Manipulating
Equilibrium Constants
1) Coefficient Rule
 If coefficients change in a chemical reaction to a certain
“n” value, raise the Keq to the same “n” value
 Stoichiometric ratios remain in the same proportion
Example 1:
 A) 2A + B  6D
 Kc = [D]6
[A]2[B]
 B) A + ½ B 
3D
 (Kc)1/2 = [D]3
OR
[A]2[B]1/2
 (Kc)1/2 = [ [D]6/[A]2[B] ]1/2
2) Reciprocal Rule
 The Keq value for a reverse reaction is the INVERSE of
the forward Keq value
 1/Keq = Keq for reverse reaction
Example 2:
 A) 2A + B  6D
 Kc = [D]6
[A]2[B]
 B) 6D  2A + B
 Kc = [A]2[B]
[D]6
 Kc for reverse = 1/Kc
3) Multiple Equilibria Rule
 If the overall chemical reaction is broken into multiple
steps and then combined…….
 Overall Keq = product of Keq values from each
chemical reaction
Example 3:
 2A + B  6D
#1 Keq
 3E + B 
#2 Keq
2A + F
__________________________________________________________________
3E + 2B  6D + F
Total Keq = (#1Keq)(#2Keq)
Le’Chatlier’s Principle
Basic Concept
 Main concept in chemical equilibrium
 Change in a variable that alters the equilibrium of a
system (chemical reaction) products a shift in the
OPPOSITE direction
 Reaction shifts to counteract the variable’s influence
 Reaction tries to get back to equilibrium state-----SO
reaction shifts
Basic Concept (cont.)
 What happens to a chemical reaction when equilibrium
shifts
 When one side of a chemical reaction is stressed, the
reaction shifts to the side of LEAST stress !
Variables disrupting
equilibrium
 1) Addition or removal of chemical compounds from the
reaction.
 2) Pressure
 3) Temperature/Heat
1) Concentration Changes
 Addition of a chemical compound
 Reactions shifts towards opposite side to reduce the
effect of this addition
 Removal of a chemical compound
 Reaction shifts towards the production of compound to
reduce the effect of removing it.
 Equilibrium constant (Kc) remains the same
2) Pressure Changes
 Focus on stoichiometric ratios in chemical reaction
 Reaction shifts in a direction aimed to REDUCE pressure
change
 Increased pressure
 Reaction will shift to side with LESS gas particles to reduce
pressure
 Decreased pressure
 Reaction will shift to side with MORE gas particles to increase
pressure
 Equilibrium constant (Kc) remains the same
3) Temperature Changes
 Increased temperature
 Reaction shifts AWAY from increased temperature since
temperature is forcing more product to be made
 Similar to adding more product to a reaction
 Decreased temperature
 Slowing production of product, stress removed from forward
reaction
 Reaction shifts TOWARDS decrease in temperature
 Equilibrium constant (Kc) does NOT remain the same
 Changes with temperature
Example 4:
A + B  AB
A) Heat A
B) Cool B
C) Add AB
D) Take out B
Example 5:
 Predict the influence of certain changes to the following
chemical reaction.
 2SO3(g)  2SO2(g) + O2(g) ΔH°= 197.84 kJ/mol
 A) increasing reaction’s temperature
 B) increasing reaction’s pressure
 C) adding more O2 at equilibrium
 D) Removing O2 at equilibrium
Homework
 Due Friday ! !
 Equilibrium II Worksheet
 LeChatlier Problem Worksheet