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4. (14pt) At the room temperature (25
°
C), ethene can be hydrogenated (add one mole of H
2
to the
double bond) to give ethane in the presence of a catalyst, as shown below:
It is known from the experiment that
D
G
°
=
-
30kcal/mol. Answer the following questions:
(A) Calculate the equilibrium constant for this reaction if there is sufficient data. If
there is not enough data to allow for such calculation, simply state so.
D
G
°
=
-
RTlnK, since we know R is gas constant = 1.98 cal mol
-1
k
-1
, T is absolute temperature, and
D
G
°
is a given number, we can calculate K. It is important to pay attention to the unit here! R's unit
is cal but
D
G
°
is usually given as kcal!
(B) Predict the sign of this
D
S
°
for this reaction. Briefly explain your reasoning.
Entropy measures degree of chaotic. Since two molecules combine to give one, the entropy has
decreased, thus the change is negative.
(C) Predict the sign of
D
H
°
for this reaction. Explain briefly how you arrive at this
conclusion.
D
G
°
=
D
H
°
-T
D
S
°
. It then follows that
D
H
°
for this reaction is negative.
C C
H
H
H
H
+ H
2
catalyst
25
°
C
C C
H
H
H
H
H
H
ethene
ethane