Oxidation reactions free energy

The ease with which this exchange can occur is given by the free energy of reaction, AG, which is the summation of the free energy of formation for the individual oxides. 

FIGURE 5.3 Standard free energy of reaction as a function of temperature —equilibrium gas pressure above oxide/carbonate or oxide/hydroxide. Data from Kingery et al. [3], with additions by T. A. Ring. 

This and the standard free energy make up the total free energy of reaction as shown in equation (5.6). In a gas mixture where Pm — 0.79 atm and the rest is an inert gas, all the metal nitrides are stable with respect to their metals, except Fe above 250 K and Cr above 1325 K. In air where Pn = 0.79 atm, this result is not true because tiie metals may also oxidize. Due to the presence of oi rgen in air, we must also consider the oxidation reactions at tiie same time as tiie nitridation reactions. This is done in the next section. 

The energy factor (enthalpy) and the probability factor (entropy) in chemical reactions. The driving force of chemical reactions—free energy. Oxidation-reduction potentials and their uses. 

The free energy of reaction (a) is favorable, and the starting materials are easily prepared. However, high purity is not achieved because fluorides often retain water or oxygen that remains in the product, and because slags such as Cap2 are admixed with the product and must be removed mechanically. In research-scale reductions, high purity fluoride [from oxide treated with HF(g)] and Ca can achieve > 99.9% pure lanthanoid metals, if the metals are further purified by vacuum fusion or distillation -. ... 

Carbides can be converted to oxides and reduced as above. Metal alloy fuels are already in suitable form and can be processed directly. Calcium is used as the reducing agent as the free energy of Reaction 1 is -6.6 kcal/mole at 1000°C. The other fuel and fission product oxides are reduced more easily than Th02, thus the reduction should be complete once the thoria is reduced. 

Manganese(III,IV) oxides are reduced by phenolic compounds an order of magnitude more quickly than Co(III) oxides, and several orders of magnitude more quickly than Fe(III) oxides. This apparent relationship between reaction-free energy and reaction rate is not likely to arise from differences in adsorption phenomena alone. Instead, it probably arises from differences in electron-transfer rate within the surface precursor complex. 

The reverse of this free-energy consuming reaction is the catalytic decomposition of hydrogen peroxide. It is also an acceptor reaction (6) and should also be subject to n-photo-catalysis. Indeed, zinc oxide catalyzes neither the decomposition nor the synthesis of hydrogen peroxide in the dark. However, on illumination with visible light, both reactions begin at once and proceed towards a common steady state. 

Figure 3. Gibbs free energies for reactions involving conversion of fluorides to oxides as a function of temperature for two different ratios of P) f/Ph,0 hf h,o ...

McNeil and Odom [16] developed a thermodynamic model to predict metal susceptibility to MIC by 8RB. If the reaction to produce the sulfide from the oxide has a negative Gibbs free energy, the reaction will take place. If the value is positive, the metal is immune to derivation by sulfides and will not be vulnerable to corrosion by 8RB. The model is limited to thermodynamic predictions as to whether a reaction will take place and does not consider metal toxicity to the organisms, tenacity of the resulting sulfide or others factors that influence corrosion rate. The following is a summary of mineralogical products... 

The free energies of reaction and activation have been calculated by the semiempirical molecular orbital method for the oxidation of ammonia and mono-, di- and trimethylamine by singlet and triplet oxygen atoms as models for oxidation by cytochrome P-450 [56]. For the non-radical oxidation (closed-shell path), the results indicate a two-step addition-rearrangement mechanism leading to both N-hydroxy and N-methoxy products via N-oxide intermediates. In the triplet path both a-C- and N-oxidation are competitive. N-oxidation via an addition mechanism seems to be favored over the H-abstraction mechanism, but no stable N-oxide radical intermediate is found on the triplet surface. 

A graphical representation of the standard free energy often proves useful. Figure 2.2 shows the evolution of the free energy of reaction as a function of temperature for the formation of a number of oxides. For the sake of comparison, the stoichiometries of the equations are expressed relative to a single mole of oxygen. Thus, for the formation of the monovalent (M2O), bivalent (MO) and trivalent oxides (M2O3) ... 

Figure 2.8 Standard free energy of reaction as a function of temperature. The dashed lines are the equilibrium gas pressure above the oxide/carbonate and oxide/hydroxide. (From Ref. 25.)...

Table 1 Oxidation potentials E° values computed from Gibbs free energy of reaction, AGf, which is related to voltage via relation AG° = nFE° AG° computed from thermochemical data °...

One can malhematically demonstrate that for any direct anodic oxidation reaction for any fuel ceU or hybrid system containing any fuel cell at any operating temperature and any pressure, the reversible work, Weiectricai. (J/mole) is equal to the change in Gibbs free energy of reaction at the standard state (STP), AG" [38, 39]. 

Again an outer-sphere process is considered operative and the difference of 10.6 kcal mol" in the free energy of reaction derives from a lower and more positive AS value for Am i as the oxidant. 

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