Chemistry 102(001) Fall 2014
Instructor: Dr. Upali Siriwardane
e-mail: upali@latech.edu
Office: CTH 311
Phone 257-4941
Office Hours: M,W 8:00-9:30 & 11:00-12:30 am; Tu,Th, F 8:00 10:00 am. or by appointment.;
Test Dates:
September 23, 2014 (Test 1): Chapter 13
October 16, 2014 (Test 2): Chapter 14 &15
November 11, 2014 (Test 3): Chapter 16 &7
November 13, 2014 (Make-up test) comprehensive:
Chapters 13-17
CHEM 102, Fall 14, LA TECH
14-1-1
Chapter 14. Chemical Equilibrium
14.1
14.2
14.3
14.4
Fetal Hemoglobin and Equilibrium 61 3
The Concept of Dynamic Equilibrium 61 5
The Equilibrium Constant (K) 61 8
Expressing the Equilibrium Constant in Terms of
Pressure 622
14.5 Heterogeneous Equilibria: Reactions Involving Solids
and Liquids 625
14.6 Calculating the Equilibrium Constant from Measured
Equilibrium Concentrations 626
14.7 The Reaction Quotient: Predicting the Direction of
Change 629
14.8 Finding Equilibrium Concentrations 631
14.9 Le Châtelier’s Principle: How a System at Equilibrium
Responds to Disturbances 641
CHEM 102, Fall 14, LA TECH
14-1-2
Chemical equilibrium
Different types of arrows are used in chemical
equations associated with equilibria.
Single arrow
Assumes that the reaction proceeds to completion
as written.
Two single-headed arrows
Used to indicate a system in equilibrium.
Two single-headed arrows of different sizes.
May be used to indicate when one side of an
equilibrium system is favored.
CHEM 102, Fall 14, LA TECH
14-1-3
Chemical Equilibrium
Branch of chemistry dealing with reactions
where reactants and products coexist in a
dynamic equilibrium
the rates of forward and backward reactions
have comparable rates reaction
CHEM 102, Fall 14, LA TECH
14-1-4
Chemical Equilibrium
Equilibrium region.
A point is finally reached where the forward
and reverse reactions occur at the same
rate.
H2 + I2
2HI
There is no net change in the concentration
of any of the species.
CHEM 102, Fall 14, LA TECH
14-1-5
Partial Pressure
Chemical Equilibrium
Kinetic
Region
Equilibrium
Region
HI
I2
H2
CHEM 102, Fall 14, LA TECH
Time
14-1-6
Complete Reaction
Equilibrium Region
Concentration
Kinetic
Time
CHEM 102, Fall 14, LA TECH
14-1-7
Equilibrium
A state where the forward and reverse
conditions occur at the same rate.
I’m in static
equilibrium.
Dynamic
Equilibrium
CHEM 102, Fall 14, LA TECH
14-1-8
Forward and Backward
Reactions
Potential
Energy
This type of plot
shows the energy
changes during
a reaction.
H
activation
energy
Reaction coordinate
CHEM 102, Fall 14, LA TECH
14-1-9
1) Chemical equilibrium is reached in a reaction
when
a) All reactions stop.
b) The forward reaction stop.
c) The concentration of products and reactants
become equal.
d) Rate of forward and reverse reaction become
equal.
e) The temperature become constant.
CHEM 102, Fall 14, LA TECH
14-1-10
2) For the equilibrium :
A(g) + B(g)
C(g) + D(g).
Are the concentrations of A,B,C, and D changing if
100 molecules per second of A and B are being
converted into C and D by the forward reaction
and 100 molecules per second of C and D are
being converted in to A and B by the reverse
reaction? Explain.
CHEM 102, Fall 14, LA TECH
14-1-11
Shifts with Temperature
N2O4(g)
colorless
2NO2(g)
Dark brown
N2O4(g)  2 NO2(g) ;  H=? (+or -)
CHEM 102, Fall 14, LA TECH
14-1-12
Melting of Ice
CHEM 102, Fall 14, LA TECH
14-1-13
3) What are examples of physical and chemical
dynamic equilibrium you have seen?
CHEM 102, Fall 14, LA TECH
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Types of Equilibria
Homogenous equilibrium: Chemical equilibrium
where reactants and products are in same
phase.
