T-2
that would be energetic enough to form an activated complex. If we compare two otherwise identical
reactions, the rate constant will be smaller for the reaction with the larger E
a
(assuming both reactions
take place at the same temperature).
These ideas are illustrated in Figure 1 for the hypothetical elementary reaction:
A + B
C
In this example, molecule A reacts with molecule B to form molecule C. The activated complex
occurs at the peak (where the potential energy is largest). As represented in the diagram, for a reaction
to occur, the molecules A and B must approach each other along the left hand side of the reaction
coordinate, plotted as the abscissa (x-axis), with enough translational energy to roll up the hump to the
activated complex at the peak.
FIGURE 1
Potential
Energy
(kJ/mol)
E
a
Reaction Coordinate for A + B
C
Energy Diagram for A + B
C
A + B
C
H
rxn
(reactants)
(products)
The activated complex can undergo further change to form molecule C. Most of the activation
energy, E
a
, is redistributed in forming the molecule of C. In Figure 1,
H
rxn
is negative. However, for
some reactions
H
rxn
is positive.