The Foundations of Chemistry

A
B 2 88nAB
B

Figure 16-10 shows plots of potential energy versus the progress of the reaction. In

Figure 16-10a the ground state energy of the reactants, A and B 2 , is higher than the ground

state energy of the products, AB and B. The energy released in the reaction is the differ-

ence between these two energies, E.It is related to the change in enthalpy, H^0 rxn(Section

15-11).

Quite often, for reaction to occur, some covalent bonds must be broken so that others

can be formed. This can occur only if the molecules collide with enough kinetic energyto

overcome the potential energy stabilization of the bonds. According to the transition

state theory,the reactants pass through a short-lived, high-energy intermediate state,

called a transition state,before the products are formed.

A
BXB88n A,B,B 88nAXB
B

reactants transition state products

A
B 2 AB 2 AB
B

The “progress of reaction” represents
how far the reaction has proceeded
along the pathwayleading from
reactants to products. This coordinate
is sometimes labeled the “reaction
coordinate.”

678 CHAPTER 16: Chemical Kinetics

Progress of reaction

A - - - B - - - B

Ea

forward

reaction

Products

AB + B

∆Erxn

Reactants

A + B 2

(a)

Progress of reaction

A - - - B - - - B

Ea

reverse

reaction Products

AB + B

Reactants ∆Erxn

EnergyA + B^2

(b)

Endothermic reaction

Exothermic reaction

Energy Ea

reverse

reaction

Ea

forward

reaction

Figure 16-10 A potential energy
diagram. (a) A reaction that releases
energy (exothermic). An example of
an exothermic gas-phase reaction is

H
I 2 88nHI
I

(b) A reaction that absorbs energy
(endothermic). An example of an
endothermic gas-phase reaction is

I
H 2 88nHI
H