The Lithium Group

The elements which constitute Group 1 of the Periodic Table are known as the alkali metals. They are lithium Li, sodium Na, potassium K, rubidium Rb, cesium Cs, and francium Fr. (Sometimes the NH4+ ion is included among these since it resembles K+ or Rb+ in many of its reactions.) All six of the elements have atoms characterized by an outer electron structure of ns1 with n representing the principal quantum number. The elements exhibit marked resemblances to each other with Li deviating the most. This deviation is assignable to the small size of Li which causes the positive charge of Li+ to be concentrated, that is, the charge density is high. All of the elements exhibit oxidation numbers of 0 and I, with exceptions being rare, such that their chemistries are dominated by the oxidation state I. The six metals are exceptionally reactive, being strong reductants, reacting with HOH at all pH values to give H2 and M+, and having hydroxides MOH which are strong and soluble. Ionic sizes in pm for the members of the group are as follows: Li (76), Na (102), K (139), Rb (152), Cs (167), and Fr (180). The E° values for the M+/M couples are as follows: Li (−3.04 v), Na (−2.71 v), K (−2.93 v), Rb (−2.92 v), Cs (−2.92 v), and Fr (about −3.03 v). a. E–pH diagram. The E–pH diagram for 10−1.0 M Li is presented in Figure 5.1. The figure legend provides an equation for the line that separates Li+ and Li. The horizontal line appears at an E value of −3.10 v. Considerably above the Li+/Li line, the HOH ≡ H+/H2 line appears, which indicates that Li metal is unstable in HOH, reacting with it to produce H2 and Li+. Note further that Li+ dominates the diagram reflecting that the aqueous chemistry of Li is largely that of the ion Li+.

The Ti Group and the 5B, 6B, 7B and 8B Heavy Elements

The elements to be treated in this chapter may be considered to be of three types. All of them show one species which dominates the water domain in the E–pH diagram. The dominant species in the E–pH diagrams and the elements which display it are as follows: (1) an insoluble oxide: Ti, Zr, Hf (Group 4B) and Nb, Ta (Group 5B), (2) a high-oxidation-state anion: Mo, W (Group 6B) and Tc, Re (Group 7B), (3) a noble metal: Ru, Rh, Pd, Os, Ir, Pt (Group 8B). These five elements all show highly stable inert oxides which occupy the majority of the water domain in their E–pH diagrams. This can be seen in Figures 13.1 through 13.5. The three 4B oxides (TiO2, ZrO2, HfO2) are insoluble in HOH, dilute acids, dilute bases, and concentrated bases, but are soluble in strong concentrated acids to give TiO+2, ZrO+2, and HfO+2. The two 5B oxides (Nb2O5, Ta2O5) are insoluble in HOH, dilute acids, and dilute bases, but Nb2O5 dissolves in concentrated bases whereas Ta2O5 does not. All the elements in their highest oxidation state are hard cations and therefore will be particularly attracted to the hard atoms F and O. a. E–pH diagram. The E–pH diagram in Figure 13.1 shows Ti in oxidation states of 0, II, III, and IV. In the legend of the diagram, equations for the lines between the species are presented. Table 13.1 displays ions and compounds of Ti. The metal appears to be very active, but a thin refractory oxide coating renders it inactive to all but extreme treatment. Ions and compounds in oxidation states of II and III are unstable with regard to atmospheric O2 and also with regard to HOH except for Ti+3 in strongly acidic solution. b. Discovery, occurrence, and extraction. Ti, named after the Titans, the mythological first sons of the earth, was discovered by Gregor in 1791 in the mineral menachanite, a variety of ilmenite. The major sources of Ti are the minerals rutile TiO2 and ilmenite FeTiO3. They are treated with Cl2 and C at elevated temperatures to generate gaseous TiCl4 which condenses to a colorless liquid at 136°C.

