Oxides standard free energy

We have already noted that the standard free energy change for a reaction, AG°, does not reflect the actual conditions in a ceil, where reactants and products are not at standard-state concentrations (1 M). Equation 3.12 was introduced to permit calculations of actual free energy changes under non-standard-state conditions. Similarly, standard reduction potentials for redox couples must be modified to account for the actual concentrations of the oxidized and reduced species. For any redox couple. 

In its general corrosion behaviour, beryllium exhibits characteristics very similar to those of aluminium. Like aluminium, the film-free metal is highly active and readily attacked in many environments. Beryllium oxide, however, like alumina, is, a very stable compound (standard free energy of formation = —579kJ/mol), with a bulk density of 3-025g/cm as compared with 1 -85 g/cm for the pure metal, and with a high electronic resistivity of about 10 flcm at 0°C. In fact, when formed, the oxide confers the same type of spurious nobility on beryllium as is found, for example, with aluminium, titanium and zirconium. 

Fig. 18.4 The standard free energy of formation of various oxides as a function of...

Equations for the Standard Free Energy of Formation and Partial Molar Free Energies of Atomic Oxygen for Plutonium Oxides (1600-2150 K), cal/mol... 

In the introductory chapter we stated that the formation of chemical compounds with the metal ion in a variety of formal oxidation states is a characteristic of transition metals. We also saw in Chapter 8 how we may quantify the thermodynamic stability of a coordination compound in terms of the stability constant K. It is convenient to be able to assess the relative ease by which a metal is transformed from one oxidation state to another, and you will recall that the standard electrode potential, E , is a convenient measure of this. Remember that the standard free energy change for a reaction, AG , is related both to the equilibrium constant (Eq. 9.1)... 

Fats such as palmitic acid are metabolized through pathways similar to the ones for the oxidation of glucose. The complete oxidation of palmitic acid has a standard free energy change of-9790 kJ/mol and produces 130 ATP molecules per molecule of palmitic acid consumed. You should be able to verily that this metabolic process has about the same efficiency as the oxidation of glucose. 

In 1944 Ellingham compiled, for the first time, diagrams depicting the temperature dependence of the standard free energies of formation of numerous oxides and sulphides. In the discussion presented here, attention will be confined to the oxide reaction... 

From the Van t Hoff isotherm it follows that for the oxidation reaction considered, the standard free energy change at temperature T is given by... 

When the right-hand side of the above equation is zero, i.e., when either T = 0 or P0j equals one atmosphere, AG° must be zero. The intersection of the standard free energy change versus temperature line with the temperature axis, when AG° = 0, gives the temperature at which the oxygen equilibrium pressure, P0i, is equal to one atmosphere. This temperature is known as the decomposition temperature of the oxide and is denoted as TD on line 1 in Figure 3.5. 

A standard free energy versus temperature diagram for oxides is often presented with a scale for oxygen pressures. Such a diagram is shown in Figure 3.6. This scale is designed to... 

In Figure 4.10 are shown the lines corresponding to the standard free energies of formation of a metal oxide and of carbon monoxide also shown is the line for the free energy change associated with the reaction... 

One of the important differences between calciothermic and aluminothermic reduction of oxides concerns the interaction between the reduced metal and the reductant. Calcium does not form stable solid solutions or alloys with the reduced metals calcium contamination in the metal is, therefore, relatively small. Aluminum, on the other hand, readily forms solid solutions with the reduced metals, and the product generally contains appreciable quantities of residual aluminum. This is not a serious problem because in many cases either a certain aluminum content is desired in the reduced metal or the residual aluminum can be effectively removed in post-reduction purification operations. The extent of the contamination of a reduced metal with the reductant can be related to factors such as the reaction temperature, the standard free energy change associated with the reaction, and the slag composition. Let the following generalized reaction be considered ... 

The standard free energy change for this reaction is generally positive at all temperatures because oxides are invariably stabler than chlorides. An exception to this rule occurs in the case of copper because cupric chloride is more stable than cupric oxide. At 500 °C, the standard free energy change (AG°) for the reaction... 

The chlorination of titanium dioxide (titania) is thus entirely feasible at 900 °C. Similarly, many other metal oxides can be converted to metal chlorides by reaction with chlorine in the presence of carbon. It should be noted that carbon itself is not easily chlorinated as the standard free energy of formation of carbon tetrachloride is positive at temperatures above 500 °C. 

If AG has a value which is more negative than that of AG, then AG° becomes more negative than the standard free energy change for the metal oxide chlorination reaction (AG ). An oxide which is difficult to chlorinate in the free state may, therefore, be chlorinated more easily when compounded into a silicate. 

The forward and reverse rate constants are thus equal at zero standard free energy. However, this will be difficult to check in practice, for both reactions are very slow, since a bond-breaking/bond-forming process endowed with a quite large internal reorganization is involved. The result is that dissociative electron transfer reactions are usually carried out with electron donors that have a standard potential largely negative to the dissociative standard potential. The reoxidation of the R, X- system is thus possible only with electron acceptors, D +, that are different from the D,+ produced in the reduction process (they are more powerful oxidants). There is no reason then that the oxidation mechanism be the reverse of the... 

A more quantitative indication of the tendency of these elements to react with oxygen may be obtained from Fig. 6.9, where, for several metals, the standard free energies of formation of their oxides are shown. The large negative values relevant to these oxides may be noticed. 

The [oxidized state] and [reduced state] are, respectively, the various items in the right and left of the reactions like Eqs. (2-la) to (2-4a). AG° can be calculated using various standard free energy dates reported fi-om literature (Vanghan and Craig, 1978 Garrels and Christ, 1965 Wang and Hu, 1989). 

As discussed earlier, the dependence of pe on the concentrations of reductants and oxidants is often small in comparison with its dependence on pH. The term in the square brackets in Equation (4.25) can therefore be replaced by pe terms giving for the approximate standard free energy change ... 

Tab. 8.1 Standard free energies, enthalpies and entropies of formation of the iron oxides at 0.1 MPa and 298 K...

Biochemical reactions are basically the same as other chemical organic reactions with their thermodynamic and mechanistic characteristics, but they have the enzyme stage. Laws of thermodynamics, standard energy status and standard free energy change, reduction-oxidation (redox) and electrochemical potential equations are applicable to these reactions. Enzymes catalyse reactions and induce them to be much faster . Enzymes are classified by international... 

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