Kinetics and thermodynamics of hydrogenation reactions

The Reaction Must Be Possible. Since catalysts affect only the rate or speed of a reaction and have nothing to do with the inherent tendency of a reaction to proceed, it is obviously of real importance to know whether or not the reaction is at all possible for the conditions of temperature and pressure chosen. The concern is then with chemical affinity and since the free energy change of a definite, specific reaction is a quantitative measure of such chemical forces, it is desirable to have free-energy data on the substances in question if such data can be had. If reliable free-energy data show the reaction to be possible, then experimental work may be started in order to find suitable catalysts that will enable the reaction to proceed at a reasonable rate. 

When all the reactants and products are in the gaseous state at a partial pressure of 1 atm, then the well-known relationship eidsts  

The term — AF is the free-energy decrease in calories for the reaction as it proceeds from left to right. It represents the difference in free-cnergy content of all the reactants and all the products. On the basis used here for free-energy values, all elements in their normal state of aggregation at 1 atm pressure and at any particular temperature T are arbitrarily 

10- to h. Free energy of formation of normal ald ydes, normal alcohols, and normal ahphatic acids in the gaseous state. 

From Figs. 10-2d and e, it can be calculated that at 400°C in the case of a larger paraffin, e.g., hexane there is a spontaneous tendency to decompose or crack, yielding a smaller paraffin and smaller olefin, e.g., propane and propylene. At 100°C, such a reaction does not tend to occur. Therefore, if a relatively large olefin were to be hydrogenated to a paraffin of the same molecular weight, apparently it would be desirable to carry out such a reduction at lower temperatures for two pertinent reasons (1) the equilibrium yield is better, (2) decomposition or side reactions are less likely to be troublesome. Because of this preferred lower-temperature operation, catalysts must be available to operate at these lower temperatures. Such hydrogenation catalysts are known, as was pointed out elsewhere in this chapter. 

---