Activation Energy
Activation energy is the minimum amount of energy required for a chemical reaction to occur. It represents the energy barrier that must be overcome for the reaction to proceed. Higher activation energy means the reaction is slower, while lower activation energy means the reaction is faster.
6 Key excerpts on "Activation Energy"
- Eugene Machlin(Author)
- 2010(Publication Date)
- Elsevier Science(Publisher)
...We do not consider homogeneous chemical reaction kinetics, a subject outside the scope of this chapter – the kinetics of processes in condensed phases. 1 Activation Energy Let us consider a process taking place in a condensed phase in which reactants existing in some metastable state proceed to a more stable product state. Atom or molecule arrangements define the reactant and product states. The transition between the initial and final states involves rearrangements of atoms or molecules and is called a reaction path. Since the initial and final states represent either metastable or stable states, the energy of the system must increase along any reaction path between them, as illustrated schematically in Figure 7.1. The assumption has been made in the foregoing that any atomic or molecular arrangement characteristic of a position along a reaction path, the equivalent of an excited state, has a unique energy associated with it. However, the energy is not the only measure of the state of the system that is important in the problem of describing the transition from the reactant state to the product state. It is also necessary that the set of momentum vectors associated with the atoms or molecules obey some criteria, to be defined, if the transition from the reactant state to the product state is to be possible. As will be shown, the free energy is the quantity that describes both these factors. Figure 7.1 Energy of system as a function of reaction path from initial to final state. The maximum energy along the reaction path relative to the energy of the reactant state is called the Activation Energy (see Figure 7.1). Thus, if the system of reactant atoms is to proceed from the reactant to the product state it must somehow be excited at least to the energy corresponding to the Activation Energy. The problem of obtaining an analytic relation for the rate at which this system can overcome the Activation Energy can be approached via several levels of sophistication...
eBook - ePubPetrochemistry
Petrochemical Processing, Hydrocarbon Technology and Green Engineering
- Martin Bajus(Author)
- 2020(Publication Date)
- Wiley(Publisher)
...(1.4) is called the Activation Energy and is generally expressed in J/mol or kJ/mol. R, the ideal gas constant, is equal to 8.31 J/(mole K). Written as in Eq. (1.5), the expression recalls a form known in thermodynamics as the Clapeyron–Clausius equation. Likewise, the constant k is expressed as a function of temperature by (1.6) The result is that (1.7) (1.8) Since thermodynamics states that (1.9) considering thermodynamic relating K c and K°, consequently (1.10) Therefore, as an initial approximation, the heat of reaction is equal to the difference between the reaction's Activation Energy in the forward and reverse directions. Generally speaking, the Activation Energy of a chemical reaction ranges between 40 and 200 kJ/mol. Any value outside this range should be considered questionable. A low observed value in particular is almost always indicative of diffusion limit skewing. The Activation Energy of thermally activated reactions is also frequently higher than that of catalyzed reactions. This is not surprising since one of the functions of a catalyst is precisely to lower the potential barrier that separates the reactants from the products. It is often preferable to use the Arrhenius equation in a different form. k o, called the frequency factor, is the value of the reaction rate constant that corresponds to an infinite temperature. The concept is therefore somewhat abstract...
eBook - ePub- Kevin R. Reel(Author)
- 2013(Publication Date)
- Research & Education Association(Publisher)
...The rate constant will double (approximately) for every 10 K increase in temperature. Activation Energy AND CATALYSTS • The Activation Energy is the amount of energy needed to cause a reaction to occur. Δ E = Σ E products − Σ E reactants • Catalysts increase the rate of a reaction by facilitating a faster pathway, or mechanism, without being used up itself during the reaction. Therefore, catalysts do not appear in a balanced chemical equation. • The new mechanism of reaction created by a catalyst has a lower Activation Energy. • In living systems, catalysts are frequently used to speed up reactions that—without the catalyst—might be too slow to be useful for the organism. REACTION MECHANISMS • Most chemical reactions involve more than one step. The combination of steps is called the mechanism of reaction, and must add to yield the total reaction. • Single steps usually involve either the decomposition of a single reactant (first order) or the collision of two reactants (second order). The probability of three reacting species colliding at the same time with the right amount of energy is extremely low. • Sometimes during a reaction mechanism, intermediates are formed and then used up; but they do not show up in the overall net reaction. Example: • The slowest step in the set of reactions in a mechanism determines the overall rate of the reaction, so it is called the rate-determining step. Example: If the total reaction were a single step, the total rate law would be Rate = k [A] [B]. However, in this multistep mechanism, the total rate depends only on the second step, which only depends on [B]. Therefore, the rate law for the total reaction is rate = k [B]....
