is able to take proton away from the latter and, again, this proton can combine with the ammonia molecule only by accepting a share in the pair of electrons possessed by the nitrogen atom of the ammonia.. The resulting two ions, NH4+and CI- then form solid ammonium chloride. All of these reactions are examples of the neutralization of an acid by a base to form a salt. In all of them, the base has provided a pair of electrons which it shares with a proton. This is the characteristic feature of the Lewis theory in which a base is defined as a substance which has available a pair of electrons which it can share co-ordinately with "something." In the cases cited above, that "something" was always a proton. Figure 4 iIlustrates a case where the "something" which accepts the share in the pair of electrons belonging to the base is not a proton. Here, the sulfur trioxide molecule accepts a share in a pair of electrons possessed by the oxygen of the calcium oxide forming SO4--ion which, together with the Ca++ ion remaining from the calcium oxide, is equivalent to the salt calcium sulfate. Protons are not involved at all and yet there is the formation of the same salt which would be produced by the action of sulfuric acid on calcium oxide. Sulfur trioxide, according to the Bronsted definition, cannot be considered to be an acid or proton donor because i t has no prot,on to donate. Yet it has produced
APRIL, 1953
Electron symbols: H-X CI - X
+
Electron symbols: Ca-0 0 - .
S - A FiguFe 4
the same effect as an acid. The Lewis theory therefore broadens the usual conception of an acid by calling any substance which is able to accept a co-ordinate share in an electron pair an acid. The Bronsted theory broadened the meaning of the term "base" by defining a base as anything which can accept or combine with proton. The older idea that a base is a substance which provides hydroxyl ion was superseded and it was realized that hydroxyl ion itself is a base and that substances like NaOH, which are commonly called bases, are merely carriers of hydroxyl ion, which is the actual base; the positive metallic ions have nothing to do directly with the basic properties. The Lewis theory goes a step further. I t emphasizes that the mechanism of the acceptance of proton by a base consists in the s h a r i n ~of a pair of electrons, both of which are provided by the base, with the proton. I t also introduces the idea that the unit which accepts the share in the electron pair need not always be a proton
but may be anything capable of sharing such an electron pair, such as sulfur trioxide cited above. Also, it appears that, in the cases of hydrochloric and sulfuric acids and other substances which are commonly called acids, it is the proton provided by these substances which is really the acid, the negative ion in each case being merely a carrier of proton just as the metallic ions were seen to be merely hydroxyl ion carriers in the case of certain substances commonly called bases. The Lewis theory, then, defines an acid as an electron pair acceptor and a base as an electron pair donor. However, in view of the fact that sulfuric acid and similar substances will probably continue to be called acids and that substances like sodium hydroxide will probably continue to be called bases, may it probably not be more practical to define an acid as an electron pair acceptor or a substance which furnishes an electron pair acceptor and a base as an electron pair donor or a substance which furnishes an electron pair donor?
