Teaching the Electronic Theory of Acids and Bases in the General Chemistry Course W. F. LUDER, W. S . MeGUIRE, a n d SAVER10 ZUFFANTI Northeastern University, Boston, Massachusetts
oxygen, and water to give practice in the use of equations. HE CLAIM has been made that while the elecThe student should now be prepared to understand tronlc theory of acids and bases is worth teaching how atomic structure affects chemical behavior, so the to students of organic chemistry (2), it is too difficult study of the periodic chart of the elements is begun. for the elementary course (1). The authors have had This, together with discussion and demonstration of several years' experience in the teaching of the G. N. subatomic phenomena with the aid of cathode-ray Lewis theory to students of general, analytical, organic, tubes, permits the establishment of a table of atomic and physical chemistry. Their experience leads them structures of the elements (3). Using this table of to disagree with the statement that the theory is di- electron configuration, the various forms of valence are cult for elementary chemistry students. In fact, they introduced and discussed in terms of the Lewis elecbelieve that the electronic theory of acids and hases is tronic formulas. not only simpler to understand but, when properly This is the logical place to use the electronic theory used, even more valuable to general chemistry students of acids and hases to best advantage, especially when than to students of organic chemistry. They believe it is followed by oxidation-reduction as the next topic. presentation of it to all chemistry students and es- The equations used as examples of the broader aspect pecially to general chemistry students is of value for the of acid-base phenomena provide excellent illustrations following reasons: of covalent and coordinate bonds. The student ac(a) It forces the students to think in terms of the quires a better understanding of the electronic theory electronic structures of atoms. of valence a t the same time that the horizon of his ideas (b) It gives them practice in visualizing the different of acids and bases is enormously broadened. An even types of valence bonds (polar, covalent, coordinate). greater degree of systematization can be achieved when, (6) It permits a more complete and broader classiafter oxidation-reduction is studied, the classification fication of acid-base reactions. of acids and oxidizing agents as electrophilic and of (d) It makes possible a correlation with oxidation- bases and reducing agents as eledrodotic (4) is taken reduction reactions. Experience has shown that with the help of the UP. tables and lecture demonstrations herewith described, ACID-BASE REACTION TABLES the electronic theory of acids and bases is no more A brief historical survey of acid-base concepts from difficult than any other. Its concept of neutralization Boyle to Arrhenius (2) serves to introduce the subject is simpler and more fundamental than for the other to the student in terms with which he may be familiar theories, and, in fact, includes them all. By permitting already. This introduction shows the student that few the correlation of oxidation-reduction phenomena definitions in chemistry have changed more often than with acid-base reactions it promotes a greater measure the definition of acids and bases. It prepares him for of system and understanding than has been possible the shock of his discovery that no adequate theory is as before. yet in general use. A short discussion of the Bronsted and solvent-system theories, including their inadeTHE GENERAL CHEMISTRY COURSE quacy and mutually contradictory features (Z), is a At Northeastern University the electronic theory of healthy antidote to the ordinary student's too proacids and bases has become an integral part of the found regard for the authority of his textbook. At first semester's work in general chemistry. After a this point a valuable scientific moral may be brought preliminary discussion of science and the scientific profitably to his attention-the importance of basing method, a brief survey of the history of chemistry, and scientific definitions upon experimental evidence. This was, of course, Lewis' first step in his clarificaa short treatment of matter and energy, the three states of matter are studied together with changes of tion of the problem of acids and bases (5). Lewis state. Consideration of the gas laws leads logically returned to the experimental definitions of acids and into the discussion of molecules and atoms. At this bases by defining an acid as any substance which, like point symbols, formulas, and equations are introduced HCl, neutralizes NaOH or any other base. A base is for the first time, followed by a study of hydrogen, any substance which, like NaOH, neutralizes HCl or INTRODUCTION
T .
any other acid. Application of this experimental definition to a large number of reactions leads to the theoretical explanation. An acid is capable of accepting a share in a lone electron-pair from a base to form a coordinate bond. The formation of this bond is the first step in all neutralization reactions. For example,
:o:
,,
:0: .. .. :o:
H
s + : 0 :H
-
:ij:
:o: .. s :o: H
:o;:
in which the sulfur atom secures eight valence electrons by sharing a lone pair from the water molecule. After a brief discussion of the idea of relativity (3) introduced by such definitions, using the amphoteric behavior of water as an example, Tables 1 and 2 are presented and discussed. Table 1 contains illustrations of three of Lewis' "phenomenological criteria" (5). (The fourth criterion is discussed and illustrated in the next section.) Table 2 contains reactions which are more closely associated with behavior in ionizing solvents such as water. With the aid of these tables, the new ideas are introduced to the student and a t the same time are correlated with information already in his possession. When these ideas are introduced to students who have not previously learned the older concepts too well, there is nothing particularly difficult about them. In fact, some explanations are easier to make in terms of the electronic theory. Consider, for example, the hydrolysis of salts. According to the electronic theory, any sufficiently strong acid or base will increase the concentration of cation or anion in an ionizing solvent. When a test with litmus shows a salt solution to be acidic or basic, a single simple equation is often all that
Description Nnrtralizotion The Arrhenius definition is a special ease The solvent-system definition is a special case The Bronsted definition is a special case The electronic theory includes all three a n d many others, c. g.
is necessary by way of explanation. For example, the hydrolysis of ZnClz may be represented by the equation Zn++
+ HOH
-
(ZnOH)+
+ H+
The explanation is that the zinc ion is acidic. Written according to the Arrhenius theory this reaction would require four equations and a confusing classification of ZnClr as a "salt of a strong acid and a weak base." The Bronsted explanation is nearly as complicated in its unnecessary introduction of the hydration of ions.