Heterogeneous equilibrium: Chemical Equilibrium
where at least one phase of a reactant or product
is different from the rest.
CHEM 102, Fall 14, LA TECH
14-1-15
4) What are the homogenous and heterogeneous
equilibrium among the following?
H2(g) + CO2(g)
NH4Cl(s)
NaCl(s)
CHEM 102, Fall 14, LA TECH
H2O(g) + CO(g)
NH3(g) + HCl(g)
Na+(aq) + Cl-(aq)
14-1-16
Law of mass Action
Defines an equilibrium constant (K) for the process
jA+kB
lC+mD
[C]l[D]m
K = ----------------- ; [A], [B] etc are
[A]j[B]k
Equilibrium concentrations
Pure liquid or solid concentrations are not written in
the expression.
CHEM 102, Fall 14, LA TECH
14-1-17
Value of K
k+
K = ------------------------------ = --rate of forward Reaction
rate of backward Reaction
k-
K = a (infinity) -> Irreversible reactions
K = 0 -> No reaction
K = between 0 and 1 -> Equilibrium reactions
CHEM 102, Fall 14, LA TECH
14-1-18
Equilibrium Expression
An equilibrium expression could be written
for any reaction
[HI]2
K = ----------- = 16 L/mol
[H2][I2]
Keq >> 1
reaction will go mainly to products
Keq ~ 1
reaction will produce roughly
equal
amounts of product and
reactant
Keq << 1
reaction will go mainly to reactants
CHEM 102, Fall 14, LA TECH
14-1-19
k is constant at a temperature
N2O4(g)
colorless
Initial
[NO2]2
Keq 
[N2O4 ]
2NO2(g)
Dark brown
@ Equilibrium
N2O4
NO2
N2O4
NO2
Keq
0.00
0.02
0.0014
0.017
0.21
0.00
0.03
0.0028
0.024
0.21
0.00
0.04
0.0045
0.031
0.21
0.02
0.00
0.0045
0.031
0.21
CHEM 102, Fall 14, LA TECH
14-1-20
5) What items goes into the numerator and
denominator in equilibrium constant
expression?
CHEM 102, Fall 14, LA TECH
14-1-21
6) What determines the exponent of each
concentration in the equilibrium constant
expression?
2H2 (g) + S2 (g)
CHEM 102, Fall 14, LA TECH
2H2S (g)
14-1-22
7) What is the difference between initial [A]i and
equilibrium [A]eq concentrations?
N2O4(g)
colorless
Initial
N2O4 NO2
0.00 0.02
0.00 0.03
0.00 0.04
0.02 0.00
CHEM 102, Fall 14, LA TECH
[NO2]2
Keq 
[N2O4 ]
@ Equilibrium
N2O4
NO2
0.0014
0.017
0.0028
0.024
0.0045
0.031
0.0045
0.031
2NO2(g)
Dark brown
Keq
0.21
0.21
0.21
0.21
14-1-23
Selected Equilibrium Constants
CHEM 102, Fall 14, LA TECH
14-1-24
8) Which will be present in the largest amounts,
reactants or products, when
a) The equilibrium constant is large?
b) The equilibrium constant is small?
CHEM 102, Fall 14, LA TECH
14-1-25
9) Write the equilibrium expression for flowing
reactions:
a) CO(g) + Cl2(g)
COCl2(g) : K =
b) N2O4(g)
2NO2(g) : K =
c) MgCO3(s)
MgO(s) + CO2(g) : K =
d) NaCl(s) + H2O (l)
CHEM 102, Fall 14, LA TECH
Na+(aq) + Cl-(aq):
K=
14-1-26
What is K (Kc) and Kp
Kc (K) - equilibrium constant calculated based on
[A]-Concentrations.
Kp- equilibrium constant calculated based on partial
pressure
Kp =
CHEM 102, Fall 14, LA TECH
14-1-27
10) What is the difference between K
and Kp?
Write K and Kp for the reaction:
CO(g) + Cl2(g)
K=
CHEM 102, Fall 14, LA TECH
COCl2(g)
Kp =
14-1-28
Reaction quotient
Any set of concentrations can be given and a Q
calculated. By comparing Q to the Kc value, we
can predict the direction for the reaction.
Q < Kc
Q = Kc
Q > Kc
Net forward reaction will occur.