The Scandium Group

The members of the Sc Group are Sc scandium, Y yttrium, La lanthanum, Ce cerium, Pr praseodymium, Nd neodymium, Pm promethium, Sm samarium, Eu europium, Gd gadolinium, Tb terbium, Dy dysprosium, Ho holmium, Er erbium, Tm thulium, Yb ytterbium, Lu lutetium, and Ac actinium. All these elements resemble each other greatly, especially in the series La–Lu (called the lanthanoids). Their slight differences may be assigned largely to size similarities, but a few oxidation state changes give rise to marked differences. The predominant oxidation state is III, but the IV state for Ce, and the II state for Eu are also important in their aqueous chemistries. The electron structures of these elements along with some other of their pertinent properties are shown in Table 12.1. Note the progression in the sizes of M+3 rising from Sc to Ac, but decreasing from La to Lu. This behavior causes Y+3 to fall in between Dy+3 and Ho+3, which results in yttrium’s chemistry usually resembling the latter lanthanoids. For this reason, Y will be treated as a lanthanoid in succeeding sections. The successive filling of the 4f electron level from La through Lu should also be noted, as well as the interesting 5d occupancy for Gd. The richest ore of Sc is the rare mineral thorveitite Sc2Si2O7, but it also occurs in very small quantities in some lanthanoid, uranium, and tungsten ores. Yttrium and the lanthanoids (abbreviated Ln), except for Pm, occur in monazite LnPO4 (mostly light lanthanoids), bastnaesite LnCO3F (mostly light lanthanoids), xenotime LnPO4 (mostly heavy lanthanoids), loparite (mostly light lanthanoids), and lateritic clays (some with mostly light lanthanoids, others with mostly heavy lanthanoids). All isotopes of Pm are radioactive and it does not occur with the lanthanoids. Exceedingly small amounts are present in uranium ores where it has been produced by the spontaneous fission of U-238. Its major source is artificial production, the longest lived isotope being Pm-145 (half life of 17.7 years). Ac is also without a stable isotope, the radioactive element resulting from the decay of naturally occurring Th and U. The longest lived Ac species is Ac-227 which has a half life of 21.77 years.

The Nitrogen Group

The Nitrogen Group of the Periodic Table contains the elements nitrogen N, phosphorus P, arsenic As, antimony Sb, and bismuth Bi. The outer electron structure ns2np3 characterizes all five of the elements, with n representing principal quantum numbers 2, 3, 4, 5, and 6, respectively. The ns2np3 indicates the possibility of oxidation states V, III, and -III. As one goes down the group, the metallic character increases, with N and P being distinctly non-metals, As a metalloid, and Sb and Bi metals. However, the major bonding in most of the compounds of the group is covalent, aqueous cationic species being formed only by Sb and Bi. A covalency of 5 is exhibited by all the elements except N, this being assignable to the considerable energy required to place 10 electrons around the atom. The pentavalent state is the most stable for P, with its stability falling off down the group, as the trivalent state stability increases. Covalent radii in pm are as follows: N (75), P (110), As(122), and Sb(143). Ionic radii (most hypothetical) in pm are these: Sb+3 (90), Sb+5 (74), Bi+3 (117), and Bi+5 (90). a. E–pH diagram. Figure 9.1 depicts the E–pH diagram for N with the soluble species (except H+) at 10−1.0 M. Equations for the lines that separate the species are displayed in the legend. The colorless strong acid nitric acid HNO3, its colorless anion nitrate NO3−, the colorless weak acid nitrous acid HNO2, its colorless anion NO2−, the colorless ammonium ion NH4+, and the colorless hypothetical compound ammonium hydroxide NH4OH are involved.

The Beryllium Group

The elements which constitute the Be Group of the Periodic Table are known as the alkaline earths. They are beryllium Be, magnesium Mg, calcium Ca, strontium Sr, barium Ba, and radium Ra. All six of the elements have atoms characterized by an outer electron structure of ns2 with n representing the principal quantum number. The elements exhibit marked resemblances to each other with Be differing considerably. This deviation is assignable to the small size of Be which causes the positive charge of Be+2 to be concentrated, that is, the charge density is high. The higher charge-density ions attack HOH to attach OH− and to liberate H+, that is, they hydrolyze readily. All of the elements exhibit oxidation numbers of 0 and II, with their chemistries being dominated by the oxidation state II. The six metals are exceptionally reactive, being strong reductants, reacting with acids, HOH, and bases at all pH values to give H2. The other product of such reactions are M+2 ions and M(OH)2, the ions being present at lower pH values and the hydroxides being present at higher pH values. The transition from M+2 to M(OH)2 occurs at increasing pH values from Be to Ra, such that the hydroxide of Sr is slightly soluble and those of Ba and Ra are soluble. These soluble hydroxides are strong bases. Ionic sizes in pm for the members of the group are as follows: Be (59), Mg (72), Ca (100), Sr (132), Ba (135), and Ra (148). The E° values for the M+2/M couples are as follows: Be (−1.97 v), Mg (−2.36 v), Ca (−2.87 v), Sr (−2.89 v), Ba (−2.91 v), and Ra (−2.91 v), indicating that they are very reactive metals. a. E–pH diagram. The E–pH diagram for 10−1.0 M Be is presented in Figure 6.1. In the figure legend are equations which describe the lines separating the species. It can be seen that Be is thermodynamically unstable with respect to H+, HOH, and OH−. However, Be is relatively inactive in the middle pH range due to a protective oxide coat. To dissolve Be, dilute acids such as HCl or H2SO4, or bases such as NaOH or KOH are required.

