Le Chatelier's Principle

Introduction

Equilibria

Learning Objectives

●To define Le Chatelier's principle and understand how the change in
equilibrium takes place according to this principle.

●To discuss optimal yield.

A.C.:2.3: explain the use of the Le Chatelier principle in predicting the effect
of changes in conditions on equilibrium

●changes in temperature

●changes in pressure

●changes in reactant concentration

●changes in product concentration

●presence of a catalyst.

Answer the following questions:

1. Define forward and reverse reaction?

2. What is activation energy?

3. What is a dynamic equilibrium?

4. What are the conditions necessary for dynamic

equilibrium?

Starter (10mins)

Open Ended

1. Define forward and reverse reaction?

Open Ended

1. What is a dynamic equilibrium?

Open Ended

What are the conditions necessary for dynamic equilibrium?

Extension

A dynamic equilibrium is established between two reactants A and B according to the equation shown: A(aq) ⇋
2B(aq).Compound A has an initial concentration of 0.8 mol/dm3 that drops to 0.4 mol/dm3 once equilibrium is
established.

Which graph for this equilibrium is correct?

Position of Equilibria

Equilibria

Equilibrium Position

⚫ It is possible to change the proportion of reactants and

products in an equilibrium mixture (to get a greater yield
of products)

⚫ Change the position of equilibrium

⚪ Proportion of products is increased – moved to the right or forward

direction

⚪ Proportion of reactants is increased – moved to the left or backward

direction

⚫ We can change this by altering the conditions

LO: To define Le Chatelier's principle and understand how the change in equilibrium takes place according to this
principle.

Le Chatelier’s Principle

‘If a system at equilibrium is disturbed, the equilibrium

moves in the direction that tends to reduce the

disturbance’

⚫ If any factor is changed which affects the equilibrium

mixture, the position of equilibrium will shift to oppose
the change

LO: To define Le Chatelier's principle and understand how the change in equilibrium takes place according to this
principle.

Concentration

⚫ Increase the concentration of either W or X, then equilibrium

must shift in the direction that reduces the concentration

⚪ We add more W, increasing the concentration of W

⚪ The only way the system can reduce the concentration of W, is by it

reacting with X to form Y + Z

⚪ This moves the equilibrium to the right

⚪ A higher proportion of Y + Z is produced

⚪ Adding more X would have the same effect

⚪ If we removed Y or Z it also causes the equilibrium to move right to

produce more Y or Z

W(aq) + X(aq) Y(aq) + Z(aq)

LO: To define Le Chatelier's principle and understand how the change in equilibrium takes place according to this
principle.

Demonstration

The two different coloured Co(II) complex ions, [Co(H2O)6]2+ and [CoCl4]2-, exist
together in equilibrium in solution in the presence of chloride ions:

[Co(H2O)6]2+(aq)(pink) + 4Cl-(aq) ⇌ [CoCl4]2-(aq)(blue) + 6H2O(l)

Open Ended

How does increasing the concentration of a reactant affect the position of equilibrium in a reversible reaction?

Open Ended

How does decreasing the concentration of a product affect the position of equilibrium in a reversible reaction?

Multiple Choice

For the reaction below, which change would cause the equilibrium to shift to the right?

CH4(g) + 2H2S(g) ↔ CS2(g) + 4H2(g)

• Decrease the concentration of H2S.

• Decrease the concentration of CH4.

• Decrease the concentration of CS2.

• Increase the concentration of CS2

Overall Pressure

⚫ Only affects reactions involving gases

⚫ Increasing the pressure of a gas means there are more molecules of it in a

given volume (equivalent to increasing concentration)

⚫ Position of equilibrium must move to decrease the pressure of the overall

system

⚪Moves to the left (fewer molecules, exerts less pressure)

⚪If we decrease the pressure, then will move to the right (more molecules, exerts more pressure)

⚫ Because of the nature of the colours of the two molecules we can detect the

shift in equilibrium based on colour

Remember: increasing pressure, increases the concentration of all the reactants and products by the same amount

N2O4(g) 2NO2(g)

Dinitrogen tetraoxide, colourless

1 mole

Nitrogen dioxide, Brown

2 moles

LO: To define Le Chatelier's principle and understand how the change in equilibrium takes place according to this
principle.

