Environmental Engineering Reference
In-Depth Information
For example, the forward reaction of the reversible reaction (RX. 5.21) is exothermic
with
mol −1 ; heat is a product: N 2 (g) + 3H 2 (g) !
2NH 3 (g) + 92 kJ.
If we lower the temperature, the equilibrium shifts to produce more heat. Since the
formation of ammonia is exothermic, this favors the production of more ammonia.
So, if we increase the temperature of a reaction at equilibrium, to minimize the effect
of this change, the equilibrium shifts to the side where the heat is absorbed, to the right if
the reaction is endothermic, and to the left if it is exothermic. For exothermic reactions,
an increase in temperature decreases the value of the equilibrium constant, K eq ,whereas
for endothermic reactions, an increase in temperature increases the K eq value.
Effect of Change in Pressure : A change in pressure due to a change in volume of a
gaseous system leads to a shift of the equilibrium. According to Avogadro
Δ r H=
92 kJ
s principle,
which states that under the same condition of temperature and pressure, equal volumes
of all gases contain the same number of molecules, we can divide gas-phase reactions
in three groups:
'
1. Reactions for which the number of moles of the products is greater than the
amount of moles of the reactants (
Δ
n > 0). According to Avogadro
'
s law, these
kind of reactions take place with a volume increase:
ðÞ !
2CH 4 g
ðÞ
+O 2 g
2CO g
ðÞ
+4H 2 g
ðÞ Δ
n=6
3=3
ð
RX
:
5
:
22
Þ
2. Reactions by which the volume decreases (
Δ
n < 0):
ðÞ !
2NH 3 g
ðÞ Δ
n=2
4=
2
N 2 g
ðÞ
+3H 2 g
ð
RX
:
5
:
21
Þ
Δ
3. Reactions where
n = 0, i.e., without a volume change:
ðÞ !
2HI g
ðÞ Δ
n=2
2=0
H 2 g
ðÞ
+I 2 g
ð
RX
:
5
:
23
Þ
An increase in system pressure due to decreasing volume causes the reaction to shift to
the side with the least moles of gas. A decrease in pressure due to increasing volume
causes the reaction to shift to the side with the most moles of gas. There is no effect on a
reaction where the number of moles of gas is the same on each side of the reaction equa-
tion. So, increasing the pressure by decreasing the volume shifts the equilibrium to the
left for reactions belonging to the first group and to the right for reactions belonging to
the second group and is without any effect for reactions belonging to the third group.
5.3 CATALYSIS
The word catalysis was coined by Berzelius in 1836 to describe the acceleration of
certain chemical reactions. It refers to materials (catalysts) that accelerate chemical
reactions without undergoing changes themselves. This is too optimistic a definition
as the properties of all real catalysts change with use. The definition is also unsatis-
factory because it implies that the acceleration is brought about without direct
 
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