Le Chatelier's Principle
-
Upload
melodykeys -
Category
Science
-
view
84 -
download
0
Transcript of Le Chatelier's Principle
Chemistry II: Le Chatelier’s Principle Dr. Melody Anak Kimi
Centre for Pre-University Studies
Universiti Malaysia Sarawak
This OpenCourseWare@UNIMAS and its related course materials are licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
Le Chatelier’s Principle
By the end of this topic you should be able to:
explain Le Chatelier’s principle
explain the factors affecting chemical equilibrium
apply Le Chatelier’s principle in predicting the equilibrium
shift
If the external conditions for a reversible reaction
are changed, the equilibrium will shift into a new
equilibrium position so as to maintain the equilibrium
constant for the reaction.
Le Chatelier’s Principle
A
A B
B
C
C
D
D
Le Chatelier’s Principle
External stresses
Presence of catalyst
Change in concentration
of products or reactant
Change in pressure
(gases) including the
presence of noble gases
Change in temperature
(A) Effect Of Catalyst On Equilibrium
increases the rate of
reaction
-provides an alternative reaction mechanism
with lower activation energy, Ea
-reduces the time taken to reach equilibrium
no effect on the values of
the equilibrium constant,
KC or Kp
Only reduces the time to reach KC or Kp
provides a mechanism with
lower Ea, therefore
increasing the value of the
reaction rate constant, k
provides a mechanism with lower Ea,
therefore increasing the value of the reaction
rate constant, k
As shown in the Arrhenius equation:
With lower Ea, the value of k will be higher,
thus increasing the rate of reaction.
RT
Ea
Aek
(A) Effect Of Catalyst On Equilibrium
According to the Le Chatelier’s Principle;
(B) Effect Of Concentration On Equilibrium
Concentration of reactant increases, the equilibrium
position will shift in the direction that reduces the
concentration of this reactant (shift to the right). Thus,
increases the concentration of the product.
Concentration of the product increases, the equilibrium
position will shift in the direction that reduces the
concentration of this product (shift to the left). Thus,
increases the concentration of the reactant.
(B) Effect Of Concentration On Equilibrium
The system is
no longer in
equilibrium
Increase in the
forward rate of
reaction
[H2] and [I2]
decreases while [HI]
increases
New equilibrium position
is established when the
rate of the forward
reaction = the rate of the
reverse reaction
When the new
equilibrium is
established, the
amount of H2 present is
more than the initial
amount
The final position of
the equilibrium (after
the added H2) is
different from the
initial equilibrium
position because the
equilibrium has
shifted to the left
H2 (g) + I2 (g) 2HI (g)
Consider the following
reversible reaction in
equilibrium
Add more
⇌
t1 t2 t3
[H2
] [I2]
[HI
]
Equilibrium
reached at t1
H2 added
at t2
New equilibrium
reached at t3
(B) Effect Of Concentration On Equilibrium
H2 (g) + I2 (g) 2HI (g)
⇌
In the hydrolysis of ethyl ethanoate (an ester),
CH3COOC2H5 (l) + H2O (l) CH3COOH (l) + C2H5OH (l)
ethyl ethanoate ethanoic acid ethanol
the equilibrium position can be shifted to the right (thus, increasing the formation of products) by:
– Removing ethanoic acid
– Removing ethanol
– Increasing the concentration of ethyl ethanoate
(B) Effect Of Concentration On Equilibrium
⇌
Example
Consider the following equilibrium system:
Ag+(aq) + Fe2+(aq) Ag(s) + Fe3+(aq)
a) Write the expression for the equilibrium constant, KC.
b) State what happens when the following changes are made after the system has reached equilibrium: i. Adding more aqueous silver nitrate
ii. Adding more iron(III) nitrate
iii. Adding more silver
⇌
Example
a) Kc = [Fe3+]
[Ag+][Fe2+]
b) The following changes occur:
i. the equilibrium shifts to the right and more Ag and
Fe3+ will be produced.
ii. the equilibrium shifts to the left and more Ag+ and
Fe2+ are produced.
iii. there is no net change in equilibrium position
because Ag does not appear in the equilibrium
expression.
For equilibrium systems involving gases
Effect of Pressure on Equilibrium
increase in pressure increase the rate of reaction
(exothermic or endothermic)
change in pressure will
affect the position of
equilibrium
provided the reaction involves an overall
change in volume (i.e., the number of moles
of gaseous reactants is different from the
number of moles of gaseous products)
change in the
pressure
does not affect the equilibrium
constant, (Kc or Kp)
Effect of Pressure on Equilibrium
Three methods of changing the pressure of an equilibrium system at a
constant temperature:
Adding or removing a gaseous
reactant or product equivalent to the effect of changes in
concentration on equilibrium
Changing the volume of the
system
total pressure decreased by
increasing the volume of the system,
or increased by decreasing the
volume of the system
Adding noble gas to the system
increases the pressure of the system
without changing the volume BUT
does not affect the equilibrium
constant, KP
When changes to the pressure occur by changing the
volume of the system,
Le Chatelier’s Principle predicts the following:
Effect of Pressure on Equilibrium
total pressure on the
equilibrium
reaction takes place in the
direction that decreases the
total number of moles of gas
total pressure on the
equilibrium
reaction takes place in the
direction that increases the
total number of moles of gas
When the piston is pushed downwards after the system reached equilibrium;
– Total system pressure increases.
