Activation energy slow reaction

Several studies [7e, 9, 10, 17] show that the first reaction (ki) is the slow step and thus the activation energy, for production of T(C6)H is the activation energy for reaction 1. Recent theoretical calculations of the barrier to reaction 1 suggest a 0.4 eV (9 kcal/mole) barrier between the base-paired pro-... 

Futher, different reactions require different amounts of activation energy. A reaction which requires a higher activation energy is slow at ordinary temperature. Consider the two ... 

Noon values are J43a = 4 x 10 and J43b = 1 x 10. The value of the rate coefficient for R48, K48 = 1.5 x 10 [Morris and Niki (177)], has already been discussed (see Section IV.D.2). In the lower troposphere, the two loss paths—photolysis and OH reaction—are comparable, while in the middle and upper troposphere, where n(OH) drops off, photolysis is dominant. At the same time, the destruction of CH by OH, which has a large activation energy, slows greatly, resulting in a significant reduction in the production of H2C=0 and therefore in n(H2C=0) itself. 

It is, however, possible to induce explosions in these systems by the use of additives which are frequently referred to as sensitizers. Thus Ashmore has shown that the addition of 0.5 mm Ilg of NO to 50 mm Ilg of an equimolar mixture of H2 + CI2 lowers the critical explosion temperature from 400 to 270°C. The explosion in this case is still, however, a thermal explosion, and it has been shown that the lowering of the explosion temperature was produced by an increase in the concentration of Cl atoms, not by a change in the chain mechanism. This increase in concentration of Cl atoms was produced by the replacement of the slow, high-activation-energy initiation reaction, M + CI2 2C1 + M(E > 57 Real), by the much-lower-activation-energy reaction, NO + CI2 NOCl + C1(jE = 22 Real). 

FIGURE 12-3 Activation Energies and Reaction Enthalpies, (a), (b), Large E, slow reaction, (c) Small Ea, fast reaction, (a), (b), hH < 0, large equilibrium constant ... 

They suggested that reactions (34), (35), and (36) are slow compared to reaction (37) and the activation energies of reactions (34) and (37) are larger than those of reactions (31) and (33). From the absorption rate study using a stirred vessel, they derived the forward rate constant of reaction (33) to be 1.4 x 10 M 1 sec-- - at 25°C. The rate constants of other reactions, the rate law, and the products of all reactions involved have not yet been reported. 

Such a substituent lowers the symmetry from C2 to C,. In this point group the original bonds are 2a and a", and the final bonds are also 2a and a". From Fig. 6 one can see that the troublesome bz orbital, which causes the forbiddeness in Dewar benzene, becomes a, just as the ai bond orbital becomes a. The reaction is formally allowed. Adding two chlorine atoms returns the symmetry to C2v again, and the rate falls off drastically. Both a symmetry factor and an electronic factor of some other kind are manifest. Nevertheless the allowed reactions are still remarkably slow and have very substantid activation energies. The reactions are all very exothermic to the extent of about 60 kcal. There is no obvious reason why an activation energy should exist at all, if it were not for the lingering effect of the symmetry forbiddeness of the symmetrical parent compound. 

While some reorganisation reactions are spontaneous and immediate, others such as the interchange of different ester groups on tetrahedral phosphates are extremely slow and have high activation energies. Reorganisation reactions occur in polyphosphate melts (Chapter 5.4), and they also occur with pentacoordinated derivatives (Chapter 13.4). 

At intermediate system temperatures, different reaction sequences lead to chain branching. The activation energy of reaction (3) is quite high ( 17 kcal/mol), so when the temperature falls below about 1000 K, its rate becomes quite slow. Instead, reaction (4) is most important, since it has almost no temperature dependence, producing hydroperoxy radicals HO2. The dominant chain branching sequence in this temperature range is the series of reactions... 

The rate (or speed) of reaction is measured by the amount of reactant nsed up, or the amount of product formed, in a certain period of time. Reactions with low activation energies go faster than reactions with high activation energies. Some reactions go very fast, while others are very slow. For any reaction, the rate is affected by changes in temperature, changes in the concentration of the reactants, and the addition of catalysts. 

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