Activation energy reduced

In a closed system and at a fixed temperature, the thermodynamic equilibrium constant of any reaction has a fixed value. A catalyst has an impact on the reaction rate by lowering the activation energy, reducing the required time to achieve the thermodynamic equilibrium and it can have an impact on the reaction channel by favoring one of several possible transition states, but a catalyst does not influence the thermodynamic equilibrium itself. That means the maximal achievable yield cannot be higher than that predicted by thermodynamics. An important consequence of this limitation is that for a comparison of different catalysts the experimental conditions must not allow that the thermodynamic equilibrium is reached. Typical reaction conditions for this case would be low reactant flow (setup then resembling a closed system) and/or a temperature allowing a very fast reaction. If the experimental conditions allow the reaction to approach the thermodynamic equilibrium, a comparison of different catalysts is usually impossible. For any kinetic studies the... 

Using catalysts does not only lower the activation energy, reduce the energy consumption and improve the process efficiency, but can also improve the selectivity and quality of the products produced. 

According to Lewis et al. [30], the rate constant of the imidization is increased and the activation energy reduced when the reaction is performed with microwaves rather than with conventional heating. In contrast to most other studies, however, Mijovic et al. found no difference in curing rates upon investigating epoxide systems and polyimides [31]. 

Fig. XVIII-13. Activation energies of adsorption and desorption and heat of chemisorption for nitrogen on a single promoted, intensively reduced iron catalyst Q is calculated from Q = Edes - ads- (From Ref. 130.)...

After 60 minutes of aimealing, all the Pt has reacted to fonn Pt2Si. Almost immediately thereafter the reaction between Pt2Si and Si to fonn PtSi starts and after a fiirther 60 minutes all the Pt2Si has reacted, resulting in a stable PtSi film on Si. The data of silicide thickness versus ramped temperature can be plotted in reduced fonn in an Arrhenius-like plot to give the activation energy [6, 14] ... 

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. 

The viscosity of liquid silicates such as drose containing barium oxide and silica show a rapid fall between pure silica and 20 mole per cent of metal oxide of nearly an order of magnitude at 2000 K, followed by a slower decrease as more metal oxide is added. The viscosity then decreases by a factor of two between 20 and 40 mole per cent. The activation energy for viscous flow decreases from 560 kJ in pure silica to 160-180kJmol as the network is broken up by metal oxide addition. The introduction of CaFa into a silicate melt reduces the viscosity markedly, typically by about a factor of drree. There is a rapid increase in the thermal expansivity coefficient as the network is dispersed, from practically zero in solid silica to around 40 cm moP in a typical soda-lime glass. 

A stronger bond between the nucleophilic atom and carbon is reflected in a more stable transition state and therefore a reduced activation energy. Since the 8 2 process is concerted, the strength of the partially formed new bond is reflected in the energy of the transition state. 

This mechanism can reduce the overall activation energy of the reaction in at least two ways. The partial transfer of a proton to the carbonyl oxygen increases the electrophilicity of the carbonyl. Likewise, partial deprotonation of the amino group increases its nucleophilicity. 

A = rate constant (pre-exponential factor from Arrhenius equation k = A exp (-E /RT), sec (i.e., for a first order reaction) B = reduced activation energy, K C = liquid heat capacity of the product (J/kg K)... 

In general, the activation energies for both cationic and anionic polymerization are small. For this reason, low-temperature conditions are normally used to reduce side reactions. Low temperatures also minimize chain transfer reactions. These reactions produce low-molecular weight polymers by disproportionation of the propagating polymer ... 

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