WHY NOT MODERNIZE the TEXTBOOKS ALSO? II. Hydrolysis and I t s Relation to I o n i c Charge and Radius LAURENCE S. FOSTER Brown University, Providence, Rhode Island
A
solve in a polar solvent; why water is such an unusual liquid; by what processes the ions of the salt become cal Society, in his contribution to the symposium on "Theories and Teaching of Acids and Bases," hydrated; and what is meant by acidic and basic propProfessor Briscoe reminded us that in this connection erties. He should, thus, be familiar with the fundai t is the duty of teachers of chemistry to present the mental concepts in the modem theory of electrolytic student with all the pertinent facts as well as the vari- solutions. These topics have received adequate treatous theories which have been advanced for their inter- ment in many articles which have appeared in the EDUCATION and elsewhere, as pretation. "With all the facts before him, the student JOURNAL OF CHEMICAL can then examine critically all theories that have been well as in numerous textbook^.^ An equally important offeredto explain the chemical characteristics and reac- topic, but much less frequently mentioned, is the influtions of acids and bases."' Surely, few will disagree ence of the size of ions and the magnitude of their charge with this. How does it happen, then, that seventeen upon the properties of the compounds containing them.s years after the publication of the Br#nsted-Lowry view- Attention might also be focused upon the idea of ionpoint, textbooks still introduce the concept of proton- deformation or polarization, but this phenomenon is transfer only half-heartedly into their discussions of still somewhat obscure and produces only secondary neutralization, and fail entirely to point out its applica- effects upon the tendency toward hydrolysis. If the tion to an equally important converse process, hydroly- course is developed so that these fundamental topics are sis? Apparently, this topic presents to textbook writers logically presented, no difficulty need be anticipated in what appear to be insuperable difficulties, and almost as extending the viewpoint to the study of hydrolytic phenomena. .. a moue revert to the admittedly simder, * thev , . but less logical, earlier viewpoint. The answer lies, I believe, in WHAT HAPPENS WHEN SALTS DISSOLVE? the failure of authors to follow the precepts expressed When salts come in contact with a solvent with a by Professor Briscoe. If all the pertinent facts were made available, the student would have no more diffi- high dipole moment, the crystal lattice is broken down. culty in understanding the modem theory of hydrolysis the individual positive and negative, ions become surthan he has with other aspects of atid-base equilibria. rounded by clusters of solvent molecules, and the forces It is possible that because of its difficulty the topic of of attraction between the ions are reduced.' The exhydrolysis has no place in textbooks designed for stu- planation of the reactions of salts in aqueous solution dents who are studying chemistry for the first time, but must take into account the formation of hydrated ions. if it is included and theories to explain i t are discussed, certainly the explanations should be consistent with the FORCES OP ATTRACTION BETWEEN THIRD PERIOD POSITIVE IONS AND WATER MOLECULES treatment afforded other equilibria existing in aqueous Latimer and Hildebrands have tabulated the forces solutions. The purpose of this article is to summarize an approach based upon recently evolved ideas concern- exerted by the positively charged central ion upon the ing atomic structure, crystalline solids, complex com- negative end of attached water dipoles and have shown ~ounds,strone electrolytes, and acids and bases which that in passing from sodium to chlorine, as the radius of enable one toutreat h$drolytic phenomena systemati- - 2 See particularly, HILDEERAND, "Principles of chemistry." cally. 4th ed., The Macmillan Company, New York City. 1940. As has often been pointed out, to modernize the ap- Chapters 5,s. 13, 18 (62%6), 20 ($13, 14, and is). a HILDEBRAND, 09. cit., pp. 26&72; CARTLEDGE, "Introducproach to the theory of electrolytic solutions one must tion to inorganic chemistry." Ginn and Company, Boston, 1935, do more than change a few words in his lecture notes, Chapters 8 ($15 and 16). 14 ($19). and 22 (652-7). 4 f i e ~ m , 'some uses of the polar molecule concept in eleThe end to be reached must be anticipated from the first chemistry," J. CHEM.EDUC., 13, 122 (1936), especially day of the course. Before investigating the theories of mentary 3, p. 127; HuGGrNs, "Some contributions of crystal structure rehydrolysis, the student needs to possess a knowledge of ,,,,,I, to chemistry teaching," ibid.. 13, 160 (1936). what constitutes a salt; what happens when salts dis- especially p. 161, "Ions." LATMEK AND HILDEBRAND, "Reference book of inorganic 1 Bnrsco~, "Tesching the new concepts of acids and bases in chemistry," revised ed., The Macmillan Company, New York City. 1940, p. 27. general chemistry," J. CHEM.Ennc., 17, 128 (1940). 509
T THE Baltimore meeting of the American Chemi-
