Catalysed reactions activation energy

The effect of temperature on the non-catalysed reaction was difficult to disentangle, for at lower temperatures the autocatalytic reaction intervened. However, from a limited range of results, the reaction appeared to have an experimental activation energy of c. +71 kj moh. ... 

The unsaturation present at the end of the polyether chain acts as a chain terminator ia the polyurethane reaction and reduces some of the desired physical properties. Much work has been done ia iadustry to reduce unsaturation while continuing to use the same reactors and hoi ding down the cost. In a study (102) usiag 18-crown-6 ether with potassium hydroxide to polymerise PO, a rate enhancement of approximately 10 was found at 110°C and slightly higher at lower temperature. The activation energy for this process was found to be 65 kj/mol (mol ratio, r = 1.5 crown ether/KOH) compared to 78 kj/mol for the KOH-catalysed polymerisation of PO. It was also feasible to prepare a PPO with 10, 000 having narrow distribution at 40°C with added crown ether (r = 1.5) (103). The polymerisation rate under these conditions is about the same as that without crown ether at 80°C. 

With trifluoroacetic acid as solvent, toluene and o-xylene gave second-order kinetics and for the activation energy for toluene was 12.7 (from data at 1.6 and 25.2 °C), i.e. considerably less than for the zinc chloride-catalysed reaction in acetic acid330. 

FIRST-ORDER RATE COEFFICIENTS AND ACTIVATION ENERGIES FOR REACTION OF RX WITH BENZENE IN PhN02 CATALYSED BY A1C13352... 

At any particular temperature, more molecules will possess sufficient energy to overcome the activation energy for the catalysed reaction compared with the uncatalysed reaction. The distribution of molecular energies of the gas molecules does not change at constant temperature. 

Figure 8.29 Reaction profiles for an uncatalysed reaction (upper single curve) and catalysed (curve with a double maximum). The two solid arrows represents a for the catalysed reactions note how 1 ( a for the first reaction step) is so much larger than the activation energy for the second step, 2 ...

A new, more general, way to combine ab initio quantum mechanical calculations with classical mechanical free-energy perturbation approach (QM/FE approach) to calculate the energetics of enzyme-catalysed reactions and the same reaction in solution has been reported." The calculated free energies were in fairly good agreement with the experimental data for the activation energies of the first test case, amide hydrolysis in trypsin and in aqueous solution. 

The enzyme appears to catalyse the reaction by predisposition of these hydrogen bonding interactions and the calculated activation energy is 53 kJ moh This is a significant lowering of the activation barrier compared to the barrier found in the solution reaction (92 kJ moh ) and represents an enzymatic rate acceleration of one million fold. In another theory study [14] which modelled this reaction in a simulated water matrix, the electrostatic interaction between F and the positively charged sulphur (R3S+) of SAM 8 was deduced to confer significant stability to the reaction complex. These theory studies also support a... 

The application of Absolute Rate Theory to the interpretation of catalytic hydrogenation reactions has received relatively little attention and, even when applied, has only achieved moderate success. This is, in part, due to the necessity to formulate precise mechanisms in order to derive appropriate rate expressions [43] and, in part, due to the necessity to make various assumptions with regard to such factors as the number of surface sites per unit area of the catalyst, usually assumed to be 10 5 cm-2, the activity of the surface and the immobility or otherwise of the transition state. In spite of these difficulties, it has been shown that satisfactory agreement between observed and calculated rates can be obtained in the case of the nickel-catalysed hydrogenation of ethylene (Table 3), and between the observed and calculated apparent activation energies for the... 

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