Chemistry - A Molecular Science

Chapter 9 Reaction Energetics

defined by the three hydrogen atoms. In order for the hydroxide ion to reach the carbon atom, the hydrogen atoms must be pushed ap

art; that is, the H-C-H bond angles must open

up from their tetrahedral values. Figure 9.6 shows the potential energy of the system along the reaction coordinate, which is some comb

ination of C-I and C-O distances and H-C-H

bond angles. Initially, the OH

1- ion is far from the CH

I molecule, and the bonds of the 3

two reactants dictate the energy. As the OH

1- ion enters the plane of the three H atoms, it

forces them apart, the C-I bond begins to le

ngthen and weaken, and the C-O bond begins

to form. As a result of all of these changes,

the energy rises sharply. When the H-C-H

bond angles reach 120

o and the CH

unit is planar, the energy is at a maximum, and a very 3

unstable species, [ICH

(OH)] 3

1-, has formed.

C

H

I

H

H

1-I+

+OH

1-

transition

state

H HH

C

OH

E (f)a

E (r)a

I

H

H

H

OH

C

1-

DE

Energy

Reaction Coordinate

Figure 9.6 Reaction energy diagram for CH

I + OH 3

1-^

There are no gases involved in the reaction, so

ΔH =

ΔE.

The species present at the energy maximum is called the

transition state

for the

reaction because it is the species through which

the reactants make the transition to the

products. The energy required to form the transition state from the reactants is known as the

activation energy

for the forward reaction, E

(f), and the energy required to reach the a

transition state from the products is the activation energy for the reverse reaction, E

(r). a

Once formed, the transition state immediat

ely proceeds to lower energy by either

continuing to form products or by returning

to reactants. Either direction is equally

probable. Thus, we can represent the reaction as

1-

1-

1-

333

ICH + OH

[ICH OH]

I + H COH

UU

The double arrows (

U

) indicate that the process can proceed through the transition state

from either direction;

i.e.

, [ICH

(OH)] 3

1-

can be produced by collisions between ICH

and 3

OH

1- or by collisions between I

1- + H

COH 3

.

In order for a reaction to take place, the collision between reactants must be

sufficiently energetic to form the transition state. Although the reaction we are discussing is exothermic, it will not occur if th

e energy of the collision between OH

1- and CH

I is less 3

than the activation energy for the reaction.

Remember that thermal energy (~RT) is only

~3 kJ/mol at room temperature. Consequently,

many reactions must be heated to get them

started even if they are exothermic. For example, gasoline reacts spontaneously with oxygen in a very exothermic reaction. Yet the reaction does not occur in the absence of a flame or spark to provide the energy require

d to overcome the activation energy for the

reaction. Thus, the activation energy is a barri

er that must be overcome if reaction is to

take place. Consequently, the activation energy is often referred to as an

energy barrier

.

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State

University