Peter Keusch, University of Regensburg
German version
I should have recommended something simpler."
Alphonso X, the Wise of Spain (1223-1284)
Albert Einstein
 | Both the Arrhenius and the Eyring equation describe the temperature dependence of reaction rate. Strictly speaking, the Arrhenius equation can be applied only to the kinetics of gas reactions. The Eyring equation is also used in the study of solution reactions and mixed phase reactions - all places where the simple collision model is not very helpful. The Arrhenius equation is founded on the empirical observation that rates of reactions increase with temperature. The Eyring equation is a theoretical construct, based on transition state model. |
The bimolecular reaction
According to the transition state model, the reactants are getting over into an unsteady intermediate state (AB ‡) on the reaction pathway:
E: Potential energy Transition state: Maximum of energy in the path way |
There is an 'energy barrier' on the pathway between the reactants (A, B) and the product (C). The barrier determines a 'threshold energy' or minimum of energy necessary to permit the reaction to occur. It is called 'activation enthalpy' ('activation energy').
Fig. 1 shows the energy of the molecules along the reaction coordinate which measures the progress of the reaction. Along the flat region at the left, the particles are approaching each other. They possess kinetic energy and their potential energy is constant. The beginning of the rise in the curve signifies that the two molecules have enough energy to have an effect on each other. During the approach, the particles slow down as their kinetic energies furnish the potential energy to climb the curve. If the reacting particles possess sufficient energy they can ascend the left side of the 'barrier' all the way up to the summit. Attaining of the summit can be interpreted as follows: The approaching reactant molecules had sufficient kinetic energy to overcome the mutual repulsive forces between the electron clouds of their constituent atoms and thus come very close to each other. An 'activated complex' AB ‡ or 'transition state' is formed at the potential energy maximum. The high-energy complex represents an unstable molecular arrangement, in which bonds break and form to generate the product C or to degenerate back to the reactants A and B. Once the energy barrier is surmounted, the reaction proceeds downhill to the product. Principles of the transition state theory: - There is a thermodynamic equilibrium between the transition state and the state of reactants at the top of the energy barrier. - The rate of chemical reaction is proportional to the concentration of the particles in the high-energy transition state. |
The process if the reaction (1) can be divided into two stages (2):
· a direct step downhill from the transition state to the product
· the equilibrium between the reactants and the transition state.
The change in the concentration of the complex AB ‡ over time can be described by the following equation:
· Due to the equilibrium between the 'activated complex' AB ‡ and the reactants A and B, the components k1 · [A] · [B] and k-1 · [AB ‡] cancel out. Thus the rate of the direct reaction is proportional to the concentration of AB ‡:
k2 is given by statistical mechanics:
kB = Boltzmann's constant [1.381·10-23 J · K-1]
T = absolute temperature in degrees Kelvin (K)
h = Plank constant [6.626·10-34 J · s]
k2 is called 'universal constant for a transition state' (~ 6 · 10-12 sec-1 at room temperature).
· Additionally, [AB ‡] can be derived from the pseudo equilibrium between the transition state molecule AB ‡ and the reactant molecules by application of the mass action law:
K ‡ = thermodynamic equilibrium constant
Due to the equilibrium that will be reached rapidly, the reactants and the activated complex decrease at the same rate. Therefore, considering both equation (5) and (6), equation (4) becomes:
Comparing the derived rate law (1) with expression (7) yields for the rate constant k of the overall reaction
· Additionally, thermodynamics gives a further description of the equilibrium constant:
DG ‡ = free activation enthalpy [kJ · mol-1]
DS ‡ = activation entropy [J · mol-1 · K-1]
DH ‡ = activation enthalpy [kJ · mol-1]
Figure 2: Enthalpie of activation |
DH ‡ is the difference between the enthalpy of the transition state and the sum of the enthalpies of the reactants in the ground state. It is called activation enthalpy (Fig. 2).