The Construction of E–pH Diagrams

In order to construct an E–pH diagram one needs to follow eight basic steps: (1) Select the species of the element involved which contain one or more of the following entities: the element, oxygen, and hydrogen. This is best done by reading the descriptive chemistry of the element in a good inorganic text and identifying the species, both soluble and insoluble, which persist, at least for several minutes, in aqueous solution. (2) Starting at the lower left-hand corner of an E–pH framework, arrange the selected species in vertical order of increasing oxidation number of the element. Then, if there are different species with the same oxidation number, arrange them in horizontal order of decreasing protonation (increasing hydroxylation). If there is only one species of a given oxidation number, this species extends across the entire pH range for the purposes of diagram construction. (3) Draw in border lines between the species, that is, the lines representing the transformation of a species to another species. You will not know exactly where these lines occur but the approximate regions are sufficient for the purposes of diagram construction. (4) Write equations for the transformations that have been indicated. Some of them will involve electrons and therefore will be half-reactions. Such equations must always be written as reductions, that is, with the electrons on the left. In addition, no reaction should contain the OH− ion; only the H+ and/or HOH instead. (5) From appropriate tabulations, obtain the standard free energy values (ΔG° in kJ/mole) of every species in the equations. These ΔG° values are to be employed in the following relationship which applies to each of the above equations. . . . ΔG° (reaction) = ∑ΔG° (products) − ∑ΔG° (reactants) (6) . . . (6) The ΔG° (reaction) values for each equation are to be converted into E° values for those equations containing electrons and into K values for those equations which do not. This is done by use of the following expressions: . . . E° = ΔG° /−96.49n log K = ΔG° /−5.7 (7/8) . . . where n represents the number of electrons in an equation.

E–pH Diagrams

This volume is intended to employ E–pH diagrams to describe the inorganic solution chemistry of the chemical elements. Such diagrams are very useful in numerous fields of investigation, including electrochemistry, analytical chemistry, inorganic chemistry, geochemistry, environmental chemistry, corrosion chemistry, hydrometallurgy, water chemistry, agricultural chemistry, toxicology, biochemistry, chemical engineering, materials science, health physics, and nutrition. It is assumed that the reader is acquainted with the following major topics which are treated in elementary chemistry: stoichiometry, equilibrium, acid–base phenomena, solubility, complexation, elementary thermodynamics, and electrochemistry. In 1923, W. M. Clark and B. Cohen published a paper in which they introduced the idea of plotting the electromotive force as referred to the hydrogen electrode E against the pH for several chemical systems. In 1928, Clark continued to develop this graphical presentation in his text on the determination of pH. The utility of the method was further extended by numerous other investigators such as M. Pourbaix, G. Valensi, G. Charlot, T. P. Hoar, R. M. Garrels, N. de Zoubov, J. Van Muylder, E. Deltombe, C. Vanleugenhaghe, J. Schmets, M. Maraghini, P. Van Rysselberghe, A. Moussard, J. Brenet, F. Jolas, K. Schwabe, J. Besson, W. Kunz, A. L. Pitman, J. N. Butler, P. Delahay, H. Freiser, H. A. Laitinen, L. G. Sillen, P. L. Cloke, and others. In 1963, M. Pourbaix in collaboration with N. de Zoubov published Atlas d’equilibres electrochimiques, a collection of E–pH diagrams for 90 chemical elements. This volume was translated into English in 1966 by J. A. Franklin and published as Atlas of Electrochemical Equilibria in Aqueous Solutions. Subsequently other investigators published computer programs for constructing the diagrams: L. Santoma; B. G. Williams, and W. H. Patrick; P. B. Linkson, B. D. Phillips, and C. D. Rowles; K. Osseo-Asare, A. W. Asihene, T. Xue, and V. S. T. Ciminellie; D. R. Drewes; M. Mao and E. Peters; H-H. Huang and C. A. Young; J. P. Birk and Laura L. Tayer; G. P. Glasby and H. D. Schulz; and Q. Feng, Y. Ma, and Y. Lu.

