Transcript Le Chatelier s Principle

Title: Lesson 2 Le Chatelier’s Principle
Learning Objectives:
– Understand the impact of Le Chatelier’s Principle
– Use Le Chatelier’s Principle to explain the effect of changes on a system at
equilibrium
– Complete some short experiments to observe the effect of Le Chatelier’s
Principle
Recap
The sequence of diagrams represents
the system as time passes for a gas
phase reaction in which reactant X
is converted to product Y.
Which statement is correct?
A.
At t = 5 days the rate of the
forward reaction is greater than
the rate of the backward reaction.
B.
At t = 7 seconds the reaction has
reached completion.
C.
At t = 10 minutes the system has
reached a state of equilibrium.
D.
At t = 5 days the rate of the
forward reaction is less than the
rate of the backward reaction.
Diagram 1
t = 7 seconds
Diagram 2
t = 5 minutes
Diagram
t = 10 minutes
Time, t
X=
Y=
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Diagram 4
t = 5 days
Le Chatelier’s Principle

This describes how a dynamic chemical equilibrium
responds to disturbances
‘A system at equilibrium when subjected to a change
will respond in a way as to minimize the effect of the
change’

This applies to changes in:



Concentration
Pressure
Temperature
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Changes in Pressure

Increasing Pressure:


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Decreasing Pressure:

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Shifts equilibrium to the side with fewest gas molecules
This has the effect of reducing the pressure increase
Shifts equilibrium to the side with more gas molecules
This has the effect of increasing the pressure
Equilibrium constant is not affected
2N2(g) + 3H2(g) ⇌ 2NH3(g)
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H = -91.8 kJ mol-1
Pressure and equilibrium
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Pressure and equilibrium
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Changes in Concentration

Decrease in [Reactant] or increase in [Product]



Increase in [Reactant] or decrease in [Product]



Equilibrium shifts to the left
This has the effect of increasing [Reactant] and decreasing [Product]
Equilibrium shifts to the right
This has the effect of decreasing [Reactant] and increasing [Product]
Equilibrium constant is not affected
Ethanol
CH3CH2OH(aq)
+
+
Ethanoic Acid
CH3CO2H(aq)
⇌
⇌
Ethyl ethanoate
+
CH3CH2OOCCH3(aq) +
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water
H2O(l)
Concentration and equilibrium
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Changes in Temperature

Exothermic Reaction:

Increasing temperature shifts equilibrium to left
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
Decreasing temperature shifts equilibrium to the right


This is the ‘hot’ side of the reaction, so temperature increases
Endothermic Reaction:

Increasing temperature shifts equilibrium to the right


This is the ‘cold’ side of the reaction, so temperature decreases
Decreasing temperature shifts equilibrium to the left


The left is the ‘cold’ side of the reaction, so temperature decreases
This is the ‘hot’ side of the reaction, so temperature increases
Equilibrium constant is affected


Exothermic Reaction: Higher Temp  Lower Kc; Lower Temp  Higher Kc
Endothermic Reaction: Higher Temp  Higher Kc; Lower Temp  Lower Kc
2N2(g) + 3H2(g) ⇌ 2NH3(g)
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H = -91.8 kJ mol-1
Temperature and equilibrium
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Temperature and equilibrium
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Le Chatelier’s principle: summary
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Change of conditions
Effect on equilibrium position
(L = left ; R = right)
ADDITION of reagent
L  R to remove some of the added reagent
REMOVAL of reagent
R  L to replace some of the removed reagent
ADDITION of product
R  L to remove some of the added product
REMOVAL of product
L  R to replace some of the removed product
INCREASE temperature
 ENDOTHERMIC direction to absorb energy
and so LOWER temperature
DECREASE temperature
 EXOTHERMIC direction to release energy
and so RAISE temperature
INCREASE pressure
 Direction which DECREASES number of
gas molecules to LOWER P
DECREASE pressure
 Direction which INCREASES number of
gas molecules to RAISE P
Addition of Catalysts
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

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Catalysts speed up the reaction by providing an alternate reaction pathway that
has a transition state with a lower activation energy.
Forward and backward reactions pass through the same transition state, the
activation energy is lowered by the same amount for each reaction.
Rate is increased for both reactions by the same factor.
Catalysts have no effect on the position of the equilibrium.
Will not increase the equilibrium yield of product but will speed up the
attainment of the equilibrium state and product will be formed more quickly.
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NB What about
addition of a
catalyst?
Concn / mol dm-3
Products
This does NOT
disturb the
equilibrium
position
 SAME eqm
position but
sooner
 a catalyst causes
forward and
backward
reactions to be
speeded up
EQUALLY
1 = without catalyst
2
1
2 = with catalyst
Reactants
Time 
How would the equilibrium position of each of the following
reactions be affected by each of the given change of conditions?
i.e. would it shift L  R, R  L or remain unchanged?
(SELECT AND CLICK THE APPROPRIATE BUTTON)
1.
3H2(g) + N2(g) ⇌ 2NH3(g) ; -ve HR
(a) INCREASING temperature
(b) DECREASING temperature
(c) INCREASING pressure
(d) DECREASING pressure
(e) Adding a CATALYST
2.
LR
RL
No Change
LR
RL
No Change
LR
RL
No Change
LR
RL
No Change
LR
RL
No Change
LR
RL
No Change
LR
RL
No Change
LR
RL
No Change
LR
RL
No Change
LR
RL
No Change
2H2O2(g) ⇌ 2H2O(g) + O2(g) ; +ve HR
(a) INCREASING temperature
(b) DECREASING temperature
(c) INCREASING pressure
(d) DECREASING pressure
(e) Adding a CATALYST
How would you make each of the following reactions produce more
products at equilibrium? i.e. how would you cause the equilibrium position
to shift L  R? (SELECT AND CLICK THE APPROPRIATE BUTTON)
1.
CH4(g) + H2O(g) ⇌ CO(g) + 3H2(g) ; +ve HR
INCREASE T
2.
DECREASE P
DECREASE T
INCREASE P
DECREASE P
2CO(g) + 2NO(g) ⇌ 2CO2(g) + N2(g) ; -ve HR
INCREASE T
4.
INCREASE P
2H2(g) + CO(g) ⇌ CH3OH(g) ; -ve HR
INCREASE T
3.
DECREASE T
DECREASE T
INCREASE P
DECREASE P
2X(g) + Y(g) ⇌ 2Z(g) ; +ve HR
INCREASE T
DECREASE T
INCREASE P
DECREASE P
R  L shift is favoured by opposite changes in P and T
CORRECT because the forward
reaction is ENDOTHERMIC
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CORRECT because the forward
reaction is EXOTHERMIC
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CORRECT because the forward reaction
HAS A REDUCTION IN THE NUMBER
OF GAS MOLECULES FROM L  R
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CORRECT because the forward reaction
HAS AN INCREASE IN THE NUMBER
OF GAS MOLECULES FROM L  R
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BACK TO QUESTIONS
CORRECT
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INCORRECT
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Summary of effects of concentration,
pressure, temperature and catalysts
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Solutions
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Investigating Le Chatelier’s Principle

Follow the instructions here
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You should aim to complete at least 3 of the four
experiments.
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Recap

Le Chatelier’s Principle explains how equilibrium systems
respond to changes

Any disturbance to a system will be minimised but not
eliminated.
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