Le Chatelier's Principle

Chemistry II: Le Chatelier’s Principle Dr. Melody Anak Kimi

Centre for Pre-University Studies

Universiti Malaysia Sarawak

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Le Chatelier’s Principle

By the end of this topic you should be able to:

explain Le Chatelier’s principle

explain the factors affecting chemical equilibrium

apply Le Chatelier’s principle in predicting the equilibrium

shift

If the external conditions for a reversible reaction

are changed, the equilibrium will shift into a new

equilibrium position so as to maintain the equilibrium

constant for the reaction.


A

A B

B

C

C

D

D


External stresses

Presence of catalyst

Change in concentration

of products or reactant

Change in pressure

(gases) including the

presence of noble gases

Change in temperature

(A) Effect Of Catalyst On Equilibrium


increases the rate of

reaction

-provides an alternative reaction mechanism

with lower activation energy, Ea

-reduces the time taken to reach equilibrium

no effect on the values of

the equilibrium constant,

KC or Kp

Only reduces the time to reach KC or Kp

provides a mechanism with

lower Ea, therefore

increasing the value of the

reaction rate constant, k

provides a mechanism with lower Ea,

therefore increasing the value of the reaction

rate constant, k

As shown in the Arrhenius equation:

With lower Ea, the value of k will be higher,

thus increasing the rate of reaction.

RT

Ea

Aek


(B) Effect Of Concentration On Equilibrium

According to the Le Chatelier’s Principle;


Concentration of reactant increases, the equilibrium

position will shift in the direction that reduces the

concentration of this reactant (shift to the right). Thus,

increases the concentration of the product.

Concentration of the product increases, the equilibrium

position will shift in the direction that reduces the

concentration of this product (shift to the left). Thus,

increases the concentration of the reactant.


The system is

no longer in

equilibrium

Increase in the

forward rate of

reaction

[H2] and [I2]

decreases while [HI]

increases

New equilibrium position

is established when the

rate of the forward

reaction = the rate of the

reverse reaction

When the new

equilibrium is

established, the

amount of H2 present is

more than the initial

amount

The final position of

the equilibrium (after

the added H2) is

different from the

initial equilibrium

position because the

equilibrium has

shifted to the left

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

Consider the following

reversible reaction in

equilibrium

Add more

⇌

t1 t2 t3

[H2

] [I2]

[HI

]

Equilibrium

reached at t1

H2 added

at t2

New equilibrium

reached at t3


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

⇌

In the hydrolysis of ethyl ethanoate (an ester),

CH3COOC2H5 (l) + H2O (l) CH3COOH (l) + C2H5OH (l)

ethyl ethanoate ethanoic acid ethanol

the equilibrium position can be shifted to the right (thus, increasing the formation of products) by:

– Removing ethanoic acid

– Removing ethanol

– Increasing the concentration of ethyl ethanoate


⇌

Example

Consider the following equilibrium system:

Ag+(aq) + Fe2+(aq) Ag(s) + Fe3+(aq)

a) Write the expression for the equilibrium constant, KC.

b) State what happens when the following changes are made after the system has reached equilibrium: i. Adding more aqueous silver nitrate

ii. Adding more iron(III) nitrate

iii. Adding more silver

⇌

Example

a) Kc = [Fe3+]

[Ag+][Fe2+]

b) The following changes occur:

i. the equilibrium shifts to the right and more Ag and

Fe3+ will be produced.

ii. the equilibrium shifts to the left and more Ag+ and

Fe2+ are produced.

iii. there is no net change in equilibrium position

because Ag does not appear in the equilibrium

expression.

(C) Effect Of Pressure On Equilibrium

For equilibrium systems involving gases

Effect of Pressure on Equilibrium

increase in pressure increase the rate of reaction

(exothermic or endothermic)

change in pressure will

affect the position of

equilibrium

provided the reaction involves an overall

change in volume (i.e., the number of moles

of gaseous reactants is different from the

number of moles of gaseous products)

change in the

pressure

does not affect the equilibrium

constant, (Kc or Kp)


Three methods of changing the pressure of an equilibrium system at a

constant temperature:

Adding or removing a gaseous

reactant or product equivalent to the effect of changes in

concentration on equilibrium

Changing the volume of the

system

total pressure decreased by

increasing the volume of the system,

or increased by decreasing the

volume of the system

Adding noble gas to the system

increases the pressure of the system

without changing the volume BUT

does not affect the equilibrium

constant, KP

When changes to the pressure occur by changing the

volume of the system,

Le Chatelier’s Principle predicts the following:


total pressure on the

equilibrium

reaction takes place in the

direction that decreases the

total number of moles of gas

total pressure on the

equilibrium

reaction takes place in the

direction that increases the

total number of moles of gas


N2O4 2NO2

1 mol of N2O4 2 moles of NO2

left right

⇌

When the piston is pushed downwards after the system reached equilibrium;

– Total system pressure increases.