– This pressure must be lowered to reach a new equilibrium.
– This condition favours the direction which produces a smaller total number of mols.
– The equilibrium position shifts to the left & more N2O4 are produced.
Effect of Pressure on Equilibrium
N2O4 2NO2 ⇌
When the piston is pulled upwards after the system reached equilibrium;
– Total system pressure decreases.
– This pressure must be increased to reach a new equilibrium.
– This condition favours the direction which produces a higher total number of mols.
– The equilibrium position shifts to the right& more NO2 are produced.
Effect of Pressure on Equilibrium
N2O4 2NO2 ⇌
Example
Explain the effect of pressure on the following equilibrium
system:
2SO2 (g) + O2 (g) 2SO3 (g)
⇌
Solution:
2SO2 (g) + O2 (g) 2SO3 (g)
Increasing pressure will shift the equilibrium to the right hand side because the forward reaction is accompanied by a decrease in the total number of moles of gaseous particles.
Conversely, decreasing pressure will shift the equilibrium to the left hand side, because the reverse reaction is accompanied with an increase in the total number of gaseous particles.
Example
⇌
(D) Effect Of Temperature On Equilibrium
Increasing the temperature not only increases the rate
constant, k but also changes the equilibrium experession, KC
From the Arrhenius equation, when
the temperature of a system
increases, the rate of reaction also
increases
For a reversible reaction, the
forward and reverse reaction
rates increases, BUT NOT to
the same extent
Consider the following reversible reaction;
A + B C + D H = +ve
The forward reaction (with reactants A & B) is
endothermic.
Also, the reverse reaction (with reactants C & D) is
exothermic.
If the H above is –ve, the opposite is true.
(D) Effect Of Temperature On Equilibrium
⇌
(D) Effect Of Temperature On Equilibrium
The following changes to the equilibrium position are
predicted by the Le Chatelier’s Principle:
If the temperature of the
system
the equilibrium will shift in the
direction of the endothermic
reaction
if the temperature of the
system
the equilibrium will shift in the
direction of the exothermic
reaction
Consider the following reversible reaction,
H2 (g) + I2 (g) 2HI (g) ΔH= ‒13.0 kJ mol‒1
According to Le Chatelier’s Principle:
Increasing the temperature will shift the equilibrium position to the left (endothermic reaction) where heat is absorbed to lower the temperature.
─ the concentrations of H2 (g) and I2 (g) increase.
(D) Effect Of Temperature On Equilibrium
⇌
Consider the following reversible reaction,
H2 (g) + I2 (g) 2HI (g) ΔH= ‒13.0 kJ mol‒1
According to Le Chatelier’s Principle:
Decreasing the temperature will shift the equilibrium position to the right (exothermic reaction) where heat is released to increase the temperature.
─ the concentrations of HI (g) increases.
(D) Effect Of Temperature On Equilibrium
⇌
Consider the following reversible reaction,
N2O4 (g) 2NO2 (g) ΔH= +57 kJ mol‒1
According to Le Chatelier’s Principle:
Increasing the temperature will shift the
equilibrium position to the right (endothermic
reaction) where heat is absorbed to lower the
temperature.
─ the concentration of NO2 (g) increases.
(D) Effect Of Temperature On Equilibrium
⇌
Consider the following reversible reaction,
N2O4 (g) 2NO2 (g) ΔH= +57 kJ mol‒1
According to Le Chatelier’s Principle:
– Decreasing the temperature will shift the
equilibrium position to the left (exothermic
reaction) where heat is released to increase the
temperature.
─ the concentration of N2O4 (g) increases.
(D) Effect Of Temperature On Equilibrium
⇌
(D) Effect Of Temperature On Equilibrium
The effect of temperature on the equilibrium constant,
KC is shown by the van’t Hoff equation:
= the equilibrium constant
= the heat of forward reaction
= a constant for the particular reaction
(D) Effect Of Temperature On Equilibrium
The van’t Hoff equation shows that:
Gradient =
Exothermic reaction
For exothermic reactions, the
equilibrium constant decreases
with increasing temperature
Gradient =
Endothermic reaction
For endothermic reaction, the
equilibrium constant increases
with increasing temperature
Summary
At equilibrium, when there is an increase in:
The concentration, the equilibrium will change until Q = KC.
The pressure, the direction that produces the lower total mol of
gas will be favoured.
The temperature, the endothermic direction will be favoured.
The opposites are true when there are decreases in the factors
above.