2.6.
Apply Le Chatelier’s principle to explain the effects
of change of temperature, concentration and
pressure on a system in equilibrium.
Chemical Equilibrium
Reversible Reactions:
A chemical reaction in which the products
can react to re-form the reactants
Chemical Equilibrium:
When the rate of the forward reaction equals
the rate of the reverse reaction and the
concentration of products and reactants
remains unchanged
2HgO(s) 2Hg(l) + O2(g)
Arrows going both directions ( ) indicates equilibrium in a
chemical equation
POSITION OF EQUILIBRIUM: LE CHATELIER’S PRINCIPLE
If one or more factors that
affect an equilibrium are
changed, the position of
equilibrium shifts in the
direction which opposes the Henry Le Chatelier
change.
Le Chatelier Translated:
When you take something away from a system at
equilibrium, the system shifts in such a way as to
replace what you’ve taken away.
When you add something to a system at
equilibrium, the system shifts in such a way as to
use up what you’ve added.
Factors that Affect Equilibrium
Concentration
Temperature
Pressure (for gaseous systems only)
The presence of a catalyst
POSITION OF EQUILIBRIUM: LE CHATELIER’S PRINCIPLE
Position of equilibrium refers to the relative amounts of
reactants and products in an equilibrium mixture.
• If the concentration of reactants increases, the position of
equilibrium moves to the right.
• If the concentration of products increases, the position of
equilibrium moves to the left.
Le Chatelier’s principle relates to changes in concentration,
pressure and temperature.
Catalysts have no effect on the position of equilibrium or the
value of Kc. They only speed up the rate of reaction.
Questions
1. State Le Chatelier's Principle.
An important reaction for the commercial production
of hydrogen is given below:
CO( g ) H 2 O( g ) CO2 ( g ) H 2 ( g ) H ve
How will the equilibrium position shift in each of
the following cases?
i. Gaseous carbon dioxide is removed
ii. The pressure is increased on the system
iii. The temperature is increased
2. What is meant by the term dynamic equilibrium?
Consider the following system in dynamic equilibrium
N 2 O( g ) NO2 ( g ) 3NO( g ) 155.7 kJ
a. Explain the effect if any of the following changes
on the above system in equilibrium:
i. The temperature is increased
ii. The pressure is reduced
iii. The concentration of
b. The partial pressure of NO2 and N2O4 at
equilibrium are 0.4 atm and 6.0 atm
respectively .
1
2 NO2 ( g ) N 2 O4 ( g ) 58kJmol
i. Write and expression for Kp
ii. Calculate the value for Kp
3. The equation below represents the reaction for the
production of ammonia.
N2(g) + 3H2(g) 2NH3(g) ΔH= - 92kj.mol-1
Equilibrium is established with the gases having
the following partial pressures in atmosphere.
a. Write an expression for the
gas p/atm equilibrium constant for the
reaction.
N2 2 b. Calculate a value for the
equilibrium constant and state
H2 3 its units.
c. Explain the effect of the
NH3 3 increase temperature on the
value of the equilibrium
constant.