– This pressure must be lowered to reach a new equilibrium.

– This condition favours the direction which produces a smaller total number of mols.

– The equilibrium position shifts to the left & more N2O4 are produced.


N2O4 2NO2 ⇌

When the piston is pulled upwards after the system reached equilibrium;

– Total system pressure decreases.

– This pressure must be increased to reach a new equilibrium.

– This condition favours the direction which produces a higher total number of mols.

– The equilibrium position shifts to the right& more NO2 are produced.


N2O4 2NO2 ⇌

Example

Explain the effect of pressure on the following equilibrium

system:

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

⇌

Solution:

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

Increasing pressure will shift the equilibrium to the right hand side because the forward reaction is accompanied by a decrease in the total number of moles of gaseous particles.

Conversely, decreasing pressure will shift the equilibrium to the left hand side, because the reverse reaction is accompanied with an increase in the total number of gaseous particles.

Example

⇌

(D) Effect Of Temperature On Equilibrium


Increasing the temperature not only increases the rate

constant, k but also changes the equilibrium experession, KC

From the Arrhenius equation, when

the temperature of a system

increases, the rate of reaction also

increases

For a reversible reaction, the

forward and reverse reaction

rates increases, BUT NOT to

the same extent

Consider the following reversible reaction;

A + B C + D H = +ve

The forward reaction (with reactants A & B) is

endothermic.

Also, the reverse reaction (with reactants C & D) is

exothermic.

If the H above is –ve, the opposite is true.


⇌


The following changes to the equilibrium position are

predicted by the Le Chatelier’s Principle:

If the temperature of the

system

the equilibrium will shift in the

direction of the endothermic

reaction

if the temperature of the

system

the equilibrium will shift in the

direction of the exothermic

reaction

Consider the following reversible reaction,

H2 (g) + I2 (g) 2HI (g) ΔH= ‒13.0 kJ mol‒1

According to Le Chatelier’s Principle:

Increasing the temperature will shift the equilibrium position to the left (endothermic reaction) where heat is absorbed to lower the temperature.

─ the concentrations of H2 (g) and I2 (g) increase.


⇌


H2 (g) + I2 (g) 2HI (g) ΔH= ‒13.0 kJ mol‒1


Decreasing the temperature will shift the equilibrium position to the right (exothermic reaction) where heat is released to increase the temperature.

─ the concentrations of HI (g) increases.


⇌


N2O4 (g) 2NO2 (g) ΔH= +57 kJ mol‒1


Increasing the temperature will shift the

equilibrium position to the right (endothermic

reaction) where heat is absorbed to lower the

temperature.

─ the concentration of NO2 (g) increases.


⇌


N2O4 (g) 2NO2 (g) ΔH= +57 kJ mol‒1


– Decreasing the temperature will shift the

equilibrium position to the left (exothermic

reaction) where heat is released to increase the

temperature.

─ the concentration of N2O4 (g) increases.


⇌


The effect of temperature on the equilibrium constant,

KC is shown by the van’t Hoff equation:

= the equilibrium constant

= the heat of forward reaction

= a constant for the particular reaction


The van’t Hoff equation shows that:

Gradient =

Exothermic reaction

For exothermic reactions, the

equilibrium constant decreases

with increasing temperature

Gradient =

Endothermic reaction

For endothermic reaction, the

equilibrium constant increases

with increasing temperature

Summary

At equilibrium, when there is an increase in:

The concentration, the equilibrium will change until Q = KC.

The pressure, the direction that produces the lower total mol of

gas will be favoured.

The temperature, the endothermic direction will be favoured.

The opposites are true when there are decreases in the factors

above.

Science

Le Chatelier's Principle