Sulfur dioxide equilibrium position

Solutions of hexaphenylethane in liquid sulfur dioxide conduct electricity, suggesting an ionization into triphenylmethyl positive and negative ions. Since the spectrum of triphenylmethide ion was missing from the spectrum of the solution the following equilibrium was postulated ... 

Gas phase oxidation of sulphur dioxide is kinetically inhibited and virtually impossible without a catalyst at any temperature. At ordinary temperatures the reaction is so slow that, in practical terms, it does not occur at all. Increasing the temperature increases the rate of reaction, but simultaneously the position of the equilibrium shifts unfavourably -away from sulphur trioxide and towards sulfur dioxide and oxygen. Without a catalyst, the temperature needed to make the system react at a practical speed is so high that a very poor conversion [i.e. very little SO3 production] is obtained."... 

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. 

A catalyst is a substance, which speeds up a chemical reaction but is not itself consumed in the reaction. However, it cannot alter the equilibrium position of a chemical reaction (i.e., the relative proportions of sulfur dioxide and sulfur trioxide present after the reaction). Thus, the reaction rate for an equilibrium reaction, such as that represented by Eq. 9.24, is the speed with which equilibrium is reached not the speed to complete conversion). 

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. 

Figure 22.6 shows schematically the motion corresponding to the four normal modes of carbon dioxide (linear) and the three normal modes of sulfur dioxide (nonlinear), and shows the frequencies divided by the speed of light, given in cm . These values are sometimes called frequencies and the unit is called wave numbers. The arrows in the diagrams show the direction of motion of each nucleus away from its equilibrium position during one-half cycle of the concerted motion. As each nucleus oscillates, it first moves in the direction indicated and then reverses. 

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