Sulfur free energy change

In order to appreciate the relationship between the free energy change of a chemical reaction and the cell emf, a reaction such as that of zinc metal with dilute sulfuric acid is considered ... 

The sulfur compounds RSH and disulfides are highly reactive, but RS and RSR are not. Nitro compounds usually react at diffusion-controlled rates. Aromatic compounds also fit into the Hammett (1940) equation when log k is plotted against free-energy change due to polar effects log(k/kg) = ap. Anbar... 

Figure 11.18. The correlation between rates and overall free energy change for the oxidation of a series of phenanthroline complexes of Fe(III) by Ce(IV) in sulfuric acid (open circles) and Ce(IV) in perchloric acid (closed circles). (Adapted from Shriver et al., 1990.)...

The E.M.F. of a lead storage battery containing 2.75 molal sulfuric acid was found to be 2.005 volt at 25 C. The aqueous vapor pressure of the acid solution at this temperature is about 20.4 mm., while that of pure water is 23.8 mm. The mean ionic activity coefficient of the sulfuric acid is 0.136. Calculate the standard free energy change of the cell reaction at 25 C and check the values from tabulated free energy data. 

The group of microorganisms known as autotrophic bacteria obtain energy from the oxidation of a variety of inorganic compounds (ammonia, nitrite, sulfide, ferrous iron, hydrogen, carbon monoxide, sulfur, thiosulfate).The free energy changes and over-all efficiencies of a number of these processes have been calculated. We may take as an example the oxidation of nitrite to nitrate performed by Nitrobacter... 

Cyanide and sulfite ions dissolve elemental sulfur to form thiocyanate and thiosulfate. The free energy change is favorable as E decreases and H decreases. 

It will be observed that according to the suggested cell reaction, two molecules of sulfuric acid should be removed from the electrolyte and two molecules of water formed for the discharge of two faradays of electricity from the charged cell. This expectation has been confirmed experimentally. Further, it is possible to calculate the free energy of this change thermodynamically in terms of the aqueous vapor pressure of sulfuric acid solutions the values should be equal to — 2FEy where E is the E.M.F. of the cell and this has been found to be the case. 

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