Activation Energy of the Reverse Reaction

8 Activation Energy of the Reverse Reaction and Kinetic Energy Release 

By merely considering thermodynamic and kinetic models we may be able to understand how ions are formed and what parameters are effective to determine their further fate in the mass spectrometer. However, the potential energy surfaces considered in this context only reached up to the transition state (Fig. 2.6). Without explicitly mentioning it, we assumed the curves to stay on the same energetic level between transition state and the products of ion dissociation, i.e., the sum of heats 

The total excess energy, Eextot, of the precursor ion relative to the heats of formation of the products in their ground state, comprises the excess energy in the transition state, Eex, plus the activation energy of the reverse reaction, Eq  

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The reaction temperature can have a large effect on the activities of the catalysts. Figure 4 shows the concentration of methyl ester with the solid base and acid catalysts at different temperatures after 10 hours reaction. The results show that the concentration of methyl ester with MgO-AUOa catalyst decreased more quickly than that with Ti02-S04 and CaO catalysts. This means that the activation energy of the reverse reactions with Mg0-Al203 catalysts is higher. 

According to Eq. (14.3), the activation energies of the reverse reactions wiU afso differ by a fraction of this work (but this time with the same sign) ... 

If the enthalpy of this reaction is 54 kJ, what would be the activation energy of the reverse reaction ... 

From the previous discussion about the temperature sensitivity of reaction rate as a function of activation energy, we can understand why the chemical equilibrium constant of an exothermic reversible reaction decreases with increasing temperature. An exothermic reaction has a negative heat of reaction, since the activation energy of the reverse reaction exceeds that of the forward reaction. As temperature increases, the reverse reaction increases relatively more rapidly than the forward reaction, which means that at chemical equilibrium we have relatively more reactants than products and a lower equilibrium constant. 

C) activation energy of the reverse reaction and the potential energy of the activated complex... 

The activation energy of the reverse reaction is always greater than the activation energy of the forward reaction since the reaction is exothermic. Therefore the reverse reaction will increase more quickly with an increase in temperature than will the forward reaction. Temperature may still dominate for reactor productivity, but in the opposite direction compared with the irreversible case, since conversion increases with lower temperature. However, when the temperature becomes too low, both reaction rates slow down such that we cannot achieve the desired production rate with this variable alone. Instead, the concentrations of nC4 and iC dominate the rate through the relationship imposed by the equilibrium constant 12 = CiC/C c. 

A E, is the difference between the energy of the products and the energy of the reactants. The activation energy of the reverse reaction is Ear-AE. These energy levels are represented in an energy diagram such as the one shown below for the reaction NO2 + CO NO + CO2. This is an exothermic reaction because the products are lower in energy than the reactants. 

The first reaction in the chain is the limiting reaction of the Zeldovich mechanism and its stimulation by vibrational excitation of N2 molecules, discussed in Section 6.2. The second reaction is stimulated by vibrational excitation of CO molecules due to the relath ely high energy barrier of the reaction and the absence of activation energy of the reverse reaction (see Sections 2.7.2 and 2.7.3). 

A certain reaction has a AH = -75 kJ and an activation energy of 40 kJ. A catalyst is found that lowers the activation energy of the forward reaction by 15 kJ. What is the activation energy of the reverse reaction in the presence of this same catalyst ... 

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