128 CHAPTER 3 Alkenes• Thermodynamics and KineticsThe gas constant Ris thought to be
named after Henri Victor Regnault
(1810–1878), who was commissioned
by the French Minister of Public
Works in 1842 to redetermine all the
physical constants involved in the
design and operation of the steam
engine. Regnault was known for his
work on the thermal properties of
gases. Later, while studying the
thermodynamics of dilute solutions,
van’t Hoff (p. 194) found that R
could be used for all chemical
equilibria.The Gibbs standard free-energy change has an enthalpy component
and an entropy component:The enthalpyterm is the heat given off or the heat consumed during the
course of a reaction. Atoms are held together by bonds. Heat is given off when bonds
are formed, and heat is consumed when bonds are broken. Thus, is a measure of
the bond-making and bond-breaking processes that occur as reactants are converted
into products.If the bonds that are formed in a reaction are stronger than the bonds that are broken,
more energy will be released as a result of bond formation than will be consumed in
the bond-breaking process, and will be negative. A reaction with a negative
is called an exothermic reaction. If the bonds that are formed are weaker than those
that are broken, will be positive. A reaction with a positive is called an
endothermic reaction.
Entropy is defined as the degree of disorder. It is a measure of the freedom
of motion of the system. Restricting the freedom of motion of a molecule decreases its
entropy. For example, in a reaction in which two molecules come together to form a
single molecule, the entropy in the product will be less than the entropy in the reac-
tants because two individual molecules can move in ways that are not possible when
the two are bound together in a single molecule. In such a reaction, will be nega-
tive. In a reaction in which a single molecule is cleaved into two separate molecules,
the products will have greater freedom of motion than the reactant, and will be
positive.PROBLEM 15a. For which reaction will be more significant?- or
- or
b. For which reaction will be positive?
A reaction with a negative has a favorable equilibrium constant;
that is, the reaction is favored as written from left to right because the products are
more stable than the reactants. If you examine the expression for the Gibbs standard
free-energy change, you will find that negative values of and positive values of
contribute to make negative. In other words,the formation of products with
stronger bonds and with greater freedom of motion causes to be negative.PROBLEM 16a. For a reaction with and calculate the
and the equilibrium constant (1)at 30°C and (2)at 150°C.
b. How does change as Tincreases?
c. How does change as Tincreases?Values of are relatively easy to calculate, so organic chemists frequently eval-
uate reactions only in terms of that quantity. However, you can ignore the entropy term
only if the reaction involves only a small change in entropy, because then the
term will be small and the value of ¢H°will be very close to the value of ¢G°.Ignor-T¢S°¢H°Keq¢G°¢G°¢H°=-12 kcalmol-^1 ¢S°=0.01 kcalmol-^1 ,¢G°¢S° ¢G°¢H°¢G° 1 Keq 712¢S°A+B∆C A+B∆C+DA∆B A+B∆C¢S°¢S°=(freedom of motion of the products)-(freedom of motion of the reactants)¢S°¢S°1 ¢S° 2¢H° ¢H°¢H° ¢H°≤H°=(energy of the bonds being broken)-(energy of the bonds being formed)¢H°1 ¢H° 2¢G°=¢H°-T¢S°1 ¢S° 21 ¢G° 2 1 ¢H° 2Entropy is the degree of disorder of a
system.
The formation of products with stronger
bonds and with greater freedom of mo-
tion causes ≤G°to be negative.BRUI03-109_140r4 24-03-2003 11:53 AM Page 128