Heat of activation, apparent

Traditionally, the quantity given by-/ (3 In i0/31 /T) is tenned the apparent heat of activation at the reversible potential. The word apparent is a code word for the fact that the quantity delivered by (7.92) has a mixed meaning (because it is partly concerned with the energy barrier to the reaction and partly with the thermodynamics of the reaction occurring reversibly). 

The new factor, therefore, is equivalent to the reduction of the apparent heat of activation by 5,000 calories or so. Thus the termolecular reaction might be expected to have a heat of activation about 20,000 calories less than that of the bimolecular reaction at 1,000° abs. The predicted heat of activation is thus about 40,000 calories, the observed value being 44,000. 

Until more definite information is forthcoming about its real meaning we may call E the apparent heat of activation. 

We are now in a position to see the difference between the true and apparent heats of activation. The true heat of activation, Q, is given by... 

Relation between the true and apparent heats of activation. 

When the products of reaction exert no retarding influence, the apparent heat of activation is less than the true value by an amount A, which determines the variation with temperature of the adsorption. 

The existence of a marked retarding influence of the products has, on the other hand, the effect of increasing the apparent heat of activation. 

There is one important special case in which the apparent heat of activation becomes equal to the true value. This is when the surface is completely covered with the reactant over the whole range of temperature, and there is no retardation due to the presence of the products of reaction, cr has the constant value unity, and the variation of the observed reaction velocity is due entirely to the changing rate of the actual chemical transformation. 

The results were, in fact, negative. In the following table Eco2 represents the apparent heat of activation in the decomposition of formic acid vapour into carbon dioxide and hydrogen. The numbers in the last column represent the relative velocity of reaction at 200° C. for equal surfaces of the various catalysts, and are referred to platinum,... 

The relation between the true and apparent heats of activation is given by the equation... 

The decomposition of ammonia on the surface of platinum takes place at a speed which is inversely proportional to the pressure of the hydrogen present. The combined influence of the two terms E and A produces an apparent heat of activation of more than 100,000 calories. This is in striking contrast with the value of E true, 39,000 calories for the unretarded reaction on the surface of tungsten. The decomposition of ammonia on molybdenum is of zero order, but retarded by nitrogen the value of E according to Burk is 53,200 calories. Kunsman finds 32,000 calories Trans. Faraday Soc., 1922, 17, 621. 

Adkins and Nissen, ] for example, found that alumina prepared in different ways exhibited very varying catalytic activity towards the decomposition of formic acid. This could hardly be due simply to the different surface areas of the several preparations, since the variation was not confined to the total speed of reaction but affected also the relative speeds of the two alternative decompositions which formic acid undergoes. Moreover, the apparent heats of activation varied from one kind of alumina to another. J... 

Adkins and Nissen appear to think that because the heat of activation is dependent upon the method of preparation of the catalyst it can have no connexion with the stability of the molecule (this is quite apart from any distinction between true and apparent heat of activation). This is of course a complete misunderstanding, for the stability of the molecule is also a function of the nature of the surface upon which it is adsorbed. Otherwise catalysis would not exist. 

The presence of the reference electrode will also become apparent in electrode kinetics in the analysis of apparent heats of activation (Sect. 3.4) and the impossibility of measuring absolute electrode kinetic parameters for a half reaction such as reaction (2). 

Since the electrode potential, E, includes an undeterminable potential difference due to the reference electrode [eqn. (6)], an apparent heat of activation can be obtained at constant overpotential, regardless of the reference electrode, by replacing E with E ° + rj in eqn. (101) [36]. 

Double layer effects are also apparent on the apparent heat of activation [6, 46]... 

Derivation of the Apparent Heat of Activation at the Reversible Potential... 

Normally, as may be seen from Eq. (11), evaluation of AH can be made from plots of In kvsA/T or, taking account of any doublelayer effects, from nioVs, /T, This gives the apparent heat of activation at the reversible potential. The real but experimentally undeterminable AH at 17 = 0 differs from the apparent value by... 

As is well known, the apparent heat of activation evaluated experimentally is dependent on 17, as follows from Eq. (11). Following the form of that equation, the exchange current density i o can be written in an abbreviated practical form as... 

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