Chromium thermodynamic data

The reactivity of chromium(V) and chromium(IV) species is uncertain since there are no reliable thermodynamic data, and not much can be said at present about the structure of these species. With respect to the latter some hints can be obtained from the fact that the changeover from chromium(V) to chromium(IV) or vice versa in all cases was found to be rate determining, which seems to correlate well with the conclusion of Tong and King d that Cr(VI) and Cr(V) have coordination number four, whereas Cr(IV) and Cr(III) have six. 

Prom the following thermodynamic data, with the assumptions that the heat capacities of reaction are negligible and that standard conditions (other than temperature) prevail, calculate the temperatures above which (a) carbon monoxide becomes the more stable oxide of carbon, in the presence of excess C (6) carbon is thermodynamically capable of reducing chromia (Cr2Os) to chromium metal (c) carbon might, in principle, be used to reduce rutile to titanium metal and (d) silica (taken to be a-quartz) may be reduced to silicon in a blast furnace. 

Thermodynamic data for the equilibrium Eq. (54) have been obtained by spectrophotometric and potentiometric (pH) measurements (cf. Table XXVIII). The Kt values lie in the region 1-10, the only significant exceptions being the small values for the aqua and 1,10-phenanthroline chromium(III) systems. The enthalpy changes are small, as anticipated for reactions which involve bond breaking and bond making of similar kinds of bonds. 

Crystal field theory is one of several chemical bonding models and one that is applicable solely to the transition metal and lanthanide elements. The theory, which utilizes thermodynamic data obtained from absorption bands in the visible and near-infrared regions of the electromagnetic spectrum, has met with widespread applications and successful interpretations of diverse physical and chemical properties of elements of the first transition series. These elements comprise scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel and copper. The position of the first transition series in the periodic table is shown in fig. 1.1. Transition elements constitute almost forty weight per cent, or eighteen atom per cent, of the Earth (Appendix 1) and occur in most minerals in the Crust, Mantle and Core. As a result, there are many aspects of transition metal geochemistry that are amenable to interpretation by crystal field theory. 

The standard Gibbs energies and the standard electrode potentials for the reactions (22) and (23) were calculated according to the thermodynamic data available in the literature.10 The cathodic reactions include the reduction of chromium Cr(VI) to Cr(III) and hydrogen evolution, as presented in the following equations ... 

For iron, nickel, and chromium the experimental values of passivation and the thermodynamic data in the Pourbaix diagram are quite different because additional kinetic effects determine the regions of passivity and active dissolution for these metals. For other metals, e.g., zinc and copper the correspondence between the Pourbaix diagram and regions of stability and corrosion is better. 

Experimental thermodynamic data for ternary Cr-Fe-Si alloys are limited. [1968Che] report the activity of silicon in Cr-Fe-Si melts along sections with 12, 18 and 25 % Cr at 1600°C. It was shown that the activity exhibits a negative deviation from ideal behavior. The dependence of logioTsi on chromium concentration is linear, and extrapolation of the relationship to zero chromium content yields a value of about 0.0027. 

A. Bondar, 2006. Boron-Chromium-Nickel, in G. Effenberg, ve S. Ilyenko (Eds.), Ternary Alloy System Phase Diagrams, Crystallographic and Thermodynamic Data VolllDl, 320-343. 

Chromium(ni) continues to be the second most widely studied metal ion after cobalt(m). The question of dissociative (/a) or associative (/a) mechanisms for substitutions at chromium(m) has been clarified somewhat by comparing data for the formation and dissociation of the [Cr(H20)5X] + and [Cr(NH8)5X] + ions (X = unidentate leaving anion). Recent studies indicate that an associative mechanism is important for aqua-chromium(m) complexes, but for the [Cr(NHa)6X] + ions a dissociative-interchange mechanism is favoured. A summary of kinetics and thermodynamic data for the formation and aquation of [Cr(NH3)5X] + ions is given in Table 12. 

There are two aspects to reference redox systems. One point is the possibility of compiling electrode potentials in a variety of solvents and solvent mixtures, which are not affected by unknown liquid junction potentials. Unfortunately very frequently aqueous reference electrodes are employed in electrochemical studies in nonaqueous electrolytes. Such data, however, include an unknown, irreproducible phase boundary potential. Electrode potentials of a redox couple measured in the same electrolyte together with the reference redox system constitute reproducible, thermodynamic data. In order to stop the proliferation of—in the view of the respective authors— better and better reference redox systems, the lUPAC recommended that either ferrocenium ion/ferrocene or bw(biphenyl)chromium(l)/te(biphenyl)chromium(0) be used as a reference redox system [5]. 

Actually, the drop of pH is related to more complex reactions and species. Thus, in more sophisticated models, several hydrolysis reactions and metal chloride formation are taken into account but the selection of species and reactions is somewhat different from model to model. Oldfield and Sutton [94] and Watson and Postlethwaite [2] considered only hydroxides as the product of cation hydrolysis. Sharland [96] introduced simple metallic chlorides. The most complete set of species and reactions has been used by Bernhardsson et al. [4], which made available the thermodynamic data of a large number of species, including several iron, nickel, chromium, and molybdenum polycations as well as metal chlorides and hydroxychlorides. Gartland [19] used a more limited set of species (Table 10.3) selected among the Bernhardsson data. According to their experimental results, Hebert and Alkire [95] included Al(OH) " as the hydrolysis product in their model of the crevice corrosion of aluminum alloys. 

Table 11.14 Thermodynamic data for chromium(lll) species at 25 °C and comparison with data available in the literature.

The thermodynamic data utilised for chromium metal, the chromium ion and the solid phase, Cr203(s), are listed in Table 11.17. The metal and ion data were used to derive the data listed in Table 11.14. 

Chromium compounds number in the thousands and display a wide variety of colors and forms. Examples of these compounds and the corresponding physical properties are given in Table 1. More detailed and complete information on solubiUties, including some solution freezing and boiling points, can be found in References 7—10, and 13. Data on the thermodynamic values for chromium compounds are found in References 7, 8, 10, and 13. 

Ball J. W. and Nordstrom D. K. (1998) Critical evaluation and selection of standard state thermodynamic properties for chromium metal and its aqueous ions, hydrolysis species, oxides, and hydroxides. J. Chem. Eng. Data 43, 895-918. 

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