'
LECTURE DEMONSTRATIONS
The fourth criterion of acid-base behavior given by Lewis (5) is "Acids and bases may be titrated against one another by the use of substances, usually colored, known as indicators." A description of lecture demonstrations illustrating this criterion follows. I. Water Used as So1nrenl.-The indicator used in this series of experiments is methyl violet dissolved in water. A . Acids (HCI, H2SOn,HOAc, HN03) Ten milliliters of water are poured into each of four 6-inch test tubes and 3 to 4 drops of the indicator are added. The violet color that results can be changed to yellow by the careful addition of a few drops of a dilute acid. The above-mentioned acids are used, one in each tube, to show this color change. The reaction is explained to the students on the basis that these acid solutions contain hydronium ions (H30)+ which are electron-pair acceptors and therefore acids. B. Bases (NH3, C5H5N,GHbOH, 1,4-Dioxane)
TABLE 1 ACID-BASEREACTION^ IN GENERAL (Hf used for simplicity) Acid Bare H+ (COCl)+ HCI Electron-pair acceptor
Product
OH-
HOH COCh NHEl NH1 Eledron-pair donor First step in neutralization-formation of a coordinate bond BCL (C&IrhN AlCb (CzH&O CrHsN SO3 CaCOa Ca(0H)z Ag(NHahf
c1-
-
-
Displacement A strong acid displaces a weaker one from combi- BC4 LNaaO BClsl NalCOp COz H+ AI(OHh Al+++ HOH nation with a base NH4+ H+ Cu(NH&++ CuC' NHa CsHaN IHaN SOrl A strong base displaces a weaker one from combi- CsHsN +SO8 CiHaO1- HOH nation with an acid HGHsO* OHNH*+ OHNHI HOH Calalysis Many acids catalyze organic reactions AICb, BFa, HF, SOa E . c. reaction between alcohols and benzoyl chloride more rapid in pyridine Bases also catalyze organic reactions Presence of acids increases speed of reaction of ionizing solvents with metali Bases also may speed solvent reactions similarly
++ +
++
+
-+
The basic characteristics of these compounds can be demonstrated by addition of a few drops of each to the four yellow solutions obtained in the above experiment. In each case the color is changed back from yellow to violet on the addition of the base. These compounds are basic because they are electron-pair donors and can therefore neutralize the (H30)+ electron-pair acceptors of the above-mentioned acids. Example: CHPCHS
/ :o: \
\
CHrCH2 (Base)
CHPCH2 (Acid)
II. Chlorobenzene (Anhyd.) Used as Solerent.-The indicator used in these experiments is methyl violet dissolved in chlorobenzene. A . Acids (HCl gas dissolved in CsHaC1, BC12 dissolved in CsH6Cl,and fuming SnC14) Ten milliliters of chlorobenzene are poured into each of three 6-inch test tubes and 3 to 4 drops of the indicator are added to each tube. The violet color that results can be changed to yellow by the addition of a few drops of an acid. The above acids are used to show this color change; one in each test tube. Here again we have substances that are acidic because they are electron-pair acceptors. B. Bases (Ethers, Acid anhydrides, Alcohols) 0
R-CHI
\
:o:
R-CH,
/
// R-CH1-C \ :o: / R-CH-C
R-CH?
0
\ :o: /
H
\
(Ethers)
0 (Anhydrides). (6)
tion takes place; a clear solution results.) A . Acids [BCla, HC1 (gas) dissolved in CC14, and AICls suspended or dissolved in CC14] Ten milliliters of anhydrous carbon tetrachloride are poured into each of three 6-inch test tubes and 3 to 4 drops of the indicator added to each tube. The violet color that results can be changed to yellow by the addition of some of tbese acids. The AlCla is not very soluble in CC14. Shaking the mixture for a considerable length of time and then filtering gives a clear filtrate which does not behave acidically. If any hydrolysis should take place, due to traces of moisture being present, the resulting HC1 gas would dissolve in the solvent and an acid reaction would be observed. If some of the original mixture of AIC13 suspended in CCL is used, however, the violet color of the indicator disappears and the resulting yellow color shows that the solid A1Cl3 is behaving as an acid. This experiment helps to prove that the AlC13 itself is acting as an acid and not any HCl resulting from hydrolysis. Enough AlC13 can be dissolved in the CClc by warming so that a clear solution of it may be used as an acid solution. The BCls and the HCl gas are readily soluble in the solvent, so no trouble is encountered in the preparation and use of tbese acidic solutions. B. Bases (Pyridine, Amines, Esters) CH = CH
7
CH-CH (Pyridine)
(Alcohols)
These substances can all be demonstrated to be bases by allowing them to react with the acids mentioned above. If these bases are added to the yellow solutions formed in Part A of this experiment, the violet color of the indicator quickly reappears. Example:
III. Carbon Tetrachloride Used as Solerent.-The indicator used is methyl violet dissolved in chlorobenzene rather than in carbdn tetrachloride. (The indicator is quite insoluble in carbon tetrachloride, but when it is added in chlorobenzene solution no precipita-
/
CHCHP (Triethylamine)
..
