Composite reactions energy profiles

Figure 1.6.3 is the reaction energy profile for a two-step reaction with two transition states, TSj and TS2. The overall reaction rate is determined by the step whose transition state has the largest energy. This step is called the rate-limiting (or rate-determining) step and is the bottleneck of the overall reaction rate. The letter I denotes the reaction intermediate. The reaction is a composite one, as shown in the scheme ... 

SIMS Secondary Ion mass spectroscopy A beam of low-energy Ions Impinges on a surface, penetrates the sample and loses energy In a series of Inelastic collisions with the target atoms leading to emission of secondary Ions. Surface composition, reaction mechanism, depth profiles... 

FIGURE 13.14 Potential energy profiles for a reaction whose activation energy is lowered by the presence of a catalyst. The activation energy for the catalyzed pathway (red curve) is lower than that for the uncatalyzed pathway (blue curve) by an amount AEa. The catalyst lowers the activation energy barrier for the forward and reverse reactions by exactly the same amount. The catalyst therefore accelerates the forward and reverse reactions by the same factor, and the composition of the equilibrium mixture is unchanged. 

It is also possible to draw a schematic energy profile for a composite reaction this will consist of the energy profiles for the individual elementary steps. For a two-step mechanism, such as that represented by Reactions 2.5 and 2.6, a possible energy profile would be as shown in Figure 2.2. Note that the horizontal axis is still labelled reaction coordinate , although this should not be taken to imply that the second step occurs immediately on completion of the first. The intermediate carbocation may undergo many, many collisions with various species before finally experiencing a successful collision with an OH ion as represented by Equation 2.6. 

Since the Arrhenius equation applies equally well to composite reactions, it is sometimes the case that an energy profile such as that in Figure 7.4 is also used to represent these reactions. Strictly this is not correct, although it can be useful as a shorthand representation. A composite reaction consists of a sequence of elementary steps each of which will have its own energy profile and activation energy. The... 

This ester resembles its methyl homologue in possessing three modes of decomposition [131]. It also supports a self-decomposition flame, the multiple reaction zones of which are clearly separated at low pressures [122, 123, 125]. Temperature and composition profiles in the low-pressure decomposition flame have been measured [133]. The products include formaldehyde, acetaldehyde and ethanol with smaller amounts of methane and nitromethane. The activation energy derived from the variation of flame speed with final flame temperature was 38 kcal. mole", close to the dissociation energy of the RO—NO2 bond. The controlling reaction is believed to be unimolecular in its low pressure regime, and the rate coefficient calculated from the heat-release profile is... 

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