Chapter 16 –
Chemical Equilibria
1.
2.
3.
4.
5.
Objectives:
Determine equilibrium constant expressions.
Know the factors which affect equilibrium.
Calculate Q and K and predict effects on equilibrium.
Perform equilibrium calculations.
Recall the Le Chatelier’s principle and predict effects on
equilibrium.
Chemical Equilibrium
Equilibrium systems are:
• DYNAMIC (in constant motion)
• REVERSIBLE (can be approached
from either direction).
Pink to blue
Co(H2O)6Cl2 ---> Co(H2O)4Cl2 + 2 H2O
Blue to pink
Co(H2O)4Cl2 + 2 H2O ---> Co(H2O)6Cl2
Dynamic Equilibrium
Fe3+ + SCN-  FeSCN2+
+
• After a period of time, the concentrations of
reactants and products are _____________.
• The forward and reverse reactions
__________ after equilibrium is attained.
Chemical Equilibrium
In Phase Changes:
In the formation of stalactites and stalagmites:
CaCO3(s) + H2O(liq) + CO2(g)
Ca2+(aq) + 2 HCO3-(aq)
Reaction Quotient &
Equilibrium Constant
Reactants concentration
decrease; then become
constant at equilibrium
Product concentration
increases; it becomes
constant at equilibrium
Equilibrium achieved
Reaction Quotient &
Equilibrium Constant
At any point in the reaction:
Q = [HI]2
[H2][I2]
At Equilibrium:
K = [HI]2 = 55.3
[H2][I2]
ALL reacting chemical systems
are characterized by their
REACTION QUOTIENT, Q.
When Q = K, the system is at
equilibrium.
In general, for a reaction:
aA + bB
cC + dD
Q = [C]c[D]d
[A]a[B]b
Product concentrations
Reactant concentrations
Reaction Quotient &
Equilibrium Constant
• For any time of chemical equilibrium of the
type:
aA + bB
cC
+
dD
• The following is a constant:
K=
• If K is known, it is possible to predict
concentrations of reactants and products.
Equilibrium Expressions
• Solids and liquids never appear in
equilibrium expressions.
S (s) + O2
(g)
NH3 (g) + H2O (l)
SO2 (g)
K=
NH4 (aq) + OH- (aq)
K=
Meaning of K
1. Can tell if a reaction if product-favored or
reactant-favored.
N2 (g) + 3 H2 (g)
2 NH3 (g)
K=
The concentration of products is ________________ than the
reactants; the reaction is _____________ favored.
AgCl (s)
Ag+(aq) + Cl –(aq)
K=
The concentration of products is ________________ than the
reactants; the reaction is _____________ favored.
Product or Reactant Favored
Meaning of K
K comes from Thermodynamics.
In chapter 19:
If DGo < 0 ; reaction is product favored.
If DGo > O ; reaction is reactant favored.
DGo = -RT lnK
If K is really big; lnK > 0 DGo is __________.
If K is really small; lnK < O DGo is __________.
Meaning of K
2. Can tell if a reaction is at equilibrium, or
predict with way it will go to reach it.
n-butane
H H H H
H—C—C—C—C—H
H H H H
K =
[iso]
[n]
iso-butane
H H H
H—C—C—C—H
H
H
H C H
H
= 2.5
Practice
•
For the isomerization of butane (
) into isobutene (
), the Kc = 2.50
at 298K. Determine if the following systems are at equilibrium and if not, in
which direction will the reaction proceed to achieve equilibrium.
a)
•
b)
c)
Is the system at equilibrium when [butane] = 0.75 M and [isobutene] = 2.60 M?
If it is not, in which direction will the reaction proceed to achieve equilibrium?
An aqueous solution of ethanol and acetic acid, each at an initial
concentration of 0.810 M, is heated to 100oC. At equilibrium, the acetic
acid concentration is 0.748 M. Calculate K for the reaction.
CH3CH2OH + CH3COOH
CH3COOCH2CH3 + H2O
A 1.000 L flask is filled with 1.00 mol of H2 and 1.00 mol of I2 at 448oC.