No change, at equilibrium.
Net reverse reaction will occur.
CHEM 102, Fall 14, LA TECH
14-1-29
11) Given the equilibrium constant,
concentration of reactants and products of
reaction mixture, How could you determine
if the reaction is at equilibrium or not?
CHEM 102, Fall 14, LA TECH
14-1-30
Terminology
Initial concentration:
concentration (M) of reactants and products
before the equilibrium is reached.
Equilibrium Concentration
Concentration (M) of reactants and products
After the equilibrium is reached.
CHEM 102, Fall 14, LA TECH
14-1-31
12) Given the concentrations of reactants and
products at the beginning, and the equilibrium
constant, how could you determine whether the
reaction will proceed in the forward or the reverse
direction to reach equilibrium?
CHEM 102, Fall 14, LA TECH
14-1-32
13) For the reaction,
5CO(g) + I2O5(s)
I2(g) + 5CO2(g)
the expression for Kc is:
the expression for Kp is:
CHEM 102, Fall 14, LA TECH
14-1-33
14) Consider the equilibrium
3H2(g) + N2(g)
2NH3(g)
at a certain temperature. An equilibrium mixture in
a 4.00-L vessel contains 1.60 mol NH3(g), 0.800
mol N2(g), and 1.20 mol H2(g). What is the value of
Kc?
CHEM 102, Fall 14, LA TECH
14-1-34
Predicting Shifts in Equilibrium
Le Chatelier’s principle
Equilibrium concentrations are based on:
• The specific equilibrium
• The starting concentrations
• Other factors such as:
• Temperature
• Pressure
• Reaction specific conditions
Altering conditions will stress a system, resulting in
an equilibrium shift.
CHEM 102, Fall 14, LA TECH
14-1-35
15) According to La Chatelier's Principle, for the
following reaction at equilibrium:
2NO(g) + Cl2(g)
2ClNO(g); H = - (exothermic)
a) If the pressure of the reaction container is decreased,
what can you say about pressure or the concentration
of Cl2?
a) If the temperature is increased, which way the
equilibrium would shift?
CHEM 102, Fall 14, LA TECH
14-1-36
Stepwise Equilibrium
[NO]2
(1)
(2)
2NO(g) + O2(g)  2NO2(g)
Add to Combine (1.) & (2.)
N2(g) + 2O2(g)  2NO2(g)
Kc =
Kc1 =
N2(g) + O2(g)  2NO(g)
[NO]2
[N2][O2]
CHEM 102, Fall 14, LA TECH

[NO2]2
[NO]2[O2]
[N2][O2]
[NO2]2
Kc2 =
[NO]2[O2]
= Kc1  Kc2
14-1-37
Calculating Stepwise Equilibrium
Add two equations with K1 and K2 to get Keq
Keq = K1 x K2
Subtract one equations with K2 from another with
K2 to get Keq
Keq = K1 / K2
Doubling K1 to get Keq
Keq = (K1)2 ;tripling Keq = (K1)3 etc.
Reversing a reaction with K1 get Keq
Keq = (K1)-1
CHEM 102, Fall 14, LA TECH
14-1-38
Stepwise Equilibrium
Consider the reactions
2NO + O2 <===> 2 NO2
K=a
2 NO2
<===> N2O4
K=b
The value of the equilibrium constant for
the reaction
2NO + O2 <===> N2O4 is
a. a + b
b. ab
c. (a/b)2
d. (ab)2
e. ab/2
CHEM 102, Fall 14, LA TECH
14-1-39
Stepwise Equilibrium
Consider the reactions
2NO + O2 <===> 2 NO2
K=a
2 NO2
<===> N2O4
K=b
The value of the equilibrium constant for the
reaction
4NO + 2O2 <===> 2 N2O4 is
a. a + b
b. ab
c. (a/b)2
d. (ab)2
e. ab/2
CHEM 102, Fall 14, LA TECH
14-1-40
16) Consider the reaction:
4NH3(g) + 5O2(g)
4NO(g) + 6H2O(g);
a) The equilibrium constant expression of the reaction K
is:
b) The equilibrium constant expression for reverse of this
reaction in terms of K is:
c) The equilibrium constant expression for of this reaction
multiplied by a factor of two and in terms of K is:
CHEM 102, Fall 14, LA TECH
14-1-41