Module 3

Lesson 13/14 – Le Chatelier’s Principle

Objectives

MustState Le Chatelier’s principle .ShouldApply 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 equilibrium5. 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?

Carbon dioxide dissolves in water

H2O + CO2 HCO3- + H+

Once equilibrium has been reached you can see no change in the gas bubbles in the container.

If you add some acid though…

Manipulating equilibria

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 + DIf 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

• This only applies to reactions involving gases

A(g) + 2B(g) C(g) + D(g)

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

• 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.

Increasing pressure

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.

In this case the forward reaction is exothermic

(ΔH = -250kJmol-1)

The reverse reaction is endothermic (+250 kJmol-1)

A(g) + 2B(g) C(g) + D(g)

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

A(g) + 2B(g) C(g) + D(g)

ΔH = -250kJmol-1

• 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.

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