ERT207
ANALYTICAL CHEMISTRY
CHEMICAL
EQUILIBRIUM
26 JAN 2011
MISS NOORULNAJWA DIYANA
YAACOB
The Concept of Equilibrium
 Do all chemical reactions go to completion?
When you start with only reactants, do you end up with
only products?
 Most of chemical reactions are reversible. They do not go
to completion (just products present).
 In fact, they will be left, at some instance, with a mixture
of reactants and products, which are in equilibrium (their
concentrations are not changing).
 At equilibrium the concentrations of the products and
the reactants are not changing.
2
Equilibrium, Continued
Rate
Initially,
only
theisforward
Because
As
Once
the equilibrium
forward
the
reactant
reaction
established,
concentration
proceeds
Eventually,
the
reaction
proceeds
reaction
takes
place.
itinthe
decreases,
makes
products
the
and
forward
reverse
and uses
reaction
reactions
reactants.
theforward
reverse
direction
as
fast
asslows.
As
the products
at in
the
same
accumulate,
rate,direction.
sothe
the
itproceed
proceeds
the
forward
reverse
concentrations
reaction
of
speeds
all materials
At
this time
equilibrium
isup.
established.
stay constant.
Rate forward
Rate reverse
Time
3
Chemical Equilibrium
N2O4 (g)
kf
kr
2NO2 (g)
Reversible reaction
As time passes, more NO2 is produced
and the brown color intensifies.
After some time, the brown color
stops changing. The concentration of
NO2 becomes constant. Same does
the concentration of N2O4.
At this instance, does the
reaction stop?
Dr. Al-Saadi
4
Chemical Equilibrium
N2O4 (g)
kf
kr
2NO2 (g)
Rate forward = kf [N2O4]
Rate backward = kr [NO2]2
The reaction is reversible, i.e. it does NOT stop but proceeds at both directions.
Reaction progress
[N2O4] is high
;
[NO2] is zero
Rate forward >> Rate backward
Rate forward
> Rate backward
Rate forward = Rate backward
Dynamic equilibrium is established
The reaction does not stop. It
continues running on both
directions at equal rates.
Dr. Al-Saadi
5
Chemical Equilibrium
N2O4 (g)
kf
kr
2NO2 (g)
Rate forward = kf [N2O4]
Rate backward = kr[NO2]2
The rates of the reaction
(forward and backward
rates) change over time.
The forward rate
gradually decreases,
while the backward rate
gradually increases
(starting from zero).
Eventually, at equilibrium,
both rates become equal.
6
Equilibrium
Constants
7
At equilibrium the rates of the
forward and reverse reactions are
equal, and the concentrations of the
reactants
and
products
are
constant.
8
The equilibrium constant (Keq) is a value
representing
the
unchanging
concentrations of the reactants and the
products in a chemical reaction at
equilibrium.
9
For the general
reaction aA + bB → cC + dD
at a given temperature
→
c
K eq
d
[C] [D]
=
a
b
[A] [B]
10
For the reaction
3H2 + N2 → 2NH3
→
2
K eq
[NH3 ]
=
3
[H2 ] [N2 ]
11
For the
reaction
4NH3 + 3O2 → 2N2 + 6H2O
→
2
K eq
6
[N2 ] [H2O]
=
4
3
[NH3 ] [O2 ]
12
The magnitude of an equilibrium constant indicates the
extent to which the forward and reverse reactions take
place.The larger the equilibrium constant, the farther to
the right is the reaction at equilibrium.
H2 + I2 → 2HI
→
2
K eq
[HI]
o
=
= 54.8 at 425 C
[H2 ] [I2 ] At equilibrium more product
At equilibrium more reactant
than
exists.
than reactant
product exists.
COCl2 → CO + Cl2
→
K eq
[CO][Cl2 ]
-4
o
=
= 7.6 x 10 at 400 C
[COCl2 ]
13
Magnitude of Equilibrium Constant
aA+bB
cC +dD
Kc = (
[C]c [D]d
[A]a [B]b
 For the above reaction, three outcomes are possible:
o The reaction goes to completion. The equilibrium
mixture will consist predominantly from products.
