Exothermic reaction 27 Hydrogen atom

First H2 is physically adsorbed on the surface. The forces are weak and the adsorption enthalpy is only slightly negative. Subsequently, chemical bonding between Ni atoms and the hydrogen atoms of H2 molecule takes place while simultaneous dissociation of the H2 molecule takes place. The chemisorption enthalpy is strongly negative (exothermic reaction). 

Even if atomic oxygen-carbon cluster reactions are rapid, as is assumed in the new neutral-neutral model, the synthetic power of this model can be recovered if exothermic hydrogen atom abstraction reactions of the sort,... 

Typically, intense chemiluminescence in the UV/Vis spectral region requires highly exothermic reactions such as atomic or radical recombinations (e.g., S + S + M - S2 + M) or reactions of reduced species such as hydrogen atoms, olefins, and certain sulfur and phosphorus compounds with strong oxidants such as ozone, fluorine, and chlorine dioxide. Here we review the chemistry and applications of some of the most intense chemiluminescent reactions having either demonstrated or anticipated analytical utility. 

Benzene and hydrogen in excess of whats needed are fed cold into the pressurized, catalyst-filled reactor. The hydrogenation reaction that takes place, attaching six hydrogen atoms each to the benzene molecules, is exothermic. 

When hydrogen atoms are added to the benzene molecule (hydrogenation), the chemical reaction gives offbeat (exothermic). 

Singlet NH inserts into the CH bonds of hydrocarbons, much like singlet methylene (see Chapter 7 in this volume). Triplet NH abstracts hydrogen atoms from hydrocarbons to form aminyl (NHp radicals and alkyl radicals in the same manner as triplet methylene, in spite of the fact that the reactions of CH2 are exothermic, whereas some reactions of NH are endothermic, depending on the alkane Absolute rate constants for many of these processes have been measured in the gas phase. However, the gas-phase chemistry of methylene is much more developed than that of imidogen. ... 

Secondly, it must be understood that each time a hydrogen atom is extracted from a hydrocarbon upon collision with fluorine, the free radical site created is subsequently, capped with fluorine by reaction with a fluorine atom or fluorine molecule. Each such step is 103 kcal mol 1 exothermic. As is well known, AC = — RT log K, thus when one is reacting fluorine with a hydrocarbon containing 10 to 20 hydrogen atoms each reaction adds 103 kcal mol 1. 

Hydroperoxides can be formed via hydrogen-atom abstraction by R02. The RO2—H bond strength is about 90 kcal. per mole consequently, many such abstractions will be endothermic. Benson (3) has estimated an activation energy of 6 kcal. per mole for exothermic abstractions and 6 + AH for endothermic reactions. The pre-exponential factors should be about 108 0 M"1 sec."1—i.e., about a factor of 2 lower than hydrogen-atom abstraction by CH3 radicals, Some typical abstraction reactions are listed in Table II. 

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