Overall Pressure

⚫ Will only change the position of equilibrium if there are different

number of molecules on either side of the equation

⚫ Pressure has no affect on the position of equilibrium in the above

reaction

⚫ Proportions of all three gases will remain the same

Remember: the rate at which equilibrium is reached will be sped up by increasing the

pressure (more collisions in a given time)

H2(g) + I2(g) 2HI(g)

2 moles

2 moles

LO: To define Le Chatelier's principle and understand how the change in equilibrium takes place according to this
principle.

Open Ended

What would happen to the position of the equilibrium when the following changes are made to the reaction below? 2HgO(s) ↔ Hg(l) + O2(g)

a- The pressure on the system increases.

Open Ended

What would happen to the position of the equilibrium when the following changes are made to the reaction below? 2HgO(s) ↔ Hg(l) + O2(g)

(a) HgO is added to the system.

Temperature

⚫ We already know that reversible reactions are

exothermic in one direction (release heat) and
endothermic in the other (take in heat)

⚪ The size of the enthalpy change is the same in both directions but

the sign changes

⚫ Exothermic reactions – negative enthalpy change

⚫ Endothermic reactions – positive enthalpy change

LO: To define Le Chatelier's principle and understand how the change in equilibrium takes place according to this
principle.

Example 1 – Exothermic Forward Reaction

⚫ Exothermic in the forward direction – heat is given out

when sulphur trioxide is formed

⚫ Heat is absorbed in the opposite direction

⚫ If we increase the temperature, the position of

equilibrium must shift to cool the system down

⚪ This will move it in the direction which absorbs heat (endothermic),

i.e. to the left

⚪ If we decrease the temperature, the position will shift to the right

(to warm the system up)

2SO2(g) + O2(g) 2SO3(g)

ΔH° = -197 kJmol-1

LO: To define Le Chatelier's principle and understand how the change in equilibrium takes place according to this
principle.

N2O4(g) 2NO2(g)

Example 2 – Endothermic Forward Reaction

⚫ Endothermic in the forward direction – heat is taken in

when nitrogen dioxide is formed

⚫ Heat is given out in the opposite direction

⚫ If we increase the temperature, the position of

equilibrium must shift to cool the system down

⚪ This will move it in the direction which absorbs heat (endothermic),

i.e. to the right

⚪ If we decrease the temperature, the position will shift to the left (to

warm the system up)

ΔH° = +58 kJmol-1

LO: To define Le Chatelier's principle and understand how the change in equilibrium takes place according to this
principle.

Multiple Choice

Le Chatelier's principle states that a system at equilibrium will shift to counteract a stress. When temperature is increased, the system will shift to favour the:

• Endothermic reaction

• Exothermic reaction

• Reaction with the higher activation energy

• Reaction with the lower activation energy

Open Ended

Predict the effect of decreasing the temperature on the position of the following equilibria.

(b) 2NH3(g) ↔ N2(g) + 3H2(g) ∆ H = 37.2 kJ

Multiple Choice

Consider the following endothermic reaction:

N2O4(g) + heat ⇌ 2NO2(g)

If the temperature of the system is decreased, the equilibrium will shift

• To the right, favouring the formation of NO2

• To the left, favouring the formation of N2O4

• There will be no shift in the equilibrium position

• The reaction will become exothermic

Catalysts

⚫ Catalysts have no effect on the position of equilibrium so

they do not alter the composition of the equilibrium
mixture

⚫ They provide an alternative route, lowering the activation

energy, affecting the forward and back reactions equally

⚫ They do allow equilibrium to be reached sooner, and are

therefore important in industry

LO: To define Le Chatelier's principle and understand how the change in equilibrium takes place according to this
principle.

Industry

⚫ A number of industrial processes involve reversible

reactions

⚫ Yield is important and we can apply Le Chatelier’s

principle to determine conditions for highest yield

⚫ However, we also need to consider:

⚪ Low temperatures and pressure – slow rate of reaction

⚪ High temperatures and pressure – expensive to run and build

suitable equipment

⚫ In most cases a compromise set of conditions is used

●

LO: To discuss optimal yield and conditions used in Habers process and Contact process in the industry.

Compromising Conditions in Industry

Explain why the following reactions have the conditions
shown

1.

C2H4(g) + H2O(g) C2H5OH(g)

ΔH = -46 kJ mol-1

⚪ 60-70 atmospheres

⚪ 300⁰C

⚪ Phosphoric (V) acid catalyst

⚪ Excess steam

2.

