Section 17.2 Factors Affecting Chemical
Equilibrium
• Describe how various
factors affect chemical
equilibrium.
• Explain how Le
Châtelier’s principle
applies to equilibrium
systems.
When changes are made to a system
at equilibrium, the system shifts to a
new equilibrium position.
Le Châtelier’s Principle
• Le Châtelier’s Principle was proposed in
1888 and states that if stress is applied to a
system at equilibrium, the system shifts in
the direction that relieves the stress.
• Stress is any kind of change in a system that
upsets the equilibrium.
Le Châtelier’s Principle (cont.)
• Adjusting the concentrations of either the
reactants or the products puts stress on a
system in equilibrium.
• Adding reactants increases the number of
effective collisions between molecules and
upsets the equilibrium.
Le Châtelier’s Principle (cont.)
• The equilibrium shifts to the right to
produce more products.
• Stress is relieved by shifting to the left,
converting products to reactants.
• What dose the longer equilibrium arrow to
the left or right mean?
• It means that, one reaction occurs faster
than the other temporarily. Once a new
equilibrium position is established, the two
reactions again occur at equal rates
Le Châtelier’s Principle (cont.)
• Changing the volume (and pressure) of an
equilibrium system shifts the equilibrium only
if the number of moles of gaseous reactants
is different from the moles of gaseous
products.
• CO(g)+ 3H2(g)⇌ CH4(g)+ H2O(g)
• If the number of moles is the same on both
sides of the balanced equation, changes in
pressure and volume have no effect on the
equilibrium.
Le Châtelier’s Principle (cont.)
The reaction
between CO and H2
is at equilibrium.
Lowering the piston
decreases the volume
and increases the
pressure.
As a result, more molecules
of the products form. Their
formation relieves the stress
on the system.
Le Châtelier’s Principle (cont.)
• Changes in temperature alter the
equilibrium position and the equilibrium
constant.
• If heat is added to an equilibrium system, the
equilibrium shifts in the direction in which the
heat is used up.
Le Châtelier’s Principle (cont.)
• Any change in temperature results in a
change in Keq.
Le Châtelier’s Principle (cont.)
• A catalyzed reaction reaches equilibrium
more quickly, but with no change in the
amount of product formed.
Section 17.2 Assessment
Which does NOT result in a shift of the
equilibrium to the right?
A. removing products
B. adding reactants
C. increasing concentration of reactants A. A
D
C
A
0%
B. B
C. C
0%
0%
0%
D. D
B
D. adding products
Section 17.2 Assessment
Any change in ____ results in a change
in Keq.
A. temperature
B. pressure
D
A
0%
C
D. concentration
A. A
B. B
C. C
0%
0%
0%
D. D
B
C. volume
A change in ____ alters both the
equilibrium position and the equilibrium
constant.
A. pressure
A
0%
D
D. density
C
C. volume
A. A
B. B
C. C
0%
0%
0%
D. D
B
B. temperature
Adding product to a chemical equilibrium:
A. does nothing
B. creates a stress and shifts
the equilibrium to the right
D
A
0%
C
D. causes more product to form
A. A
B. B
C. C
0%
0%
0%
D. D
B
C. creates a stress and shifts
the equilibrium to the left
Ethylene (C2H4) reacts with hydrogen to
form ethane (C2H6). C2H4(g) + H2(g) ↔
C2H6(g) + heat. How could you increase
the amount of hydrogen in the system?
A. Increase the heat.
A
0%
D
D. Decrease the C2H6.
C
C. Increase the C2H4.
A. A
B. B
C. C
0%
0%
0%
D. D
B
B. Decrease the heat.
Which does NOT result in a shift of the
equilibrium to the right?
A. removing products
B. adding reactants
D
A
0%
C
D. decreasing the concentration
of reactants
A. A
B. B
C. C
0%
0%
0%
D. D
B
C. increasing concentration
of reactants
• Q1:why the addition of a catalyst does not
change an equilibrium position.
• A catalyst speeds up the forward and reverse
reactions equally.
• Q2: Explain how the following stresses affect
the equilibrium system.
• PF3(g) + F2(g) ⇌ PF5(g) + heat
• If volume is decreased: The equilibrium shifts to
the right.
• Heat is increased : The equilibrium shifts to the
left.
• Q3:Explain how a system at equilibrium
responds to a stress and list factors that
can be stresses on an equilibrium system.
• If possible, the equilibrium shifts in the
direction that relieves the stress. changes
in concentration, pressure (volume), and
temperature
Q4:Explain how decreasing the volume of
the reaction vessel affects each
equilibrium.
a.2SO(g) + O2(g) ⇌ 2SO(g)
b.H2 (g) + Cl2(g) ⇌ 2HCl(g)
a. The equilibrium shifts to the right.
b.The stress has no effect on the equilibrium
Q5: Decide whether higher or lower
temperatures will produce more
CH3CHO in the following equilibrium.
C2H2(g) + H2O(g) ⇌ CH3CHO(g) ∆H° = -151
kJ
lower temperatures
Q6: The table below shows the
concentrations of Substances A and B in
two reaction mixtures. A and B react
according to the equation 2A ⇌ B; K eq
= 200. Are the two mixtures at different
equilibrium positions?
The two mixtures are at the same equilibrium
position.
End of section 17.2