Activation energy, for dissociation

Table 3.3. Correlation between Intramolecular Strain from Molecular Mechanics (MM) Calculations and Activation Energies for Dissociation of C—C Bonds in Substituted Ethanes"...

The alkali promotion of CO dissociation is substrate-specific, in the sense that it has been observed only for a restricted number of substrates where CO does not dissociate on the clean surface, specifically on Na, K, Cs/Ni( 100),38,47,48 Na/Rh49 and K, Na/Al(100).43 This implies that the reactivity of the clean metal surface for CO dissociation plays a dominant role. The alkali induced increase in the heat of CO adsorption (not higher than 60 kJ/mol)50 and the decrease in the activation energy for dissociation of the molecular state (on the order of 30 kJ/mol)51 are usually not sufficient to induce dissociative adsorption of CO on surfaces which strongly favor molecular adsorption (e. g. Pd or Pt). 

E 3,p,E = activation energy for dissociation, propagation and termination respectively (KJ/mol)... 

Usually, activation energies for dissociation are much higher than k T and situations like this, where the apparent activation energy becomes negative for the rate-limiting process, are rare. Nevertheless, the present case illustrates nicely that entropy changes may play an important role and that it is not always the activation energy that dominates the process. 

Explain in simple terms why the sticking is so low, despite the low activation energy for dissociation. 

Figure 6.1 Schematic potential energy diagram for atomic and molecular nitrogen adsorption on a clean and K-covered Fe(100) surface. Curve (a) is for N2 + Fe(100) curve (b) is for N2 + Fe(100)-K. Note the lowering of the activation energy for dissociation from 3 kcalmol-1 to zero. (Reproduced from Ref. 3).

A barrier (activation energy) for dissociation was reported by DeLeo and Fowler (1985a,b) in their early calculations. The value estimated from this simple model is about 1.5 eV, close to the experimental value of 1.3 eV estimated for the similar H—In pair (Wichert et al., 1987). In the... 

This bond formation compensates (partially) the activation energy for dissociation of the O—O bond in perester. The empirical peculiarities of anchimeric assistance decomposition are the following [3,4] ... 

The potential energy curves of a neutral molecule AB and the potential ionic products from processes 7.18-7.20 are compared below (Fig. 7.11). These graphs reveal that the formation of negative molecular ions, AB, is energetically much more favorable than homolytic bond dissociation of AB and that the AB " ions have internal energies close to the activation energy for dissociation. [65,73,75]... 

Batt, Gowenlock and Trotman carried out a detailed study of the pyrolysis and photolysis of terf-butyl nitrite and established that dimeric nitrosomethane exists in two isomeric forms, cis and trans (Scheme 5). Monomeric nitrosomethane could be generated by heating the dimer in the gas phase (the activation energy for dissociation was found to be ca 90 kJmoH )". Also ultraviolet irradiation dissociates the dimer, leaving monomeric 1. Vibrational analysis of monomeric 1 is summarized in Table 4. 

It is generally found that the activation energy for dissociation of simple diatomic molecules decrease when going left from the noble metals in the periodic table. This can be described most simply in terms of an increased interaction between the anti-bonding adsorbate states and the metal d-states. 

Trends in dissociative energies and activation energies for dissociation as a function of the number of d-electrons. The results are calculated in the Newns-Anderson model including the coupling between an adsorbate level epsilon a and the metal d-band. 

This bond formation compensates (partially) the activation energy for dissociation of the... 

The activation energy for dissociation of this bond is 190-230 kJ mol. The corresponding value for the C—O bond is somewhat higher. 

Thus a relatively simple scheme may be proposed in which the two forms of NO adsorbed on the Ru(001) surface, the relative populations of which vary with coverage, have different activation energies for dissociation. The dissociative reaction is poisoned by the reaction products below their temperature of desorption. 

At an initial relative coverage of NO of 0.15, dissociation of the bridged NO is complete by 250 K, as shown by spectrum (b) of Fig. 6. If one assumes that the (coverage-independent) value of the pre-exponential factor of the rate coefficient for dissociation is 1013 s-1, then the activation energy for dissociation of this state is approximately 16 kcal/mole. The coverage dependence of the activation barrier for dissociation of molecular NO is... 

Both kinetic and thermodynamic factors are important in determining protic character. Although the equilibrium concentration of solvated protons in a protic solvent such as water or ethanol may be very small, there is a low activation energy for dissociation or exchange of protons, and these labile protons can rapidly react with any species having an appreciable proton affinity. Aprotic solvents usually have a lower equilibrium concentration of solvated protons (perhaps by a factor of as much as 1010) and the activation energy for... 

Fig. 16. Transition states and activation energies for dissociative adsorption of carbonyl-containing organic molecules on Cuj3.

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