The Actinoid Metals

The elements making up the Actinoid Metals are those with atomic numbers from 89 through 103: Ac, Th, Pa, U, Np, Pu, Am, Cm, Bk, Cf, Es, Fm, Md, No, and Lr. The name is meant to parallel the lanthanoids. They are generally abbreviated as An. Their valence electron structures are 7s26d0−25f0−14. These elements resemble the lanthanoids somewhat, but they have a much wider variation in oxidation states. Nor do they resemble each other to the extent that the lanthanoids do, this being a result of the oxidation state variations. Ac resembles La greatly, but Th, Pa, and U resemble their vertical congeners (Hf, Ta, W) more than they resemble Ce, Pr, and Nd. From Np onwards, the resemblance to the lanthanoids increases such that by Am, the actinoid elements are behaving very similarly, showing a predominant oxidation state of III. All of this occurs because the 7s, 6d, and 5f levels are much closer in energy than the 6s, 5d, and 4f levels. Table 18.1 lists the actinoids with several of their pertinent characteristics. No stable isotopes of any of these elements exist, the last element in the Periodic Table with a stable isotope being Bi (Bi-209). However, some of the An elements have isotopes with very long half lives, which means that they are found in nature in relative abundance, most notably as Th-232 (1010.1 years), U-235 (108.8 years), and U-238 (109.7 years). Others are products of the decay of the above isotopes, so even though they are shorter lived, they persist in nature since they are continually being produced. The most important nuclides of this type are Ac-227 (21.8 years) and Pa-231 (104.5 years), both coming from U-235 decay. In U ores, very small amounts of Np-237 (106.3 years), Np-239 (2.4 days), and Pu-239(104.3 years) arise from the interaction of neutrons with U isotopes. Isotopes of the elements beyond U are produced artificially, Np and Pu by neutron capture by U, Am and Cm by multiple neutron capture by Pu, and elements beyond Cm by further neutron captures or bombardment of lower atomic number actinoids with ions of He, B, C, N, or O.

The Cu Group

The elements of this group, copper Cu, silver Ag, and gold Au, often called the coinage metals, resemble each other in some ways, particularly their tendency to nobility, but it cannot be said that the properties of Ag are intermediate between those of Cu and Au. Even though the d shell is full, the d electrons are active, particularly in Cu and Au. The most stable oxidation states are II for Cu, I for Ag, and III for Au. For Cu, Cu(I) as the simple ion Cu+ disproportionates in HOH, and Cu(III) is so powerfully oxidizing that it is reduced by HOH. Stability may be brought to Cu(I) and Cu(III) only by complexation or insolubility. For Ag, Ag(II) and Ag(III) are reduced by HOH, stability resulting only by forming complex species or insoluble compounds. For Au, the simple Au+ cation disproportionates in HOH, and Au(II) is not known. a. E–pH diagram. Figure 16.1 sets out the E–pH diagram for Cu at a soluble species concentration of 10−1.0 M. It is assumed that there is no complexing agent or any insoluble compound producing agent other than OH− or HOH. Further, almost all species are being considered in their hydrated forms, that is, the forms that they take in the presence of HOH. Oxidation states of 0, I, and II are present. The reddish-orange Cu is a fairly noble metal, and the sole Cu(I) compound is shown as yellow Cu2O since CuOH is unstable. The Cu+ ion does not appear, even though it has been entered in the construction of the diagram. This reflects its strong tendency to disproportionate into Cu(II) and Cu, as predicted by ΔG˚ values. The compound which results when OH− is added to a blue Cu+2 solution is blue Cu(OH)2, not black CuO. Cu+2 is more properly written as Cu(HOH)6+2, and just off to the left of the Cu+2/Cu(OH)2 line, hydrolyzed species like Cu2(OH)2+2 occur. The legend of the figure shows equations for the lines separating the species.

The V–Cr–Mn Group

The three elements to be treated in this chapter (V, Cr, Mn) are the third, fourth, and fifth members of the first transition series. The first two members (Sc, Ti) have been treated in previous chapters (Chapters 12 and 13). The ten elements of this first transition series (Sc through Zn) are characterized by electron activity in the 3d–4s levels. All elements in the 3d transition series are metals, and many of their compounds tend to be colored as a result of unpaired electrons. Most of the elements have a strong tendency to form complex ions due to participation of the d electrons in bonding. Since both the 4s and the 3d electrons are active, most of the elements show a considerable variety of oxidation states (Sc and Zn being exceptions). For the first five (Sc through Mn), the maximum oxidation number is the total number of electrons in the 4s and 3d levels. Complexing is often so strong that the most stable oxidation state for simple compounds may differ from that for complex compounds. a. E–pH diagram. The E–pH diagram in Figure 14.1 shows V in oxidation states of 0, II, III, IV, and V. This diagram, which involves vanadium at 10−3.0 M is somewhat oversimplified in that there are some isopolyanions present in the 4–6 pH regions. The prevalence of isopolyanions increases as the V concentration increases. This is illustrated in Figure 14.2 which has V at 10−1.0 M. Further, the cations V+2, V+3, VO+2, and VO2+ are probably aquated to satisfy a coordination number of six, and the V(OH)3 may actually be hydrated V2O3. Note that the soluble solution chemistries of V(IV) and V(V) are dominated by the VO+2 and VO2+ complex ions. Three of these cations (III, IV, V) are subject to hydrolysis, the processes setting in around pH values of just under 3, 3, and 2. The E–pH diagram indicates that elemental V is very active, but a thin coat of oxide protects it from all except strong action.