the kernel becomes smaller and the ionic charge greater, the force of attraction rises tenfold, due to conlombic forces alone. The number of attached water molecules, furthermore, is related to the size of the central ion and, for the third period elements, may be six or four. Other water molecules may be attached in a secondary layer, but the intluence of the charge of the-central ion upon them is negligible. It is instructive to examine the consequences of these inaeasingly strong forces of attraction of the central kernel for the oxygen ion of the attached molecules of water of hydration. In the structure [Na(H,O)s] I+, the forces are so weak that little distortion of the water molecule may be detected; they are not much different in structure from unattached water molecules. [Mg(H20)6]2f shows slightly acidic properties, due to the weakening of the bond holding the protons, but hydrolysis of magnesium salts is almost negligible. In the hydrated aluminum ion, [A1(HpO)s]3+, the oxygen of the water molecules is held much more firmly to the A1 kernel, due to the diminished distance between them and the increased charge. The protons attached to the oxygen ions, moreover, are repelled by the high charge of the Al kernel, and the bond holding the protons becomes so weakened that the ion, [Al(HzO)sla+,can act as a proton donor and is thus a weak acid. If we may assume the temporary existence of such hypothetical ions as [Si(H20)4]4+, [P(&O)J6f, [S(H20),]", and [CI(H20)d7+,we should expect increasing acidity in passing from silicon to chlorine. Protons would be removed with increasing ease, resulting in the formation of the following well-known structures, which are stable in aqueous solutions of a pH of 7: [Si(OH)do, [(HO)POs]I-,
the transitional elements, when the radii are practically constant, the acid-base properties depend largely upon the valence; and for a single element, why the hydrated ions become incxeasingly acidic as the state of oxidation rises. In Table 1, the crystal ionic radii and the ionic potential, 4, are given for the ions of all the normal elements, both a and b, in Groups I, 11, and I11 of the Periodic Table. The ones which retain water of hydration when their salts crystallize from aqueous solution are seen to be those of small ionic radius. Those with large values of the ionic potential are distinctly acidic.? The increasing polarizing effect of the 18-electron kernel elements is established by the earlier appearance of acidic properties in the hydrated ions of this series. All of the b elements listed, furthermore, may serve as the central atom in complex anions, a property which characterizes markedly only the last four of the a elements listed.
[S0&I2-,and [C104]I-.
IONIC POTENTIAL
+,
Cartledge has applied the term ionic potential, to the ratio of the charge of the ipn to its radius, and has found that the acid-base properties of elements depend linearly upon the square root of this parameter? Thus, for kernels with values of 4 below 2.2, the hydrated ions are not appreciably acidic; for those of & values hetween 2.2 and 3.2, the hydrated ions are weakly acidic and the corresponding hydroxides are amphiprotic; and for those of 4% values above 3.2, the hydrated ions and the corresponding hydroxides are both acidic. The ionic potential, as defined, does not take into account the polarization of the ions, and is, therefore, only approximate in its application. Even though for zinc the value of is less than for magnesium, the hydrated zinc ion, [Zn(HnO)e12+, is more acidic than the hydrated magnesium ion, [Mg(Hz0)6] This discrepancy may be traced to the presence in the zinc kernel of a completed shell of eighteen electrons, rather than eight. The ionic potential is nevertheless successfulin predicting the broad periodic variations in acid-base properties of the elements; in explaining why in a given series of
+
-
9 C m n ~ n c s I. , Am. Chem. Soc., 50, 285&72 (1928); Cf. CARTLEDGE. "Introduction to inorganic chemistry," loc. cit., p.
In?
DEFINITION OF HYDROLYSIS
In aqueous solutions, hydroly& of a salt consists of the reaction of either or both the anion or the cation with water. The reactions are independent of each other and whether the pH of the solution, as a result of these reactions, will have a value equal to, greater than, or less than 7, will depend upon the net effect of the reactions of all the ionic or'molecular species p r e ~ e n t . ~ For example, solutions may be acid due to the presence of an acidic cation, like ammonium, NHal+, which may be more strongly acidic than the accompanying anion is basic. If the anion is also moderately basic, as in ammonium acetate, the solution may be neutral even though the salt is extensively hydrolyzed. The solution, on the other hand, may be acidic due to the presence of an anion which is itself an acid, as with sodium bisnlfate, Na+(HS04)-. Alkaline solutions result when -
'
E m ~ b u sAND ANDERSON,"Modern aspects of inorganic chemistry," D. Van Nostrand Co., New York City, 1938, pp. 15565 (Water of crystallization); pp. 16&78 (Nature of the coordinate hnkage). MIDDLETON AND WILLARD."Semimicro qualitative analysis." Prentice-Hall, Inc., New York City, 1939, Chapter 9; Chapter 13, p. 237, etc.
