Chemistry 1011
TOPIC
Gaseous Chemical Equilibrium
TEXT REFERENCE
Masterton and Hurley Chapter 12
Chemistry 1011 Slot 5
1
12.5 Effect of Changes in Conditions
Upon an Equilibrium System
YOU ARE EXPECTED TO BE ABLE TO:
• Define Le Chatelier’s Principle.
• Use Le Chatelier’s Principle to predict qualitatively the
effect on an equilibrium system of changes in:
–
–
–
–
concentration (partial pressure) of individual components
total pressure of the system at constant volume
volume of the system
total thermal energy of the system
• Predict the effect on an equilibrium of adding a catalyst
• Describe industrial processes for the manufacture of
ammonia and sulfur trioxide
Chemistry 1011 Slot 5
2
Le Chatelier’s Principle
• A chemical equilibrium can be disturbed by
changing the external conditions
– Changing the pressure or volume
– Adding or removing a component
– Changing the temperature
• When an external change is made to an
equilibrium system, the system will alter so
as to oppose the change
Chemistry 1011 Slot 5
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Changing the Pressure or Volume
• Changing the pressure or volume of a system will
result in compression or expansion
• If possible, the system will change and the
equilibrium will shift so as to oppose the
compression or expansion
• This can only occur if the total number of moles or
product is different from the total number of moles
of reactant
Chemistry 1011 Slot 5
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Compressing the N2O4 – NO2
Equilibrium System
N2O4(g)
2NO2(g)
• Compressing the equilibrium system by
reducing the volume will increase the pressure
• The system will shift so as to reduce the
pressure
• The reverse reaction will take place since this
results in a decrease in the total number of
molecules
2NO2 (g)
N2O4(g)
Chemistry 1011 Slot 5
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Effect of Pressure on Equilibrium
Position
Compression
N2O4(g)
N2(g) + 3H2(g)
H2(g) + I2(g)
effect
N2(g) + O2(g)
Expansion
2NO2(g)
2NH3(g)
2HI(g)
2NO(g)
no effect
no effect
Chemistry 1011 Slot 5
no
no effect
6
Adding or removing a Gaseous
Component
• Adding a gaseous reactant or product to an
equilibrium system will disturb the equilibrium
• The system will shift so as to remove the
added species
• Removing a gaseous reactant or product from
an equilibrium system will disturb the
equilibrium
• The system will shift so as to replace the
removed species Chemistry 1011 Slot 5
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Modifying the N2O4 – NO2 Equilibrium
System by Adding/Removing Components
N2O4(g)
2NO2(g)
• Adding more N2O4 - reaction occurs in forward
direction
• Adding more NO2 - reaction occurs in reverse
direction
• Removing N2O4 - reaction occurs in reverse direction
• Removing NO2 - reaction occurs in forward direction
Chemistry 1011 Slot 5
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Confirming Le Chatelier’s Principle
• A determination of the reaction quotient
immediately after adding (or removing) a
gaseous component will confirm Le Chatelier’s
Principle
• For N2O4(g)
2NO2(g)
Kp = (PNO2)2/PN2O4
Adding NO2 will raise PNO2 and lower PN2O4
Q will be >Kp Reverse reaction will occur
Chemistry 1011 Slot 5
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Changing the Temperature
• Changing the temperature of a system will disturb
the equilibrium
• The system will change and the equilibrium will
shift so as to oppose the change in temperature
• If the temperature is raised, the reaction will
proceed in the endothermic direction until a new
equilibrium is reached at a higher temperature
• If the temperature is lowered, the reaction will
proceed in the exothermic direction until a new
equilibrium is reached at a lower temperature
Chemistry 1011 Slot 5
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Modifying the N2O4 – NO2 Equilibrium
System by Changing the Temperature
• The reaction
N2O4(g)
(colourless)
•
•
•
•
2NO2(g)
DHo = +57.2kJ
(brown)
is endothermic in the forward direction
An increase in temperature will cause the forward reaction to
take place in order to absorb the added heat (Le Chatelier)
A new equilibrium will be established at the higher
temperature
PNO2 will be greater; PN2O4 will be less
The gas mixture will become more brown
Chemistry 1011 Slot 5
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Confirming Le Chatelier’s
Principle
• The van’t Hoff equation relates the values
of the equilibrium constant for a reaction at
different temperatures to the value of DHo
K2 = DHo 1 - 1
ln
K1
R T1 T2
• If DH is +ve, then
K2 is smaller than K1 if T2 > T1
Chemistry 1011 Slot 5
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Effect of Changes in Conditions
Upon an Equilibrium System
• If the number of reactant molecules is different
from the number of product molecules, changing
the total pressure at equilibrium will change the
equilibrium composition. Kp WILL NOT change
• Adding or removing a gaseous reactant or product
species will change the equilibrium composition.