S is for the entropy, the extent of randomness or disorder in a system. The difference between the entropy of the transition state and the sum of the entropies of the reactants is called activation entropy DS ‡. DG ‡ is the Gibb's free energy change. According to equation (10) DG‡ is equal to the change in enthalpy DH ‡ minus the product of temperature T (which is in kelvin) and the change in entropy DS ‡ of the chemical system. DG ‡ may be considered to be the driving force of a chemical reaction. DG ‡ determines the spontaneity of the reaction. DG ‡ = 0 Þ system at equilibrium, no net change occurs DG ‡ > 0 Þ reaction is not spontaneous |
Combining equation (9) with expression (10) and solving for lnk yields:
The Eyring equation is found by substituting equation (11) into equation (8):
Figure 3: Determination of DH ‡ |
A plot of ln(k/T) versus 1/T produces a straight line with the familiar form y = -mx + b (Fig. 3), where
x = 1/T y = ln(k/T) m = - DH ‡/ R b = y (x = 0) DH ‡ can be calculated from the slope m of this line: DH ‡ = -m · R . |
From the y-intercept
DS ‡ can be determined and thus the calculation of DG ‡ for the appropriate reaction temperatures according to equation (10) is allowed.
A comparison between the Arrhenius equation
and the Eyring equation (13) shows, that lnA and DS ‡ on the one hand and Ea and DH ‡ on the other hand are analogous quantities. The two energies are therefore frequently used interchangeably in the literature to define the activation barrier of a reaction. The activation energy Ea is related to the activation enthalpy DH ‡ as follows
· low values of Ea and DH ‡ Þ fast rate
· high values of Ea and D H ‡ Þ slow rate
The typical values of Ea and DH ‡ lie between 20 and 150 [kJ / mol].
The study of the temperature dependence supplies the above all mechanistically important values lnA or DS ‡, equivalent in their mechanistical significance. lnA- and DS ‡-values are sensible sensors. They give informations about the degree of order in the transition state.
· low values of lnA correspond to large negative values of DS ‡ (unfavorable)
The activated complex in the transition state has a more ordered or more rigid structure than the reactants in the ground state. This is generally the case if translational, rotational, and vibrational degrees of freedom become 'frozen' on the route from the initial to the transition state. The reaction rate is slow.
· high values of lnA correspond to positive values (less negative values) of DS ‡ (favorable)
A positive value for entropy of activation indicates that the transition state is highly disordered compared to the ground state. Degrees of freedom are liberated in going from the ground state to the transition state, which, in turn, increase the rate of the reaction.
Although the determination of the activation parameters must be performed accurately, it should not pretend an excessive accuracy. The values of the activation energy and activation enthalpy are rounded to one decimal place. The value of activation entropy is basically written in whole numbers. Values of entropies DS ‡ ± 10 are written to one decimal place of accuracy. The value of lnA shall be expressed with an accuracy of two decimal places.
A precise determination of the activation enthalpy (and the other acivation parameters) requires at least three different rate constants. This means three kinetic runs at different temperatures are carried out. The temperature intervals should be at least 5°C. If the data points in the plot of ln(k/T) versus 1/T (Fig. 3) do not lie exactly on a straight line, a linear regression analysis providing the 'line of best fit' will not increase the accuracy. If the plotted points deviate significantly from the straight line, the rate constant should be determined at a further reaction temperature, since each of the three data points can be 'wrong'. Basically, it recommends to increase the accuracy of the measured values by improvement of the measuring method (accurate thermostating of the reaction mixture).
Sometimes the data points are on a curve concave or convex toward the abscissa axis (1/T axis) (Fig. 4).
Figure 4: Concave and convex Eyring Plot In the broad field of kinetics, not restricting consideration to enzyme kinetics, when nonlinear Arrhenius or Eyring plots can be observed, they are almost always concave. A concave Arrhenius or Eyring plot can be attributed to several factors. The most common interpretation is that at least two different rate-limiting reaction steps are involved. Convex Arrhenius and Eyring plots are observed in experiments on enzyme catalyzed reactions involving two competing enzymatic forms, each dominating in a different temperature range. A convex Arrhenius or Eyring plot means that Ea and DH ‡, respectively, decrease with increasing temperature. |
Chemical Kinetics
Kinetics: Characterization of Transition States
Rate Law and Stoichiometry
Convex Arrhenius plots and their interpretation