)eq
The value of
Kc is very
large
o
The reaction doesn't occur to any significant degree. The value of
Kc is very
The equilibrium mixture will consist predominantly
small
from reactants.
o
The reaction occurs to a significant degree, but not to The value of
completion. The equilibrium mixture will have both Kc is neither
large nor
reactants and products in comparable quantities.
small
14
Magnitude of Equilibrium Constant
aA+bB
Kc = (
cC +dD
[C]c [D]d
[A]a [B]b
)eq
Kc
10-2
The magnitude of Kc is
very small.
The reaction will
almost not occur
The reaction will
have an
equilibrium
mixture of both
reactants and
products.
102
The magnitude of Kc is
very large.
The reaction will go
almost to completion
15
TYPES OF EQUILIBRIA
 We can write equilibrium constants for many types
of chemical processes. The equilibria may represent :
o dissociation (acid/base, solubility)
o Formation of products (complexes)
o Reaction (redox)
o Distribution between 2 phases (water and
nonaqueous solvent
Le Châtelier's Principle
 Le Châtelier's principle states that:
If a change is imposed on a system at equilibrium,
the system will respond by shifting in the (forward or
reverse) direction that minimizes the effect of that
change. As a result, an new equilibrium position will
be reestablished.
.
18
Le Châtelier’s Principle
equilibrium
non-equilibrium
When a change is imposed on a system at equilibrium, the
system will react in the direction that reduces the amount
of change
Changes made on the system can be:
o Addition or removal of a reactant or product
(Effect of concentration)
o change in the volume and pressure of the system
(Effect of pressure).
o change in temperature (Effect of temperature).
Dr. Al-Saadi
20
Effect of
Concentration on
Equilibrium
21
 For most reactions the rate of reaction increases as
reactant concentrations increase.
 An equilibrium is disturbed when the concentration
of one or more of its components is changed. As a
result, the concentration of all species will change
and a new equilibrium mixture will be established
22
Changes in Concentration
Change
Increase in [Products]
Decrease in [Products]
Increase in [Reactants]
Decrease in [Reactants]
Shift in Equilibrium
left
right
right
left
Effect of Concentration Changes
on the Chlorine Water Equilibrium
+increase
decrease Cldecrease
increase
H
H
O
O
increase
HOCl
Cl
2
32
concentrationconcentration
concentrationconcentration
concentration
Cl2(aq) + 2H2O(l) → HOCl(aq) + H3O+(aq) + Cl-(aq)
→
toright
left
Equilibrium
Equilibriumshifts
shiftsto
24
Effect of Pressure
on Equilibrium
25
 Changes in pressure significantly affect the reaction
rate only when one or more of the reactants or
products is a gas and the reaction is run in a closed
container.
 The effect of increasing the pressure is to increase
the concentrations of any gaseous reactants or
products
26
Changes in Pressure and Volume
Change
Increase in Pressure
Decrease in Pressure
Increase in Volume
Decrease in Volume
Shift in Equilibrium
Side with fewest moles
Side with most moles
Side with most moles
Side with fewest moles
In a system composed entirely of gases, a increase in
the pressure of the container will cause the reaction
and the equilibrium to shift to the side that contains
the smallest number of molecules.
28
Increase Pressure
N2(g) + 3H2(g) → 2NH3(g)
→
1 mol
3 mol
6.02 x 1023 1.81 x 1024
molecules molecules
2 mol
1.20 x 1024
molecules
2.41 x 1024
molecules
Equilibrium shifts to the right towards fewer molecules.
29
Increase Pressure
N2(g) + O2(g) → 2NO(g)
→
1 mol
1 mol
6.02 x 1023 6.02 x 1023
molecules molecules
2 mol
1.20 x 1024
molecules
1.20 x 1024
molecules
Equilibrium does not shift. The number of molecules is
the same on both sides of the equation.
30
Change in Volume and Pressure
 Generally,
o A decrease in the volume of a reaction vessel will
cause a shift in the equilibrium in the direction
that minimizes the total number of moles.
o An increase in the volume of a reaction vessel will
cause a shift in the equilibrium in the direction
that maximizes the total number of moles.