The value of the equilibrium constant Kc for the reaction at 448oC is 50.5.
What are the equilibrium concentrations of H2, I2 and HI in moles per liter?
H2 + I2
2 HI
Manipulating K
When two reactions are added to give a third
net, the third K is the ____________ of the
first two.
S(s) + O2(g)
SO2(g) + 1/2 O2(g)
Find K for:
S(s) + 3/2 O2(g)
SO2(g)
SO3(g)
SO3(g)
Manipulating K
When a reaction is multiplied by a number to
change its coefficients, the K must
__________________.
S(s) + 3/2 O2(g)
X2:
2 S(s) + 3 O2(g)
SO3(g)
2 SO3(g)
Manipulating K
• When a reaction is reversed in direction, the
new K if found by calculating _____________.
S(s) + O2(g)
SO2(g)
SO2(g)
S(s) + O2(g)
Manipulating K
Concentration Units
We have been writing K in terms of mol/L.
These are designated by Kc
But with gases, P = (n/V)•RT = conc • RT
P is proportional to concentration, so we can write K in
terms of P. These are designated by Kp.
Kc and Kp may or may not be the same.
K using concentration and pressure units
Kp = Kc (RT)∆n
** Change in moles
For
S(s) + O2(g)
SO2(g)
∆n = 0 and Kp = Kc
For SO2(g) + 1/2 O2(g)
SO3(g)
∆n = –1/2 and Kp = Kc(RT)-1/2
Practice – Determine K
Given the following equilibria,
a) MgCO3(s)
Mg2+(aq) + CO32-(aq)
K1 = 6.8  10-6
b) MgF2(s)
Mg2+(aq) + 2 F-(aq)
K2 = 5.2  10-11
Determine the equilibrium constant for the following reaction.
c) MgCO3(s) + 2 F-(aq)
MgF2(s) + CO32-(aq)
K3 =?
Equilibrium and External Effects
• Temperature, catalysts, and changes in concentration
affect equilibria.
• The outcome is governed by
______________________________
• “...if a system at equilibrium is disturbed, the system
tends to shift its equilibrium position to counter the
effect of the disturbance.”
Henri Le Chatelier
1850-1936
Studied mining engineering.
Interested in glass and
ceramics.
Equilibrium and External Effects
• Temperature change
• Consider the fizz in a soft drink
CO2(aq)
CO2(g) + H2O(liq)
• Increase T. What happens to equilibrium position? To
value of K?
• Decrease T. Now what happens to equilibrium
position?
Equilibrium and External Effects
• Add catalyst
• A catalyst only affects the RATE of approach
to equilibrium.
• A catalyst affects both reactions forward
and reverse.
Catalytic exhaust system
Equilibrium and External Effects
• Concentration changes
– Only the equilibrium composition changes.
– Reaction adjusts to new equilibrium
“position”
Le Chatelier’s Principle
Adding a “reactant” to a chemical system.
Equilibrium shifts___________ .
Removing a “reactant” from a chemical system.
Equilibrium shifts___________ .
Le Chatelier’s Principle
Adding a “product” to a chemical system.
Equilibrium shifts___________ .
Removing a “product” from a chemical system.
Equilibrium shifts___________ .
Predict effect on K
Nitrogen Dioxide Equilibrium
Increase P in the system by reducing the volume
(at constant T).
N2O4(g)
2 NO2(g)
[NO2]2
K=
= 0.0058 at 298 K
N2O4
In gaseous system the equilibrium will
shift to the side with fewer molecules
(in order to reduce the P).
Therefore, reaction shifts ________
and P of NO2 _____________ and P of
N2O4 _____________.
Cobalt Complexes Equilibrium
• Is the following reaction endothermic, or exothermic?
In hot water, the solution looks blue, in ice-water bath
the solution looks pink.
Co(H2O)62+ (aq) + 4 Clpink
CoCl42- + 6 H2O
blue
Remember
• Go over all the contents of your
textbook.
• Practice with examples and with
problems at the end of the chapter.
• Practice with OWL tutor.
• Practice with the quiz on CD of
Chemistry Now.
• Work on your OWL assignment for
Chapter 16.