Energy requirement for reaction

Reactions proceed more quickly at higher temperature A higher fraction of collisions have total energy of colliding particles greater than the activation energy required for reaction. 

The energy barrier E t is the minimum energy requirement for reaction. If only this amount of energy is available, only one orientation out of all the possible collision orientations is successful. The probability of success rises rapidly if extra energy is... 

Q2 is the partition function for molecules with energy larger than Eo, the minimum energy required for reaction. 

A gas-phase reactivity model that assumes molecules react as a result of the collision of reactant molecules. The basic idea is that the kinetic energy of the impacting molecules exceeds the activation energy required for reaction. Classical mechanics is used to estimate the fraction of the collisions with enough energy to allow re-... 

The exact energy required for reaction (128) is a matter of debate but probably quanta at wavelengths longer than 2900 or 3000 A would not provide sufficient energy for it to happen. Nevertheless there is no sharp change in quantum yield of hydrogen formation near this wavelength. 

The activated complex lies at the top of the PE profile. The vertical distance AB corresponds to the critical energy required for reaction. The vertical distance CD corresponds to the critical energy for the back reaction. AC gives the PE change between reactants and products, approximately equal to A if of reaction. 

Figure 5.10 A potential energy contour diagram for a light atom attacking in a reaction with a late barrier, showing energy requirements for reaction and energy disposal in products...

What are the energy requirements for reaction Explain the energy disposal in the products. Predict the molecular beam contour diagram. 

We are left with a consideration of the energy factor. At a given temperature, the fraction of collisions that possess the amount of energy required for reaction depends upon how large that amount is, that is, depends upon the In our example act is 4 kcal for the chlorine reaction, 18 kcal for the bromine reaction. As we have seen, a difference of this size in the - act causes an enormous difference in the energy factor, and hence in the rate. At 275 , of every 10 million collisions, 250,000 are sufficiently energetic when chlorine atoms are involved, and only one when bromine atoms are involved. Because of the difference in act alone, then, chlorine atoms are 250,000 times as reactive as bromine atoms toward methane. 

Two studies [24,25] identify the rate-limiting step in the decomposition of lithium peroxide (LijOj), 540 to 600 K, as bond rupture in the O2 ion, for which the measured value of , is 220 kJ mol". The energy requirement for reaction is partially diminished by the decrease in the interionic distance (Li to O ) in the product (LijO). This conclusion was supported by a later isotopic study [26]. Zero-order kinetic behaviour was reported [25] for LijOj decomposition and it was suggested [24] that solid-solution formation (LijOj/LijO) occurred when or was less than 0.5. LijO sublimes below its melting point. 

Angular distribution measurements85 of AI from Ca, Sr and Ba with HI yield no information concerning the centre of mass differential cross sections due to kinematic constraint of AI to the centroid distribution. However, advantage was taken of this constraint to estimate the threshold translational energies required for reaction of 5,4 and 2-5 kcal mol-1, respectively. These values establish lower bounds to the AI bond dissociation energies. 

Table 12-11. Free-Energy Requirements for Reactions Reducing Carbon Dioxide to the Carbohydrate Level°... 

Based on thermochemical data tabulated elsewhere (45), the minimum threshold energy requirements for Reactions 50 and 51 are estimated as approximately 1.7 and 3.1 eV. Stabilized CHF26h F radicals from Reaction 47 would be scavenged by H2S. The species CHF2CH2 F is... 

Other factors in selecting a suitable catalyst that we should consider include the cost and structure of the catalyst, the toxicity of the catalyst and solvent, the ease of separation of the catalyst from the products, the energy requirement for reaction, the stability of the catalyst in process conditions, and the ease of treatment of the waste streams, in order to lead to an efficient and economic PTC process. 

The assumptions they make in order to evaluate n E) are the same as the first two assumptions of Miller et al. but their third assumption is different. They are considering reactions in the gas phase where a cage effect is meaningless. They state that the minimum energy required for reaction E, is still large compared to thermal energies and derive for the yield of an individual hot product... 

In the light of the discussion of microscopic reversibility that was given earlier in this section, it should occasion no surprise that the information-theoretic analysis can be applied to the selective energy requirements for reaction as well as to the specificity of energy disposal. However, the results that are availaUe for this treatment are largely derived either from data obtained for exothermic reactions or firom quasiclassicd trajectory calculations, rather than from direct experimental measurements. 

In the first step, the energy required for reaction is accumulated into molecule A by means of collisions with other molecules A to yield an energized molecule A, i.e., a molecule that has sufficient energy to react. This energized molecule then decomposes in the second step. 

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