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Activation Energy A Activation energy FIELDS OF STUDY that is higher in energy than the starting materials. An intermediate is a stable chemical structure formed Physical Chemistry; Inorganic Chemistry; Biochemistry during a reaction process that can often be captured and isolated by chemical means. Once this structure SUMMARY is formed, the reaction process can either progress to form products or revert back to the original reactants. The activation energy of a process is defined, and Activation energy is the energy required for a spe- its importance in chemical processes is elaborated. cific chemical reaction to occur. In a reaction, two Activation energy is a widely variable quantity in dif- reactant molecules contact each other with the en- ferent reactions but is nevertheless characteristic of ergy of their ambient states.(The ambient state of a any specific reaction process. material can be thought of as its “default” state—the state it takes at one atmosphere of pressure and what PRINCIPAL TERMS is commonly considered to be room temperature.) In the case of a spontaneous reaction, the energy of the Arrhenius equation: a mathematical function collision is sufficient to initiate the formation of the that relates the rate of a reaction to the energy transition state or intermediate. In a nonspontaneous required to initiate the reaction and the absolute reaction, the energy of the molecular collision is not temperature at which it is carried out. sufficient, and the two molecules will not interact. catalyst: a chemical species that initiates or speeds The input of some additional energy is required to up a chemical reaction but is not itself consumed drive the two molecules together so that the transi- in the reaction. tion state or intermediate is formed and the reaction chemical reaction: a process in which the mole- can proceed. The energy released in the transforma- cules of two or more chemical species interact with tion of reactants into products is generally sufficient each other in a way that causes the electrons in the to drive the reactions of other molecules in the reac- bonds between atoms to be rearranged, tion mixture. resulting: in changes to the chemical identities of Another way to look at activation energy is to think the materials. of it as the minimum amount of energy that two inter- reaction rate: how much of a particular reaction or acting molecules must gain in order to weaken bonds reaction step occurs per unit time. between atoms in both molecules so that those bonds transition state: an unstable structure formed can be rearranged. Since chemical reactions are es- during a chemical reaction at the peak of its poten- sentially processes of breaking and making bonds, tial energy that cannot be isolated and ultimately having sufficient energy to overcome the strength of breaks down, either forming the products of the the appropriate bonds is essential if there is to be any reaction or reverting back to the original reactants. reaction between the two molecules. Activation Energy in Chemical Reactions Activation Energy and Reaction Rates Activation energy can be thought of as a barrier that Reaction rates can be related directly to their ac- the reactants in a chemical reaction must overcome if tivation energies. This relationship is defined by the reaction is to proceed to the formation of prod- the Arrhenius equation, formulated in 1884 by the ucts. The molecules that are involved must rearrange Swedish scientist Svante Arrhenius (1859–1927), who to form either a transition state or an intermediate received the Nobel Prize in Chemistry in 1903.The 1 Activation energy Principles of Biology ACTIVATION ENERGY DIAGRAM the rate constant k is determined almost solely by the value of e−E/ RT, which can range over several hundred orders of magnitude, de- pending on the relative values of E and T. Activation The course of a reaction depends Energy (E ) a on the relative difference between the energy of the reactants and that A+B of the products. The greater this dif- ference is, the more impetus there is for the reaction to proceed to the formation of products. This is typi- cally illustrated in a plot of energy C+D versus the reaction coordinate, a symbolic representation of the prog- Potential Energy ress of a reaction. In the plot, the energy level of the reactants, on the left, is either higher or lower than Reaction Progress that of the products, on the right. In Arrhenius equation relates the rate constant of a re- between, a curved line rises from the energy level of action to its activation energy and the absolute tem- the reactants to a maximum value before falling to perature and has the form the energy level of the products. The difference be- tween the energy level of the reactants and this peak k = Ae−E/RT energy value represents the activation energy for the reaction, while the difference between the energy where k is the rate constant for the reaction or pro- levels of the reactants and the products represents cess; A is the pre-exponential factor, also known in the energy released in the reaction, also called its some cases as the frequency factor; E (or Ea) is the enthalpy. activation energy for the reaction or process; R is The specific rate of any individual reaction is de- the gas constant; T is the absolute temperature; termined by its activation energy. However, in mass and the mathematical constant e is the base of the quantities, the energy differences between reactants natural logarithm, so that the natural logarithm and products in the system also play a role. This can (ln) of e is equal to 1.The Arrhenius equation has be understood by considering the Boltzmann frac- been found to apply not only to chemical reactions tion e−E/RT, which describes the fraction of mol- but to physical processes as well. T he relationship ecules in the system having energy greater than E. can be most clearly seen by plotting experimen- As energy is released from several reactions, the frac- tally determined logarithmic values of k against tion of molecules present at any given time with suf- the inverse of the absolute temperature, 1/T. This ficient energy to react increases, and more reactions results in a straight line plot, from which the activa- can occur in any given time period. Each reaction re- tion energy of the reaction or process can be cal- quires the same activation energy, and the amount of culated. The pre-exponential factor A is identified energy that is available in the system to permit reac- as the value that the specific rate constant k would tions to occur may be anything from “barely enough” have if the activation energy E were zero (a sponta- to “excessive.” neous reaction).In that special case, the exponent The activation energy of a reaction can be greatly −E/RT would also be equal to zero, making e−E/RT reduced by the inclusion of a catalyst, a material that equal 1 and thus causing the value of k to be equal takes part in the reaction mechanism but is not con- to A. For different specific reactions, the value of sumed in the reaction. Catalysts function by forming A ranges over several orders of magnitude, but an activated complex with the reactants, typically 2 Principles of Biology Activation energy CATALYZED ACTIVATION ENERGY sufficient to drive many instances of the reaction in the gas mixture, Uncatalyzed Reaction Catalyzed Reaction with each subsequent occurrence releasing an equal amount of en- ergy as the enthalpy of reaction. The second example is the so- a ) (E ) a called thermite reaction, in which (E iron oxide and aluminum metal react to produce aluminum oxide and iron metal. This is a spectacular A+B reaction often demonstrated for Uncatalyzed Activation Energy Z chemistry exhibitions. Because the Catalyzed Activation Energy activation energy of the thermite reaction is very high, the reaction is very difficult to initiate and must typ- C+D ically be ignited by a burning piece of magnesium metal; once it has begun, Potential Energy however, it is essentially impossible to stop it due to the amount of energy Reaction Progress that is released. Typically, the iron metal falls out of the reaction mix- ture as a white-hot liquid. constraining the reactant molecules in an orientation that they would otherwise have to achieve through Activation Energy in Biological Systems collision with each other. This reduces the energy Activation energy applies to biochemical processes necessary to achieve that particular orientation—that as well as to physical processes. The chirping of is, the activation energy—so that the reaction can pro- crickets, for example, is dependent on tempera- ceed. When the reactant molecules are constrained in ture in a manner that is in complete accord with the activated complex with the catalyst, bonds between the Arrhenius equation. In biological systems, the certain atoms are weakened and the orientations of activation energy of processes is made a great deal atomic and molecular orbitals that must interact are lower by the catalytic action of protein molecules often brought into the proper alignment, or trajectory, called enzymes. Enzymes have well-defined three- for the new bonds to form between atoms. dimensional structural shapes that allow them to coordinate with other molecules in specific ways, Activation Energy in Action rather like the way a key works with a lock. The co- The activation energy of a reaction can range from ordination normally alters the three-dimensional exceedingly small to very large. Two examples shape of the substrate molecule or otherwise inter- serve to illustrate this point.
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