eBook - ePubBiomolecular Kinetics
A Step-by-Step Guide
- Clive R. Bagshaw(Author)
- 2017(Publication Date)
- CRC Press(Publisher)
...A great deal of effort has been directed toward quantifying the factors that contribute to A and Ea, with the ultimate goal of calculating the value of rate constants from fundamental parameters. For bimolecular reactions, the preexponential factor is limited by the number of collisions, which can be calculated knowing the effective radii of the reactant molecules [ 90 ]. The experimental value of A for bimolecular reactions is often less than that accounted for by the collision frequency alone, suggesting that other factors, such as the orientation of the collision, are also important (Section 3.5). It is easy to add an empirical orientation factor to get agreement with the experiment but much more difficult to calculate it ab initio. For unimolecular reactions in the gas phase, collisions with other molecules are an important route for transferring energy so that a molecule may acquire sufficient energy to subsequently dissociate in a first-order reaction. In solution, collisions of the reactant with solvent molecules may provide sufficient energy for individual reactant molecules to exceed Ea. The Activation Energy, Ea, implies that there is some sort of barrier that must be surmounted if a reactant molecule, A, is to successfully convert to product, B. Indeed, the transition-state theory, outlined in Equations 3.21 through 3.24, indicates that this barrier includes a term for the activation entropy as well as the activation enthalpy. An elementary reaction is commonly depicted as in Figure 3.5 a, where the height of the barrier above the ground-state free energy of reactant A is defined by Δ G ‡, while the difference between the ground-state A and B energy positions is defined by Δ G. The peak of the barrier represents the transition state, which is conventionally denoted with the ‡ symbol. This is not equivalent to the “top of the mountain” but rather the “saddle of a mountain pass,” as shown in the contour plot (Figure 3.6)...
- Robert J. Farrauto, Lucas Dorazio, C. H. Bartholomew(Authors)
- 2020(Publication Date)
- Wiley-AIChE(Publisher)
...CHAPTER 1 CATALYST FUNDAMENTALS OF INDUSTRIAL CATALYSIS 1.1 INTRODUCTION Chemical reactions occur by breaking the bonds of reactants and forming new bonds and new compounds. Breaking stable bonds requires the absorption of energy, while making new bonds results in the liberation of energy. The combination of these energies results in either an exothermic reaction in which the conversion of reactants to products liberates energy or an endothermic process in which the conversion process requires energy. In the former case, the energy of the product is lower than that of the reactants with the difference being the heat liberated. In the latter case, the product energy is greater by the amount that must be added to conserve the total energy of the system. Under the same reaction conditions, the heat of reaction (Δ H) being a thermodynamic function does not depend on the path or rate by which reactants are converted to products. Similarly, Δ G of the reaction is not dependent on the reaction path since it too is a thermodynamic state function. This will be emphasized once we discuss catalytic reactions. The rate of reaction is determined by the slowest step in a conversion process independent of the energy content of the reactants or products. 1.2 CATALYZED VERSUS NONCATALYZED REACTIONS In the most basic sense, the purpose of the catalyst is to provide a reaction pathway or mechanism that has a lower activation barrier compared to the noncatalyzed (E nc) pathway, as illustrated in Figure 1.1. Also shown is the catalyzed barrier (E Mn). In any reaction, catalyzed or noncatalyzed, the reaction sequence occurs through a series of elementary steps. In a noncatalyzed reaction, the species that participate in the reaction sequence are derived solely from the reactants. In a catalyzed reaction, the catalyst is simply an additional species that participates in the reaction sequence by lowering the Activation Energy and hence enhances the kinetics of the reaction...
- Kevin Reel, Derrick C. Wood, Scott A. Best(Authors)
- 2014(Publication Date)
- Research & Education Association(Publisher)
...Catalysts provide an alternative pathway for the reaction to follow, which is at a lower Activation Energy—thus increasing the reaction rate. As a result of the lower energy of activation, a greater number of molecules will have enough energy to form products when they collide. This is illustrated in Figure 11.2. Figure 11.2. Potential Energy Diagram In a chemical reaction, a catalyst is always recovered and only a small amount is required to increase the reaction rate. A homogeneous catalyst is in the same state of matter as the other reactants, while a heterogeneous catalyst is in a different state of matter than the reactants. A catalyst can be easily identified in a reaction mechanism because it is put in at the beginning of the reaction, and comes out on the products side at the very end. In the following mechanism, the catalyst is depicted by the letter C and the intermediate by the letter I. Step 1. A + C ↔ I fast step Step 2. A + I → C + D slow step The recovered catalyst will then be used again and again to speed up the chemical reaction. DID YOU KNOW? A catalytic converter is a component used in the exhaust of automobiles to aid in the removal of pollutants such as nitrous oxides (NO x), which are produced during the combustion of fossil fuels. Catalytic converters contain precious metals such as platinum because they can catalyze reactions that would otherwise take years to occur on their own in the environment....
