Sulfur dioxide equilibrium reaction with oxygen

Sulfur dioxide, SOj, reacts with oxygen in the presence of a catalyst (vanadium(v) oxide) to form sulfur trioxide. This reaction is carried out in a sealed container of volume S.Odm by mixing 2.0mol of sulfur dioxide and 1.4 mol of oxygen and allowing equilibrium to be established. A conversion rate of 15% is achieved at 700 K. Calculate the equilibrium constant at this temperature for this reaction. 

From the principles of thermodynamics and certain thermodynamic data the maximum extent to which a chemical reaction can proceed may be calculated. For example, at 1 atm pressure and a temperature of 680°C, starting with 1 mole of sulfur dioxide and mole of oxygen, 50% of the sulfur dioxide can be converted to sulfur trioxide. Such thermodynamic calculations result in maximum values for the conversion of a chemical reaction, since they are correct only for equilibrium conditions, conditions such that there is no further tendency for change with respect to time. It follows that the net rate of a chemical reaction must be zero at this equilibrium point. Thus a plot of reaction rate [for example, in units of g moles product/(sec) (unit volume reaction mixture)] vs time would always approach zero as the time approached infinity. Such a situation is depicted in curve A of Fig. 1-1, where the rate approaches zero asymptotically. Of course, for some cases equilibrium may be reached more rapidly, so that the rate becomes almost zero at a finite time, as illustrated by curve B. 

Exothermic reactions with a decrease in entropy reach equilibrium (AG = 0) at some temperature and reverse beyond this point. This is evident from Eq. (4.2) where the negative term AH will cancel with the positive term TAS when T gets sufficiently large. Since we already noted that such reactions are common in the chemical industry, should we expect most reactions to be reversible In principle, yes, but in practice we operate many reactors at a temperature far below the equilibrium point and therefore never notice any influence of the reverse reaction. There are, however, industrially important exceptions to this rule. The manufacture of ammonia from nitrogen and hydrogen and the formation of sulfur trioxide from sulfur dioxide and oxygen are two prominent cases. 

The position of equilibrium in a reversible reaction is not changed by the presence of the catalyst. This conclusion has been verified experimentally in several instances. For example, the oxidation of sulfur dioxide by oxygen has been studied with three catalysts platinum, ferric oxide, and vanadium pentoxide. In all three cases the equilibrium compositions were the same. 

Sulfur dioxide, SO2, one of the intermediates in the production of sulfuric acid, can be made from the reaction of hydrogen sulfide gas with oxygen gas. Write the equilibrium constant expressions for Kc and Kp for the following equation for this reaction. 

In the conventional plant sulfur dioxide is converted to sulfur trioxide in a series of three or four catalyst beds with cooling between the beds to remove the heat of reaction. The overall conversion is limited by the equilibrium for the relative partial pressures of sulfur dioxide, sulfur trioxide, and oxygen and the temperature of the converter exit gas. This equilibrium is equivalent to about 98.5% conversion. 

Another characteristic of 02 is its ability to act as a moderate one-electron reducing agent. For example, combination of 02 with 3,5-di-r-butylquinone (DTBQ) in DMF yields the semiquinone anion radical DTBSQ as the major product. The relevant redox potentials in DMF are 02/02 , E° = —0.60 V versus NHE, and DTBQ/DTBSQ , ° = —0.25 V versus NHE, which indicate that the equilibrium constant K for the reaction of O2 with DTBQ has a value of 0.8 x 10 (equation 148). Electrochemical studies of sulfur dioxide (S02/S02 , ° = —0.58 V vs. NHE) and of molecular oxygen in DMF indicate that the equilibrium constant K) for the reaction of SO2 with 02 has a value of 1.1 (equation 149). 

The XPS results indicated that there were about 3-5 at. % sulfur and 27-47 at. % oxygen incorporated onto the sulfur dioxide plasma treated LDPE substrate surfaces (Table 1). The sulfur atomic concentration reached a maximum at about 50 A from the sample surface (0 = 30°) right after the plasma treatment (Figure 3). (The uncertainty of the XPS multiplex scan for atomic concentration analysis is believed to be 0.5-1.0 at. %.) The sulfur-containing species diffused into the bulk of the polymer (> 100 A) as shown from the XPS data collected eleven days after the plasma reaction. This phenomenon is due to the mobility of the polymer surface. s After the sulfur dioxide plasma modification, the hydrophilic sulfonyl groups on the LDPE backbone diffuse away from the polymer surface toward the bulk of the material so that a lower surface energy can be attained. Because the air/LDPE interface has a low surface tension, thermodynamic equilibrium favors a hydrophobic surface. As a result, the sulfur atomic concentrations in the top 100 A of the substrates decreased with time as the sulfonyl groups diffused away from this surface layer. 

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