Exothermic reactions activation energy

Radical additions are typically highly exothermic and activation energies are small for carbon30-31 and oxygen centered32,33 radicals of the types most often encountered in radical polymerization, Thus, according to the Hammond postulate, these reactions are expected to have early reactant-like transition states in which there is little localization of the free spin on C(J. However, for steric factors to be important at all, there must be significant bond deformation and movement towards. sp hybridization at Cn. 

A highly exothermic (low activation energy) reaction will generally have a transition state that resembles the reactants 1... 

This calculation shows that the discussed reaction is very exothermic. The activation energy of this reaction calculated by the IPM method (see Chapter 6) is equal to 8.7 kJ mol 1 and rate constant is k = 7.3 x 106 L mol-1 s-1 at T= 400 K. This rate constant is close to that of the acceptance of the alkyl macroradical by the nitroxyl radical. Hence, this reaction is rapid enough to be the efficient step in cyclic chain termination in polymer. 

The bulk of them also have activation energies in the range of 8 3 kcal./mole. In fact the atom abstraction reactions of most free radicals from molecules, when exothermic, have activation energies in this same range. Table II lists a representative series of such reactions. 

The design of a CSTR with a reversible reaction involves finding the temperature that maximizes conversion. Since the reaction is exothermic, the activation energy of the... 

Addition of hydrogen to a multiple bond is called hydrogenation. Although the overall hydrogenation reaction is exothermic, high activation energy prevents it from taking place under normal conditions. This restriction may be circumvented by the use of a catalyst. 

If the reaction is exothermic, the activation energy in the forward direction is smaller than in the reverse one (see the two figures in the Study Guide in Section 13.13). 

Exothermic heat of reaction Activation energy Volumetric flow rate Rate coefficient Controller constant Density Temperature... 

Figure 16.12 shows the energy diagram for an exothermic chemical reaction. The red line represents the reaction pathway with no catalyst present. The blue line represents the catalyzed reaction pathway. Note that the activation energy for the catalyzed reaction is much lower than for the uncataiyzed reaction. You can think of the reactions activation energy as an obstacle to be cleared, as shown in Figure 16.13. In this analogy, much less energy is required for the horse and rider to clear the lower barrier than to jump the higher hurdle.

Some of the early correlation efforts are still of use, at least as a place to start, although they approach historical interest as well. Early on, Hirschfelder [J.O. Hirschfelder, J. Chem. Phys., 9, 645 (1941)] suggested that if a reaction is exothermic the activation energy should somehow be related to the dissociation energy of the bond being broken. Specifically, for metathesis reactions of the form... 

One possibility would be a very exothermic — G -type reaction, i.e., a pericyclic reaction involving an antiaromatic transition state which, as we have seen, can act as a BO hole. The essential features of such a process are indicated in Fig. 6.39. Because the ground-state reaction is very exothermic, the activation energy is relatively low in spite of the antiaromatic nature of the transition state. At this point there is a BO hole, so the reaction can pass on to the excited state surface and so end up in excited products emission of light completes the reaction. 

Figure 13-42 Temperature response for two exothermic reactions measured at the same location as the concentrations in Figure 13-24. One plot is for Randick s trial 10 (see Figure 13-41) and the other is for a reaction with a lower reaction activation energy.

Recent comprehensive literature reviews Include those by Foon and Kaufman (l8) and Jones and Skolnik (15). Hydrogen abstraction reactions by atomic fluorine exhibit unusually large exothermicities with activation energies in the range 0-2500 cal mole Since the values for many substances are less than 5OO cal mole exceptional experimental sensitivity is required. Severe kinetic complications further increase the difficulty of obtaining precise and accurate thermochemical kinetics results for these reactions (18-21). 

B. Molecule + Molecule —> Ion + Ion. For example, H2O + RCl —> ROH + CF, NH3 + RBr —> RNH, + Br". Since ions are solvated to a much greater extent than molecules, in a more polar solvent the reaction is more exothermic, its activation energy is lower, and it occurs more rapidly. In this case, an increase in the solvent polarity results in a greater increase in the reaction rate constant. For example, for the reaction... 

The cure of novolaks with hexa has been studied with differential scanning calorimetry (dsc) and torsional braid analysis (tba) (46) both a high ortho novolak and a conventional acid-cataly2ed system were included. The dsc showed an exothermic peak indicating a novolak—hexa reaction ca 20°C higher than the gelation peak observed in tba. Activation energies were also calculated. 

Important differences are seen when the reactions of the other halogens are compared to bromination. In the case of chlorination, although the same chain mechanism is operative as for bromination, there is a key difference in the greatly diminished selectivity of the chlorination. For example, the pri sec selectivity in 2,3-dimethylbutane for chlorination is 1 3.6 in typical solvents. Because of the greater reactivity of the chlorine atom, abstractions of primary, secondary, and tertiary hydrogens are all exothermic. As a result of this exothermicity, the stability of the product radical has less influence on the activation energy. In terms of Hammond s postulate (Section 4.4.2), the transition state would be expected to be more reactant-like. As an example of the low selectivity, ethylbenzene is chlorinated at both the methyl and the methylene positions, despite the much greater stability of the benzyl radical ... 

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