• Equilibrium Constant (Kc) and the Reaction Quotient (Qc)
• Le Châtelier’s Principle
(pp.354-370)
SCH4U – Grade 12 Chemistry, University Preparation
Ms. Papaiconomou & Ms. Lorenowicz
•
Law of Chemical Equilibrium
•
Using ICE charts and the equilibrium equation we can solve for
the:
• At equilibrium there is a constant ratio between the concentration of
reactants and products in any change.
• Concentrations at: I = initial, C = change, E = equilibrium
• Equilibrium constant
• Initial or Equilibrium concentrations of chemical species in a reaction
• May need to use the quadratic equation to solve problems.
• There may be more than one solution, but only one of the solutions will make sense.
•
The equilibrium constant (K) describes the extent of a reaction.
• K > 1 means equilibrium favours the products (more products made than reactants)
• K = 1 means at equilibrium there are equal concentrations of products & reactants.
• K < 1 means equilibrium favours the reactants (less products made than reactants)
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
How do you know a reaction is at equilibrium?
How can you predict the direction in which the
reaction must proceed to reach equilibrium?
◦ The reaction quotient (Qc) is an expression that is
identical to the equilibrium constant expression, but
its value is calculated using concentrations that are
not necessarily those at equilibrium.
aA + b B  cC + d D
Qc =
c
[C ] [D ]
d
a
b
[A ] [B ]



Q < K  denominator is large
◦ system attains equilibrium by moving to the right, move
towards making products
Q = K  system at equilibrium
Q > K  numerator is large
◦ system attains equilibrium by moving to the left, move
towards making reactants
A dynamic equilibrium tends to respond so as
to relieve the effect of any change in the
conditions that affect the equilibrium.


Predicts the way that an equilibrium
system responds to change
Involves:
◦ Initial equilibrium system
◦ Shifting “non-equilibrium” state
◦ New equilibrium state
Henry-Louis Le Châtelier,
1850-1936, a French
chemist and engineer
Increase concentration of
reactants
F:\Courses\SCH4U1 - Chemistry Gr12 Univ\~
Resources ~\Nelson Chemistry 12
Textbook\Chem_12\Attachments\d)_Animations\
16M11AN1.mov
Decrease concentration
of reactants
F:\Courses\SCH4U1 - Chemistry Gr12 Univ\~
Resources ~\Nelson Chemistry 12
Textbook\Chem_12\Attachments\d)_Animations\
16M11AN2.mov
Video:
F:\Courses\SCH4U1 - Chemistry Gr12
Univ\~ Resources ~\McGrawHill Chemistry
12 Textbook\elp\unit4\4_1.mov
Increase concentration of
products
F:\Courses\SCH4U1 - Chemistry Gr12 Univ\~
Resources ~\Nelson Chemistry 12
Textbook\Chem_12\Attachments\d)_Animations\
16M11AN3.mov
Decrease concentration
of products
F:\Courses\SCH4U1 - Chemistry Gr12 Univ\~
Resources ~\Nelson Chemistry 12
Textbook\Chem_12\Attachments\d)_Animations\
16M11AN4.mov

Common ion effect
◦ Involves adding an ion to a solution in which the ion
is already present in solution (concentration effect)
◦ Equilibrium shifts away from the added ion
Video:
F:\Courses\SCH4U1 Chemistry Gr12 Univ\~
Resources ~\McGrawHill
Chemistry 12
Textbook\elp\unit4\4_3a.avi



The value of the equilibrium constant changes with temperature,
because the forward and reverse reactions are affected
Le Châtelier’s Principle still holds.
The sign of the enthalpy change for the reaction is
important.
◦ Endothermic Change (ΔH >0)
 ↑ temperature, shift equilibrium to right, forming products & Kc increases
 ↓ temperature, shift equilibrium to left, forming reactants, & Kc decreases
◦ Exothermic Change (ΔH <0)
 ↑ temperature, shift equilibrium to left, forming reactants & Kc decreases
 ↓ temperature, shift equilibrium to right, forming products, & Kc increases
Video:
F:\Courses\SCH4U1 Chemistry Gr12 Univ\~
Resources ~\McGrawHill
Chemistry 12
Textbook\elp\unit4\4_3b.avi
Video:
F:\Courses\SCH4U1 Chemistry Gr12 Univ\~
Resources ~\McGrawHill
Chemistry 12
Textbook\elp\unit4\4_3c.avi


Pressure and Volume are
inversely related
at a constant temperature.
Pressure is caused by gas
molecules striking the walls of a
container, so you change pressure
by changing the number of gas
molecules.
◦ if ↓volume, ↑pressure so, shift to
more gas molecules
◦ if ↑volume, ↓pressure so, shift to
less gas molecules
◦ What if same number of gas
molecules on both sides of
equation?
 Volume & pressure have no effect
on the position of equilibrium.
Catalyst



A catalyst speeds up the rate of a reaction.
Overall effect is to lower the activation energy, which increases
the rate of reaction, of both the forward and reverse reactions.
Catalysts do not affect the position of equilibrium, only the time
taken.
Inert Gases


(N2 & Noble Gases)
Inert gases do not react with other gases.
This will not be part of the equilibrium system.
The equilibrium position of the system will not
change.
◦ The presence of the inert gas changes the probability of
successful collisions for both the reactants and products
equally, resulting in no shift in the equilibrium system.
Video:
F:\Courses\SCH4U1 Chemistry Gr12 Univ\~
Resources ~\McGrawHill
Chemistry 12
Textbook\elp\unit4\4_2.avi

Please re-read Section 7.4 (pp.354-370) and
answer:
◦ p.356 Q.26-28
◦ pp.366-367 Q.29-33
◦ p.370 Q.1-5