activation energy

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activation energy,

in chemistry, minimum energyenergy,
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Energy, Activation


the difference between the values of the average energy of particles (molecules, radicals, ions) that participate in an elementary act of a chemical reaction and the average energy of all the particles in the reacting system.

Activation energy varies widely for different chemical reactions, from a few joules per mole to about 10 J/mole. For the same chemical reaction, the value of the activation energy depends on the form of the distribution functions of the molecules with respect to the energies of their translational motion and with respect to the internal degrees of freedom (electronic, vibrational, rotational). Activation energy should be differentiated as a statistical quantity from the threshold energy, the minimum energy that must be attained by a pair of colliding particles for a given elementary reaction to occur.

According to concepts of the theory of absolute reaction rates, activation energy is the difference between the average energy of activated complexes and the average energy of the initial molecules.

The idea of activation energy originated in the 1870’s and 1880’s as a result of research by J. Van’t Hoff and S. Arrhenius regarding the influence of temperature on the rate of a chemical reaction. The constant k of the rate of a reaction is related to the activation energy E by the Arrhenius equation:

k = k0e–E/RT

where R is the gas constant, T is the absolute temperature in °K, and k0 is a constant called the pre-exponential, or frequency, factor of the rate constant. The equation, based on the molecular-kinetic theory, was later derived in statistical physics with consideration of a number of simplifying assumptions, one being that activation energy is independent of temperature. This assumption is valid for practical purposes and for theoretical calculations over comparatively narrow temperature ranges.

Activation energy can be found from experimental data by several methods. In one technique, the kinetics of a reaction are studied at different temperatures (for information on the various methods, seeREACTION RATE, CHEMICAL). A graph is plotted in coordinates of In k versus 1/T; according to the Arrhenius equation, the slope of the straight line on the graph is equal to E. For single-space reversible reactions, the activation energy of the reaction in the forward or reverse direction can be calculated if the activation energy of the reaction in the other direction is known, and the temperature dependence of the equilibrium constant is obtained from thermodynamic data. The temperature dependence of activation energy must be considered for more exact calculations.

The activation energy of complex reactions is a combination of the activation energies of the elementary stages. Sometimes the concept of “apparent” activation energy is used in addition to the true activation energy, determined from the Arrhenius equation. For example, if the rate constants of heterogeneously catalytic reactions are determined from changes in the volumetric concentrations of the initial substances and the products, then the apparent activation energy differs from the true value by the magnitude of the thermal effects that accompany processes of adsorption and desorption of the reacting substances on the surface of the catalyst. Determination of the activation energy is a very complex problem in nonequilibrium systems, for example, in plasma chemical systems. However, the formal application of the Arrhenius equation is possible in some cases.

The concept of activation energy is the most important concept of chemical kinetics; values are listed in special handbooks, and they are used in chemical technology to calculate the rate constants of reactions under various conditions.


activation energy

[‚ak·tə′vā·shən ′en·ər·jē]
(physical chemistry)
The energy, in excess over the ground state, which must be added to an atomic or molecular system to allow a particular process to take place.
References in periodicals archive ?
seems to increase, whereas the value of the activation enthalpy [DELTA]H seems to decrease.
The shift of the [Alpha] peak can be approximated by an Arrhenius equation: the corresponding apparent activation enthalpy is 4.
Starkweather (13) has proposed a criterion for identifying localized and delocalized movements: for localized movements, the activation entropy is negligible so that the activation enthalpy is given by:
From Eq 2, the activation enthalpy is then given by:
Therefore the low value of the activation enthalpy [E.
Application to tensile and compressive tests under a confinement pressure P is developed with the aim of showing how the strain-rate law can be determined from measurements of yield characteristics (yield stress, activation enthalpy and volumes).
The properties that define this arrangement are the activation enthalpy, the activation volume, and the entropy.
Under these conditions, polycarbonate behaves like a highly viscous Newtonian fluid and the activation enthalpy is 413 kJ/mole.
The activation enthalpy was calculated from the area in the plot of loss modulus, E[double prime], vs.