Organic Chemistry

126 CHAPTER 3 Alkenes• Thermodynamics and Kinetics

Thermodynamics

The field of chemistry that describes the properties of a system at equilibrium is called

thermodynamics. The relative concentrations of reactants and products at equilibri-

um can be expressed numerically as an equilibrium constant, (Section 1.17). For

example, in a reaction in which mmoles of A react with nmoles of B to form smoles

of C and tmoles of D, is equal to the relative concentrations of products and reac-

tants at equilibrium.

The relative concentrations of products and reactants at equilibrium depend on their

relative stabilities:The more stable the compound, the greater is its concentration at

equilibrium. Thus, if the products are more stable (have a lower free energy) than the

reactants (Figure 3.3a), there will be a higher concentration of products than reactants

at equilibrium, and will be greater than 1. On the other hand, if the reactants are

more stable than the products (Figure 3.3b), there will be a higher concentration of re-

actants than products at equilibrium, and will be less than 1.

Several thermodynamic parameters are used to describe a reaction. The difference

between the free energy of the products and the free energy of the reactants under stan-

dard conditions is called the Gibbs free-energy change The symbol °indi-

cates standard conditions—all species at a concentration of 1 M, a temperature of

25 C°, and a pressure of 1 atm.

From this equation, we can see that will be negative if the products have a lower

free energy—are more stable—than the reactants. In other words, the reaction will

¢G°

¢G°=(free energy of the products)-(free energy of the reactants)

1 ¢G° 2.

Keq

Keq

n B

[products]

[reactants]

m A  s C t D

Keq 

[C]s [D]t

[A]m [B]n

Keq

Keq

The more stable the compound, the

greater is its concentration at

equilibrium.

Free energy

Progress of the reaction Progress of the reaction

reactants

products reactants

products

a b

−∆G°+∆G°

an exergonic reaction

∆G° is negative

Keq > 1

an endergonic reaction

∆G° is positive

Keq < 1

Figure 3.3N

Reaction coordinate diagrams for

(a) a reaction in which the products

are more stable than the reactants

(an exergonic reaction) and (b) a

reaction in which the products are

less stable than the reactants (an

endergonic reaction).

ABC

Free energy

Progress of the reaction

A B+ A+BC

free energy

of the

reactants

free energy

of the

products

transition state:

the highest

energy state

on the reaction

pathway

C

Figure 3.2N

A reaction coordinate diagram. The

dashed lines in the transition state

indicate bonds that are partially

formed or partially broken.

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