the cations or anions are more strongly basic than the conjugate substances are acidic. The terms acid and base are no longer restricted to describe neutral molecules but may be applied to positively charged ions, neutral molecules, or negatively charged ions, if they act as acids or bases in the particular mixture under consideration, and hydrolysis refers to reactions in which water enters as an active reagent. APPLICATION OF THE MODERN VIEWPOINT TO THE STUDY O F TliE HYDROLYSIS OF THE CHLORIDES OF THE THIRD PERIOD METALS
Hydrolysis of Aluminum Chlorid~Reaction of the Hydrated Aluminum Ion with Water If we dissolve anhydrous AlCL in water, it immediately becomes hydrated and from the solution, under suitable circumstances, we are able to crystallize AlCla6HzO which could be written [A1(H20)6]3+.3C1-. That is, in the crystal lattice the Al kernel is found to be m o u n d e d by 6 molecules of water and is entirely separate from the CI- ions which are located in adjacent parts of the crystal. When A1CL.6Hz0 is dissolved in water, the solution'is found to test acidic by the use of litmus or other indicators. This is explained 3+ ion is an acid and has by the idea that the [Al(H20)6] reacted as follows with water: And perhaps in a second reaction: [Al(OH)(H,O)sl2+
+ HsO F? [AI(OH)a(HzO)d'+ + HaO1+
The final solution contains, in addition to the [AI(H20)6]a + ions, [Na(H20)~]I + ions (written simply Nal+in the equations) and an equivalent number of hydrated CI1- ions.
Hydrolysis of the Chlorides of Magnesium and Silicon The magnesium ion, [Mg(HzO)s]2+, is so weak an acid that only two protons are lost even in the presence of excess of the strongly basic OH1- ion. Consequently, [Mg(HzO)4(OH)z] V s precipitated on the introduction of sodium hydroxide solution into a solution of MgCln 6Hz0, but is not redissolved on the addition of excess NaOH. SiC4, on the other hand, is completely hydrolyzed when i t comes in contact with water, due to the fact that the silicon ion, [Si(H~0)~1 4+, is distinctly acidic and loses four protons to water molecules which are sufficiently basic to take them. Addition of sodium hydroxide solution causes the disappearance of the precipitate of silicic acid in accordance with the following ionic equation: [Si(OH),I0+ 4Na1+ f 40H1- s [Si0414-+ 4 H ~ 0+ 4Nal+ Reaction of NHJ1 with Sodium Aluminate The explanation of the formation of a precipitate of [Al(OH)a(HnO)aIOon the addition of N H C l to a solntion of sodium aluminate is as follows: The NH,Cl solution contains the ions NHhl+ and Cll-, in addition to the ions H301+and OH1- which are always present to a very small extent in HzO. The NH4'+ ion is a fairly strong acid and if present in sufficient quantities can cause the precipitation of [A~(OH)~(HZO)~] by giving up protons to the aluminate iqn, [Al(OH)o]3-, ac.. cording to the equation:
This reaction does not go any farther, evidently, since [A1(OH)a(HzO)a]O,aluminum hydroxide, an insoluble substance, does not precipitate. In other words, HpO as a base is not strong enough to remove more than two protons from each molecule of the acid [A1(H,0)6]~+. The NH4" ion is not sufficiently acidic, however, to If we add NaOH, we observe the precipitation of causethe precipitate to redissdve and,form[Al(H~0)6] 8+. aluminum hydroxide, [Al(OH)a(HzO)a] and upon addition of excess NaOH, disappearance of tbe precipitate. Prevention of Precifiitation of Magnesium Hydroxide by Addition of Ammonium Chloride This is ex~lainedbv the fact that the OH1- ion is a much stronger base than the HzO molecule and can reIt is well known that addition of an aqueous solution move enough protons from the [AI(H20)6] ion first to of ammonia, NHa, will precipitate [M~(HZO)~(OH)~] O O and then, cause the precipitation of [Al(H@)a(H20)3] from solutions of magnesium salts, but that the presence upon addition of more OH1- ions, to dissolve the of ammonium chloride in the ammonia solution will preamphiprotic aluminum hydroxide precipitate. The vent the formation of the precipitate. Aqueous amhydrated aluminum ion continues to act as an acid and monia solution contains as bases ammonia molecules loses protons until it finally amves a t a structure hav- and a very small percentage of hydroxyl ions which are ing the composition [AI(OH)6]a-. formed in the reversible reaction: If hvdrochloric acid is added to the solution of sodium [NH,I1+ + [OHILNHa HsO alumiiate. which contains, thus. bvdrated Na+ ions. which we.may neglect, and W ( O ~ j 6 l ions, the pre: which proceeds to the right to only a very slight extent. cipitate [ A ~ ( H Z ~ ) ~ ( ~reappears. H ) J I ~ Upon addition ~h~~~ are, however, enough basic particles present of more hydrochloric acid, the precipitate dissolves (both N H ~ and OH'-) to remove two protons from each again, due to the formation of [Al(HzO)6]3+according of the [Mg(HaO)6]Z + ions and cause precipitation of to the following ionic equations: [Mg(&O)~(OH)z] Y Addition of ammonium chloride to the aqueous ammonia solution causes the concentra3Na1+ + [AI(OH).Je 3H801+ 3C11- ci [AI(OH)8(HnO)slo 3H20 3Na1+ 3CI1- tion of the strongly acidic ammonium ions, NHdl+,to be greatly increased. They furnish protons to any basic [AI(OH).(HBO).J~ 3H,01+ + 3CL1- F? [ A I ( H ~ o ) ~ ]+ ~ +3 ~ 1 1 -+ 3x20 ions and molecules present in the magnesium solution
"
+
+
+
+
+
+
+
and counteract the effect of the ammonia molecules. The same result would be obtained by the addition of dilute hydrochloric acid to the mixture. As a consequence the formation of the basic hydrated magnesium hydroxide is prevented.