CO(g) + 2H2(g) CH3OH(g) ΔH = -91 kJmol-1

⚪ 80-100 atmospheres

⚪ 250⁰C

⚪ Copper catalyst

LO: To discuss optimal yield and conditions used in Habers process and Contact process in the industry.

Ethanol – C2H5OH

⚫ Reaction is reversible

⚫ Sped up by phosphoric acid catalyst

⚫ Reactants and products all gaseous at temperature used

ΔH° = -46 kJmol-1

Le Chatelier’s

⚪High pressure, move to the right (fewer

molecules)

⚪Low temperature, move to the right

⚪Excess steam, move to the right

Practical Problems

⚪Low temperature reduce reaction rate (although

compensated by catalyst)

⚪High pressures cause ethene to polymerise, increases

costs of building plant and energy to run

⚪Too much steam dilutes catalyst

Compromise conditions:

⚫570K, 6500KPa

⚫5% ethanol, but un-reacted ethene is separated and recycled over the catalyst again and again
until about 95%

Learn the
DETAILS

Methanol – CH3OH

CO(g) + 2H2(g) CH3OH(g)

⚫ Methanol is used as a chemical feedstock (a starting material for other chemicals)

⚫ Used to make methanal (formaldehyde) which has many uses including making plastics

⚫ Can also be added to petrol and used as a fuel (advantage as methanol fires can be put

out using water)

ΔH° = -91 kJmol-1

Le Chatelier’s

⚪High pressure, move to the right (fewer

molecules)

⚪Low temperature, move to the right

Practical Problems

⚪Low temperature reduce reaction rate

⚪High pressures increases costs of building plant and

energy to run

Compromise conditions:

⚫500K, 10000KPa

⚫Produces around 5-10% yield

⚫Uses copper catalyst

Learn the
DETAILS

Multiple Choice

The following factors affect the position of equilibrium EXCEPT

• Concentration
• Pressure
• Temperature
• States of matter

Multiple Choice

Equilibrium for 2CrO₄^2- + 2H⁺ → Cr₂O₇^2- + H₂O. What happen when H⁺ ion are added to the system?

• Position of equilibrium will shift to left
• Color of system will turn from orange to yellow
• K_c will be higher
• Equilibrium will shift to right to decrease the added H⁺ ion

Multiple Choice

Equilibrium for 2CrO₄^2- + 2H⁺→Cr₂O₇^2- + H₂O. What happen when OH^- ions are added to the system?

• Position of equilibrium will shift to right
• Color of system will turn from orange to yellow
• K_c will decrease
• OH^- ions will not react, and thus no change is seen.

Multiple Choice

For CoCI₄^2- +6H₂O →Co(H₂O)₆²⁺ + 4CI^-. What happen when CI^- ions are added?

• Position of equilibrium will shift to right
• Color of system will turn from blue to pink
• Concentration of reactants and products remain unchanged
• Equilibrium shift to left to reduce the added CI^- ions

Multiple Choice

For CoCI₄^2- +6H₂O →Co(H₂O)₆²⁺ + 4CI^-. What happen when H₂O ions are added?

• Position of equilibrium will shift to left
• Color of system will turn from blue to pink
• K_c will change and increase as H₂O is added
• Equilibrium shift to left to reduce the added H₂O

Multiple Choice

For N₂O₄ (colorless)→ 2NO₂ (brown). By reducing pressure, equilibrium is shift to right. WHY?

• Shift to the right will increase the rate of reaction
• Color of system will turn from colorless to brown.
• More particles on the right, so K_c will increase
• More particles on the right, so pressure can increase again

Multiple Choice

For N₂ + 3H₂ →2NH₃ . When pressure is increased the equilibrium shift to right. Why?

• To increase the amount of products
• To reduce the pressure, as right has less number of molecule
• K_c will increase when it is shifted to the right
• So it will increase the rate of reaction

Learning Objectives

●To understand meaning of reversible reaction, forward reaction,
backward reaction and dynamic equilibrium.

●To identify the conditions required for dynamic equilibrium

●To define Le Chatelier's principle and understand how the change in
equilibrium takes place according to this principle.

●To discuss optimal yield and conditions used in Habers process and
Contact process in the industry.

A.C.A.C.:2.3: explain the use of the Le Chatelier principle in predicting the effect of changes in conditions on equilibrium

• changes in temperature

• changes in pressure

• changes in reactant concentration

• changes in product concentration

• presence of a catalyst.