:CI:
\
(Acid)
\
CHCH-N:
iO:
CHSCH, (Ethylpropionate)
These substances can be demonstrated to be bases by the fact that when they are added to the yellow solutions resulting in Part A , the violet color of the indicator reappears. Example:
/cH=cH CH
(Base)
CH8CH2
\
/ CH \\
// CHCH-C \
CH-CH
\
N:
//
+
A1:ci:
.. . .
:CI: ..
-
/CH=CH CH
\\
CH-CH
\
N:
//
:ci: ..
AI:C~:
..
..
:CL:
..
IV. Discussion.-In all these experiments it can very readily be demonstrated to the students that the acidic substances are those that contain atoms capable of accepting a share in an electron-pair and that the bases are those substances containing atoms capable of donating a share in an electron-pair. The student can readily see that hydrogen chloride behaves as an acid regardless of whether i t is dissolved
TABLE 2 ACID-BASEREACTIONS IN IO~IZINC SOLVENTS (Hydration of the solvent cation usually not shown) Description Ionization According t o the solvent-system theory According to the Br6nsted theory Both are special cases of the electronic theory Other examples: (I denotes solvent) Note that an acid reacting with an ionizing solvent increases the concentration of solvent cations; a base increases the anion concentration
Acid
Barc
AIClr HCl Electron-pair acceptor Sot COI HOH (s) HOH (s) BC4 SeOCI. (s)
Hydrolysis of Salts Reaction of cation acid with solvent (when anion NHI+ Zni+ is weak base, e. g.. ClReaction of anion base with solvent (when cation HOH is weak acid, c. g., Naf HOH Amphoteric Behavior Water is amphoteric (it can donate or accept an HOH electron-pair) HCI Aluminum hydroxide is amphoteric (donates or AI(OH)8 accepts an electron-pair) H+
Produd
++
AICLCOCll COCli C1HOH HsOf Electron-pair donor Coordinate bond formation followed bv ionization H+ HSO,HOH (s) HOH (s) H + +HCOsNHs OHNH4+ CsHsN OH-+ CsHsNH+ SeOCl+ BCI.SeOCI* (s) (CaHdrN C1(GHS)~NS~OCI+
+
+
HOH HOH CN -
+ + HaOC+ NHs H + + ZnOHt OH-
+ HCN
HOH OHAI(OH).
A[+++
+ HOH
Complex Ions
in water, chlorobenzene, carbon tetrachloride, or any other solvent. (It is pointed out, however, that in water solutions of the common acids i t is the H+ or H30+ that behaves acidically, whereas in the organic solvents i t is the molecules of the compounds that behave as acids.) The same may be said for all other compounds that are capable of accepting electron-pairs. The student can readily be made to understand that compounds such as pyridine, alcohols, ethers, and many other substances act as bases whether dissolved in water, chloroform, carbon tetrachloride, chlorobenzene, or any other solvent that will dissolve them, because fundamentally these bases are compounds that are capable of donating an electron-pair. The experiments described above are only a few of the many acid-base titrations that the capable instructor can devise to meet the needs of his class and to aid him in presenting this material to his students.
of oxygen as the element necessarily involved in all oxidations. Likewise, the presence of a particular element (oxygen and later hydrogen) was once made the condition of acid behavior. Now, however, large groups of data can be systematized through the application of the electronic theory of acids and bases. When this is done, both k i d s of behavior can be further classified into electrophilic (acids and oxidizing agents) and electrodotic (bases and reducing agents) (4). This permits a notable increase in the correlation of experimental data. Since general chemistry students are already expected to be familiar with oxidation-reduction reactions and the writing of electronic structural formulas, there seems to be no inherent reason why the Lewis theory of acids and bases should be difficult for them. Actually it not only provides greatly needed practice in using many of the concepts now taught, but i t also promotes a much greater degree of understanding of chemistry as a science.
CONCLUSION
One of the important aims of a science is to promote the maximum degree of correlation among the experimental facts with which i t is concerned. In chemistry a large body of experimental data is already interrelated through the electronic theory of oxidation-reduction. At one time the theory was limited to the consideration
LITERATURE CITED
S. S . R.).
