Thermodynamics, Metallurgy, and Coordination Chemistry
Some basic concepts of thermodynamics help us in understanding the theory of metallurgical transformations. Gibbs energy is the most significant term here. The change in Gibbs energy, ΔG for any process at any specified temperature, is described by the equation: ΔG = ΔH – TΔS
where, ΔH is the enthalpy change and ΔS is the entropy change for the process. For any reaction, this change could also be explained through the equation: ΔG = – RTlnK
where, K is the equilibrium constant of the ‘reactant – product’ system at the temperature, T. A negative ΔG implies a +ve K in equation. This can happen only when the reaction proceeds towards products. From these facts, we can make the following conclusions:
- When the value of ΔG is negative in the equation, only then will the reaction proceed. If ΔS is positive, on increasing the temperature (T), the value of TΔS would increase.
- If reactants and products of two reactions are put together in a system and the net ΔG of the two possible reactions is –ve, the overall reaction will occur. So the process of interpretation involves coupling of the two reactions, getting the sum of their ΔG and looking for its magnitude and sign. Such coupling is easily understood through Gibbs energy (ΔG) vs T plots for the formation of the oxides (thermodynamic principle of metallurgy).
Iron, Steel, and Other Metallurgical Applications
Iron and steels are the most important construction materials. Their production is based on the reduction of iron oxides, the removal of impurities, and the addition of carbon and alloying metals such as Cr, Mn, and Ni. Some compounds are manufactured for special purposes such as TiO for the pigment industry and MnO2 for use in dry battery cells. The battery industry also requires Zn and Ni/Cd. The elements of Group 11 are still worthy of being called the coinage metals, although Ag and Au are restricted to collection items, and the contemporary UK ‘copper’ coins are copper-coated steel. The ‘silver’ UK coins are a Cu/Ni alloy. Many of the metals and/or their compounds are essential catalysts in the chemical industry. V2O5 catalyses the oxidation of SO2 in the manufacture of sulphuric acid. TiCl4 with A1(CH3)3 forms the basis of the Ziegler catalysts used to manufacture polyethylene (polythene). Iron catalysts are used in the Haber process for the production of ammonia from N2/H2 mixtures. Nickel catalysts enable the hydrogenation of fats to proceed. In the Wacker process, the oxidation of ethyne to ethanal is catalysed by PdCl2. Nickel complexes are useful in the polymerisation of alkynes and other organic compounds such as benzene. The photographic industry relies on the special light-sensitive properties of AgBr.
Coordination Compounds: Bonding Theories
Werner was the first to describe the bonding features in coordination compounds. But his theory could not answer basic questions like:
- Why only certain elements possess the remarkable property of forming coordination compounds?
- Why do the bonds in coordination compounds have directional properties?
- Why do coordination compounds have characteristic magnetic and optical properties?
Many approaches have been put forth to explain the nature of bonding in coordination compounds viz. Valence Bond Theory (VBT), Crystal Field Theory (CFT), Ligand Field Theory (LFT) and Molecular Orbital Theory (MOT). We shall focus our attention on elementary treatment of the application of VBT and CFT to coordination compounds.
Bonding in Metal Carbonyls
The homoleptic carbonyls (compounds containing carbonyl ligands only) are formed by most of the transition metals. These carbonyls have simple, well-defined structures. Tetracarbonylnickel(0) is tetrahedral, pentacarbonyliron(0) is trigonal bipyramidal, while hexacarbonyl chromium(0) is octahedral.