Kp WILL NOT change
• Changing the temperature will change the
equilibrium composition. Kp WILL change
Chemistry 1011 Slot 5
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Effect of Catalysts on
Equilibrium
• Adding a catalyst will not alter the
equilibrium concentrations of reactants or
products. Kp WILL NOT change
• Adding a catalyst WILL result in a reaction
reaching equilibrium more quickly
Chemistry 1011 Slot 5
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Applying Le Chatelier’s Principle
– The Haber Process
N2(g) + 3H2(g)
2NH3(g) DH = -92kJ
2
(P
)
NH3
= 6.0 x 105 at 25oC
Kp =
PN2 x (PH2)3
• The number of product molecules is 2, the number
of reactant molecules is 4
• The forward reaction is exothermic
– The value of Kp decreases as temperature rises
– At 227oC Kp = 0.10
• The activation energy for the forward reaction is
>150kJ
Chemistry 1011 Slot 5
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Choosing the Best Conditions
• At 25oC the equilibrium favours NH3, but at 25oC the
reaction rate is almost zero
• High temperatures are required in order to have a
reasonable number of reactant molecules with energy >
activation energy
• While the rate will increase at higher temperatures, the
equilibrium yield of ammonia will be lower
• Raising the pressure both favours a higher equilibrium
yield of ammonia and increases the rate
• Adding a catalyst will result in a lower activation energy
Chemistry 1011 Slot 5
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The Haber Process Compromise
•
•
•
•
•
Moderate temperature – 450oC
High pressure – 200 to 600 atm
Carefully selected catalyst
Extra nitrogen
Reactants recycled as ammonia removed
from system
Chemistry 1011 Slot 5
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Applying Le Chatelier’s Principle
– The Contact Process
Sulfur is burned in air
S(s) + O2(g)
SO2(g)
Sulfur dioxide is reacted with more oxygen
using a catalyst
SO2(g) + 1/2O2(g)
SO3(g) DH = -98.9kJ
Sulfur trioxide is reacted with water
SO3(g) + H2O(l)
H2SO4(l)
Chemistry 1011 Slot 5
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The SO2 - SO3 Equilibrium
• The forward reaction is exothermic – higher
temperatures favour reactants, low
temperatures preferred
– (at 200oC Kp = 1.0 x 106; at 600oC Kp = 10)
• Low temperatures result in very low rates high temperatures are required if reactant
molecules are to overcome the actvation
energy barrier
• High pressures favour products and result in
faster rates
Chemistry 1011 Slot 5
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The Contact Process Compromise
• Temperature not so high as to favour reactants, but
high enough to result in rapid rate
• Use of a carefully selected catalyst
– Pass reactant mixture over catalyst beds at moderate
temperatures – 450oC to 600oC
– First pass at high temperature (600oC) results in rapid
attainment of equilibrium with 80% conversion of to
– Second pass at results in 99% conversion
• (Note: SO3 will not react with water! It must be
dissolved in concentrated H2SO4. The resulting
mixture is then diluted)
Chemistry 1011 Slot 5
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