Carbon free energy change

Let us now consider the reduction of a metal oxide by carbon which is itself oxidised to carbon monoxide. The reaction will become energetically feasible when the free energy change for the combined process is negative (see also Figure i.i). Free energies. 

The fixation of carbon dioxide to form hexose, the dark reactions of photosynthesis, requires considerable energy. The overall stoichiometry of this process (Eq. 22.3) involves 12 NADPH and 18 ATP. To generate 12 equivalents of NADPH necessitates the consumption of 48 Einsteins of light, minimally 170 kj each. However, if the preceding ratio of l ATP per NADPH were correct, insufficient ATP for COg fixation would be produced. Six additional Einsteins would provide the necessary two additional ATP. Prom 54 Einsteins, or 9180 kJ, one mole of hexose would be synthesized. The standard free energy change, AG°, for hexose formation from carbon dioxide and water (the exact reverse of cellular respiration) is +2870 kj/mol. 

The salt is a colorless crystalline solid which is virtually insoluble in all common organic solvents. It reacts slowly with chloroform and carbon tetrachloride to give thallium(I) chloride 25), gives a characteristic red coloration with carbon disulfide, and undergoes the Diels-Alder reaction with maleic anhydride 110). It is rapidly decomposed by acids, but is stable to water this latter fact has been interpreted (55) in terms of the small free energy change for the reaction... 

Table 3 Free energy changes in the ionic dissociation of carbon-carbon cr bonds in some hydrocarbons at 25°C. ...

C20-0065. Calculate the standard free energy change at 25 °C for reduction of ZnO to Zn using carbon and using carbon monoxide. 

Figure 8.1. Free energy change for steam reforming and related reactions, including those leading to deposition of carbon on the catalyst. The plot clearly illustrates why steam reforming needs to be carried out at high temperatures.

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... 

In this, to the free energy change for the reaction involving the interaction of chlorine and the metal oxide, is added the large value of the free energy of formation of carbon dioxide from its constituent elements. 

At 800 °C, the standard free energy change for the reaction is -45.60 kj. Carbon for reduction and chlorine for chlorination are provided by certain compounds like carbon tetrachloride, and these may be used. Using carbon tetrachloride, the chlorination may be conducted at a lower temperature (650-700 °C) according to the reaction ... 

According to the free energy change associated with the pertinent reaction, nickel will form nickel tetracarbonyl at low temperatures, and this carbonyl will become unstable and revert back to nickel and carbon monoxide at moderate temperatures. The Mond process for refining nickel is based on these features. In this process, impure nickel is exposed to carbon monoxide gas at 50 °C, whereby volatile nickel tetracarbonyl (Ni(CO)4) forms. No impurity present in the crude nickel reacts with carbon monoxide. Since formation of the... 

If the combination of oxygen and carbon at the anode is included in the electrochemical reaction then the free energy change is much smaller. For the reaction... 

Warshel is to utilize a formula identical to (11.22) in this chapter to compute the free energy change. They employed an empirical valence bond (EVB, below) approach to approximately model electronic effects, and the calculations included the full experimental structure of carbonic anhydrase. An H/D isotope effect of 3.9 1.0 was obtained in the calculation, which compared favorably with the experimental value of 3.8. This benchmark calculation gives optimism that quantum effects on free energies can be realistically modeled for complex biochemical systems. 

Thermodynamically, since the free-energy change has a positive value it can only proceed when energy is supplied from the sun. The photosynthetic process occurs on a vast scale, fixing 2 x 1011 tonnes per annum of carbon as carbohydrate, representing 10 times the energy consumption of the entire human race. 

Another three-carbon compound, 1,3-bisphospho-glycerate (Fig. 13—4), contains an anhydride bond between the carboxyl group at C-l and phosphoric acid. Hydrolysis of this acyl phosphate is accompanied by a large, negative, standard free-energy change (AG ° =... 

The free energy change, when all the reactants and products are in their standard states (1 M oxaloacetate dianion and pyruvate anion, 10-7 m hydrogen ion, and 1 atm C02), is —7.4 kcal/mole. The negative value of AG° means that the reaction proceeds spontaneously under these conditions. However, some of the concentrations are not very realistic. At pH 7, carbon dioxide is present partly in the form of the bicarbonate anion, rather than as gaseous C02. To take this into account, we can add the standard free energy change... 

To illustrate, let s calculate the standard free-energy change for the reaction in Worked Example 17.6—the reduction of iron(III) oxide with carbon monoxide ... 

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