31
Change in Volume and Pressure
 Generally,
o A decrease in the volume of a reaction vessel will
cause a shift in the equilibrium in the direction
that minimizes the total number of moles.
o An increase in the volume of a reaction vessel will
cause a shift in the equilibrium in the direction
that maximizes the total number of moles.
32
Effect of Temperature
on Equilibrium
33
The rate of the reaction that absorbs
heat is increased to a greater extent,
In a reversible
and
the equilibrium
reaction,
shifts
the to
rates
favor
of
Whenreaction.
the temperature
ofthe
aprocess
system
both
that
the
forward
Whenand
the
reverse
is
raised,
the rate
increases.
reactions
endothermic,
areof
thereaction
increased
forward
(left
by an
to
increase
right)
reaction
in temperature.
is increased. When the
process is exothermic, the reverse
(right to left) process is increased.
34
Heat may be treated as a reactant
in endothermic reactions.
oC moles CO
At room
temperature
very
little
COCO
forms.
At 1000
moles
2 
C(s) + CO2(g) + heat → 2CO(g)
→
Equilibrium shifts to right
35
Change in Temperature
 Unlike the case with concentration and pressure
changes, the change in temperature of a chemical
reaction can change the value of the equilibrium
constant.
It makes the reaction faster or slower, depending on
the enthalpy change (ΔH) “heat” accompanying the
reaction.
36
Change in Temperature
N2O4(g)
2NO2(g) ΔH= 58.0 kJ/mol
 Let’s apply here Le Châtelier's principle to the heat
absorbed as a reactant.
Heat + N2O4(g)
2NO2(g)
Adding heat means the reaction will be shifted to
the right. Also, addition of heat means an increase in
temperature.
 In general, increasing the temperature of
endothermic reactions shifts it to the right. While
decreasing the temperature of endothermic
reactions shifts it to the left.
37
Change in Temperature
N2O4(g)
yellow
2NO2(g)
ΔH= 58.0 kJ/mol
brown
yellow
Heating
brown
38
Change in Temperature
 Consider the following exothermic reaction:
CoCl42- + 6H2O
Co(H2O)62+ + 4Cl- + Heat
blue
pink
Heating
Cooling
CoCl42- and Co(H2O)62+
ions at equilibrium
39
Change in Temperature
 The increase in temperature favors endothermic
reactions.
 The decrease in temperature favors exothermic
reactions.
 The change in temperature not only affects the
equilibrium position, but also alters the value of the
equilibrium constant.
40
Changes in Temperature
Change
Increase T
Decrease T
Endo. Rx
K decreases
K increases
Exo. Rx
K increases
K decreases
Effect of Catalysts
on Equilibrium
42
A catalyst is a substance that influences
the rate of a reaction and can be
recovered essentially unchanged at the
end of the reaction.
A catalyst does not shift the
equilibrium of a reaction. It affects
only the speed at which the equilibrium
is reached.
43
Energy Diagram for an Exothermic Reaction
Activation
A
catalyst
catalyst energy:
speeds
does not
up
the achange
minimum
reactionthe
energy
by energy
lowering
required
ofthe
a
for a reaction
activation
reaction.
energy.
to occur.
44
AlCl3
PCl3(l) + S(s) → PSCl3(l)
In the little
Very
presence
thiophosphoryl
of a catalystchloride
the reaction
is formed
is complete
in the
in
absence
a
few seconds.
of a catalyst because the reaction is so slow.
MnO2
2KClO3(s) → 2KCl + 3O2(l)
Δ
The laboratory preparation of oxygen uses manganese
dioxide as a catalyst to increase the rate of the reaction.
45
AlCl3
PCl3(l) + S(s) → PSCl3(l)
In the little
Very
presence
thiophosphoryl
of a catalystchloride
the reaction
is formed
is complete
in the
in
absence
a
few seconds.
of a catalyst because the reaction is so slow.
MnO2
2KClO3(s) → 2KCl + 3O2(l)
Δ
The laboratory preparation of oxygen uses manganese
dioxide as a catalyst to increase the rate of the reaction.
46