pears in the preceding period. In the other states of oxidation, attained by a few of the rare earth elements, the differencesare greatly accentuated and rapid means of separating the rare earths are based upon such possibilities of altering the ionic potentials.
THE ACID-BASE PROPERTIES O F MANGANESE DERIVATIVES
THE ACID PROPERTIES OF STANNIC CHLORTDE
Manganese may exist if1 states of oxidation of I+, 2+, 3+, 4+, 6+, and 7+. For each of these valences, and depending upon the coiirdination number and the type of linkage involved, there is a corresponding ionic radius. In general, the higher the charge of the ion, the smaller is its radius; so with Mn2+,C.N. = 6, we have an ion of fairly low charge and a radius of 0.91 A.; with Mn3+, the radius is 0.7 A,; for Mn4+, 0.52 A.; for Mn7+, 0.46 A.g Since the radius and the charge of the Mn2+ ion are about the same as for the Zn2+ion, r = 0.74 A,, the hydrated ions, [Mn(H20)a12+and [Zn(HzO)6]Z+, are of approximately the same acidity, and, while hydrolysis occurs, it does not proceed very far. On the other hand, the [Mn(H20)a]7+ ion is so acid that all of the protons are repelled, and the structure degenerates to produce the familiar permanganate ion, [MnO4I1-. Manganese, in the oxidation state of 7+, is similar to chlorine in the same oxidation state in that it forms only cations, and it is only in this condition that manganese has any properties comparable to those of chlorine.
Stannic chloride solutions are acid. This has been ' ~ terms of the electron interpreted by G. N. L e ~ i s in donor-acceptor theory as due to the tendency of the tin atom to form a complex with a coordination number of six. Thus, according to Lewis, when stannic chloride is dissolved in water, the solution is acidic because it contains an acid substance, SnCl,. The acidity arises from the attachment of two molecules of water to the tin kernel, which dissociate to yield [SnC14(0H)z122H1+, or, by the direct addition of two hydroxyl ions, leaving 2Ha01+ions in excess in the solution. The reaction is undoubtedly not limited to these processes, and the solution probably contains other complexes, such as (SnC16)2-. The reactions of stauuic chloride with water, however, are controlled by the same forces which operate to determine the extent of hydrolysis in other instances, and the equilibria into which the sixcoordinated complex enters obey the same rules.
+
SUMMARY
It is possible that the phenomenon of hydrolysis is one which should not be discussed in elementary courses because of the difficult theoretical treatment required The ionic radii of the rare earth elements become for its adequate elucidatiqn. If it is to be introduced, progressively shorter in passing from cerium (atomic however, it can profitably be tied up with the concept number 58) to lutecium (atomic number 71) and, in the of the formation of aqua-complexes, and the influences usual state of oxidation of 3+, there is an increase in of the ionic potentials. For students in beginning acidity of the hydrated ions parallel to the decrease in courses in chemistry, it is more beneficial to give an size. The radii of the ions of the later members of the adequate representation of the facts than to insist upon group are of the same order of magnitude as that of a definite theoretical interpretation. The choice of yttrium, 0.93 A,, and precipitate with it in the so-called theory can be left to the judgment of the properly yttrium submou~,even thoudfvttrium normally ap- grounded student. THE ACID-BASE PROPERTIES O F HYDRATED RARE EARTH IONS
