REACTIONS: THERMODYNAMIC ASPECTS, KINETICS, AND CATALYSIS - Biomass as a Sustainable Energy Source for the Future - page 134

(a) (b)

T 1

T 2

T 3

E 1 E a E E a E

FIGURE 5.1 Maxwell-Boltzmann distribution and activation energy: (a) number of particles
with an activation energy greater than E a and (b) temperature influence (Source: Reproduced
with permission from Pasquetto and Patrone, 1999, vol. 1. © Zanichelli editore S.p.A).

E a ( → ) XY E a ( → ) YX

Y

Δ r H

X

Reaction path

FIGURE 5.2 Relationship between the activation energy (E a ) and reaction enthalpy ( Δ r H),
plotted versus the reaction coordinate. The highest energy position (peak position) represents
the transition state.

increases) and flattens (as the area under the distribution is constant). This shows the
huge effect of the temperature on the reaction rate, which can double or even triple
with a temperature increase of only 10 C.
When two molecules with a kinetic energy equal to or greater than the activation
energy collide, they reach a higher energy content (peak position of Figure 5.2) called
transition state in which the bonds of the molecules are instable.
As shown in Figure 5.2, the energy of this state is greater than both the energy
of the reactants and the energy of the products. The difference between the
energy of the reactants and the energy of the products is the standard enthalpy
of reaction.