Le Chatelier’s principle and more...
Quick review of equilibrium
Feature of Equilibrium State
Explanation
Equilibrium is dynamic
The reaction has not stopped
but the forward and reverse
reaction are still occurring
Equilibrium is achieved in a
closed system
A closed system prevents
exchange of matter with the
surroundings
The concentrations of reactants
and products remain constant
They are being produced and
degraded at an equal rate
The macroscopic properties do
not change
Color and density are properties
that depend upon
concentrations
Equilibrium can be reached from The same equilibrium mixture
either direction
will result under the same
conditions, regardless of the
starting point.
Catalyst
• the same process still has to happen,
catalysts just help out by lowering the
activation energy
• increase the RATE of a reaction….and
therefore the decrease the time in which
equilibrium is reached
– they speed up the forward and reverse reactions
equally
• therefore decreases the time required for the system
to achieve equilibrium
• less time equals $$$ when making chemicals
Le Chatelier’s principle and more...
– Nice video- 20 minutes
– Another good one- 15 minutes
– states when a system in chemical equilibrium is
disturbed by a change, the system shifts in a way
that tends to counteract this change of variable
– a change imposed on an equilibrium system is
called a stress
• a stress usually involves a change in the
temperature, pressure, or concentration
• the equilibrium always responds in such a way
so as to counteract the stress
Stress 1. Temperature change
McGraw Hill Flash animation
Haber Process again
DH = + 92 kJ
N2 (g) + 3H2 (g)


2NH3 (g)
DH = - 92 kJ
• this is the ONLY STRESS THAT WOULD ACTUALLY CHANGE KC
• increasing temperature
– favors the “cold side”/endothermic/the reaction that
needs heat
– adding heat favors the reaction to the left since it needs
+92 kJ
– Kc decreases
DH = + 92 kJ
N2 (g) + 3H2 (g)


2NH3 (g)
DH = - 92 kJ
• decreasing temperature
– favors the “hot side”/exothermic
• the reaction is already giving off heat it
doesn’t need so cooling down is good
• Kc increases
McGraw Hill Flash animation
Stress 2. Pressure change
N2 (g) + 3H2 (g)


2NH3 (g)
Haber Process again
DH = - 92 kJ
• an increase in pressure causes the equilibrium to
shift in the direction that has the fewer number
of moles
– results in a decrease in N2 and H2 and an increase in
NH3
• an decrease in pressure causes the equilibrium
to shift in the direction that has the most
number of moles
– results in a an increase in N2 and H2 and an decrease
in NH3
• does NOT affect the equilibrium constant Kc
Stress 3. Concentration change
McGraw Hill Flash animation
Haber Process again
N2 (g) + 3H2 (g)
Kc =


2NH3 (g)
DH = - 92 kJ
[NH3]2
[N2] [H2]3
• the equilibrium responds in such a way so as to
diminish the increase or equalize the ratio
–
increasing concentration of reactants shifts the
reaction to the right (forward, more product)
– increasing concentration of products shifts the reaction
to the left (reverse, more reactants)
• does NOT affect the equilibrium constant Kc
Practice Problem
backwards Contact Process
• Predict the effect of the following changes on the
reaction in which SO3 decomposes to form SO2 and O2.
2 SO3(g)


2 SO2 (g) + O2 (g) Ho = 197.78 kJ
• increasing the temperature of the reaction
– shifts right
• increasing the pressure on the reaction
– shifts left
• adding more O2 when the reaction is at equilibrium
– shifts left
• removing O2 from the system when the reaction is at
equilibrium
– shifts right
Le Chatelier’s Principle –
Summary
Change
Effect on Equilibrium
Change in Kc?
Increase concentration
Shifts to opposite side
No
Decrease concentration
Shifts to same side
No
Increase pressure
Shifts to side with least
moles of gas
No
Decrease pressure
Shifts to side with most
moles of gas
No
Increase temperature
Shifts in endothermic
direction
Yes
Decrease temperature
Shifts in exothermic
direction
Yes
Add a catalyst
No change
No
Le Châtelier’s Principle
• Changes in Concentration
Remove
Add
Remove
Add
aA + bB
cC + dD
Change
Shifts the Equilibrium
Increase concentration of product(s)
left
Decrease concentration of product(s)
right
Increase concentration of reactant(s)
right
Decrease concentration of reactant(s)
left
Le Châtelier’s Principle
If an external stress is applied to a system at equilibrium, the system adjusts in such a way
that the stress is partially offset as the system reaches a new equilibrium position.
• Changes in Concentration
N2 (g) + 3H2 (g)
2NH3 (g)
Add
NH3
14.5
Le Châtelier’s Principle
• Changes in Volume and Pressure
(Only a factor with gases)
A (g) + B (g)
Change
C (g)
Shifts the Equilibrium
Increase pressure
Side with fewest moles of gas
Decrease pressure
Side with most moles of gas
Increase volume
Side with most moles of gas
Decrease volume
Side with fewest moles of gas
• For each of the following reactions, predict
how the equilibrium will shift as the
temperature is increase
N2(g) + O2(g)  2NO(g)
∆H = +kJ mol-1
Right
2SO2(g) + O2(g)  2SO3(g) ∆H = -kJ mol-1
left
Effect of:
Concentration
Pressure
Temperature
Catalyst
Position of
Value of Kc
Equilibrium
Changes
No change
Changes if
No change
reaction involves
a change in the
number of gas
molecules
Changes
Changes
(depends on if
exothermic or
endothermic)
No change
No change
Haber Process (don’t think you need to know this
anymore/I will not test you on it)
• N2 (g) + 3H2 (g)  2NH3 (g)
– good catalyst is iron
– optimum temp is 450°C
DH = - 92 kJ
• good for kinetic theory (molecules moving fast,
more collisions with more energy)
• bad for equilibrium, reaction is exothermic
– optimum pressure 250 atm
• good for kinetics (more collisions)
• favors the products (only 2 mol vs. 4 in the
reactancts)
Contact Process (don’t think you need to know this
anymore/I will not test you on it)
2 SO2(g) + O2(g)  2 SO3(g) Ho = - 197.78 kJ
– good catalyst is V2O5
– optimum temp is 450°C
• good for kinetic theory (molecules moving fast,
more collisions with more energy)
• bad for equilibrium, reaction is exothermic
– optimum pressure is 2 atm
• not good for kinetics (less collisions)
• low pressure favors reactants
– however, this is a very efficient yield (99%) even at low
pressure
» therefore, avoids the need for expensive and
dangerous equipment