The Foundations of Chemistry

The activation energy, Ea,is the kinetic energy that reactant molecules must have to
allow them to reach the transition state. If A and B 2 molecules do not possess the neces-
sary amount of energy, Ea, when they collide, reaction cannot occur. If they do possess
sufficient energy to “climb the energy barrier” to reach the transition state, the reaction
can proceed. When the atoms go from the transition state arrangement to the product
molecules, energy is released.If the reaction results in a net releaseof energy (see Figure
16-10a), moreenergy than the activation energy is returned to the surroundings and the
reaction is exothermic. If the reaction results in a net absorptionof energy (see Figure 16-10b),
an amount less than Eais given off when the transition state is converted to products and
the reaction is endothermic. The netrelease of energy is Erxn.
For the reverse reaction to occur, some molecules on the right (AB) must have kinetic
energy equal to the reverse activation energy, Ea reverse, to allow them to reach the tran-
sition state. As you can see from the potential energy diagrams in Figure 16-10,

Ea forwardEa reverseEreaction

As we shall see, increasing the temperature changes the rate by altering the fraction of
molecules that can get over a given energy barrier (Section 16-8). Introducing a catalyst
increases the rate by providing a different pathway that has a lower activation energy
(Section 16-9).
As a specific example that illustrates the ideas of collision theory and transition state
theory, consider the reaction of iodide ions with methyl chloride.

I
CH 3 Cl88nCH 3 I
Cl

Many studies have established that this reaction proceeds as shown in Figure 16-11a. The
Iion must approach the CH 3 Cl from the “back side” of the CXCl bond, through the
middle of the three hydrogen atoms. A collision of an Iion with a CH 3 Cl molecule from
any other direction would not lead to reaction. But a sufficiently energetic collision with

Figure 16-11 (a) A collision that

could lead to reaction of I


CH 3 Cl to give CH 3 I
Cl. The I

must approach along the “back side”

of the CXCl bond. (b) Two

collisions that are not in the

“correct” orientation to cause a

reaction.

I

I–

I–

I– Cl–

Before collision Transition state After reaction

Cl Cl I

Cl Cl

(a)

(b)

Remember that Erxnrelates product

energy to reactant energy, regardless of

the pathway. Erxnis negative when

energy is given off; Erxnis positive

when energy is absorbed from the

surroundings.

The CH 3 Cl and CH 3 I molecules each

have tetrahedral molecular geometry.