Module 3
Lesson 13/14 – Le Chatelier’s
Principle
Objectives
Must
State Le Chatelier’s principle .
Should
Apply Le Chaterlier’s principle to deduce the effect
of changing temperature, concentration or
pressure on a homogeneous system in equilibrium.
Could
Explain the importance in the chemical industry of a
compromise between chemical equilibrium and
reaction rate.
Starter - Quiz
1. Draw the symbol we use in an equation to
show that it can go forward or backwards.
2. Describe an example of a reaction that is
irreversible (can only go one way).
3. Describe an example of a reaction that is
reversible (can go either way).
4. What is the definition of dynamic equilibrium
5. At what point (beginning middle or end) is
the rate of the forward reaction the highest?
Reminder of dynamic equilibrium
• Dynamic equilibrium animation
Position of the equilibrium
• Equilibrium can “lie” to the left or right.
• Meaning that once equilibrium has been
reached (the point where there is no more
change) there could be more products or
more reactants.
Affecting the amount of product or
reactant
• Why might you want to affect the amount of
product?
Manipulating equilibria
Carbon dioxide dissolves in water
Once equilibrium has been reached you can see
no change in the gas bubbles in the container.
If you add some acid though…
The specification says you must be able
to:
Use le Chatelier’s principle to describe and
predict, in a homogeneous reaction, the
qualitative effects on the position of
equilibrium of changes in the following
conditions:
concentration, temperature, pressure;
A Statement of Le Chatelier's Principle
• If a dynamic equilibrium is disturbed by
changing the conditions, the position of
equilibrium moves to counteract the
change.
The three things that effect the
position of an equilibrium are:
1. Pressure
2. Concentration
3. Temperature
Using Le Chatelier's Principle with a
change of concentration:
A + 2B
C+D
If we change the concentration of a reagent (for example A).
According to Le Chatelier, the position of equilibrium will move
in such a way as to counteract the change.
If you increase the concentration of A the equilibrium will move
to the right
What would happen if you changed the conditions by
decreasing the concentration of A?
According to Le Chatelier, the position of equilibrium will move
so that the concentration of A increases again.
A + 2B
C+D
Equilibrium moves to the left
Removing a product
If you removed C as soon as it was formed, the position of
equilibrium would move to the right to replace it.
If you kept on removing it, the equilibrium position would keep
on moving rightwards - turning this into a one-way reaction.
A + 2B
C+D
Using Le Chatelier's Principle with a change of
pressure
A(g) + 2B(g)
C(g) + D(g)
• This only applies to reactions involving
gases
Increasing pressure
• According to Le Chatelier the position of equilibrium
will move so that the pressure is reduced again.
• Pressure is caused by gas molecules hitting the sides
of their container. The more molecules you have in
the container, the higher the pressure will be.
• The system can reduce the pressure by reacting in
such a way as to produce fewer molecules
Increasing pressure
• In this case, there are 3 molecules on the left-hand side of the
equation, but only 2 on the right. By forming more C and D,
the system causes the pressure to reduce.
A(g) + 2B(g)
C(g) + D(g)
Equilibrium moves to the right
Decreasing pressure
• The equilibrium will move in such a way that the
pressure increases again.
• It can do that by producing more molecules.
A(g) + 2B(g)
C(g) + D(g)
Equilibrium moves to the left
What happens if there are the same number of
molecules on both sides of the equilibrium?
• In this case, increasing the pressure has no effect
whatsoever on the position of the equilibrium.
• If you have the same numbers of molecules on both
sides, the equilibrium can't move in any way that will
reduce the pressure again.
Using Le Chatelier's Principle with a change of
temperature
It matters whether the reaction is exo or endo thermic.
A(g) + 2B(g)
C(g) + D(g)
In this case the forward reaction is exothermic
(ΔH = -250kJmol-1)
The reverse reaction is endothermic (+250 kJmol-1)
Increasing Temperature
• According to Le Chatelier, position of equilibrium will
move so that the temperature is reduced again.
• Suppose the system is in equilibrium at 300°C, and
you increase the temperature to 500°C.
• How can the reaction counteract the change you
have made? How can it cool itself down again?
• To cool down, it needs to absorb the extra heat that you have
just put in.
• In the case we are looking at, the back reaction absorbs heat.
• The position of equilibrium therefore moves to the left. The
new equilibrium mixture contains more A and B, and less C
and D.
A(g) + 2B(g)
C(g) + D(g)
ΔH = -250kJmol-1
Decreasing Temperature
• The equilibrium will move in such a way that the temperature
increases again.
• Suppose the system is in equilibrium at 500°C and you reduce
the temperature to 400°C.
• The reaction will tend to heat itself up again to return to the
original temperature.
• It can do that by favouring the exothermic reaction. The
position of equilibrium will move to the right.
A(g) + 2B(g)
C(g) + D(g)
ΔH = -250kJmol-1
Temperature Summary
• Increasing the temperature of a system in dynamic
equilibrium favours the endothermic reaction. The system
counteracts the change you have made by absorbing the
extra heat.
• Decreasing the temperature of a system in dynamic
equilibrium favours the exothermic reaction. The system
counteracts the change you have made by producing more
heat.
• This isn't in any way an explanation of why the position of
equilibrium moves in the ways described. It is only a way of
helping you to work out what happens
A competition with the rate…
• The rate is increased with increasing
concentration and temperature – however
this may cause the equilibrium to lie in favour
of the products.
• The final conditions will be a compromise
between the two.
• It is a common exam question to ask you to
comment on conditions in terms of
favourability for rate and for equilibrium.
Homework
• Complete the past paper questions and hand
in for next lesson.
Past paper questions
Mark scheme
Past paper question
Past paper question
Mark scheme
Mark scheme