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The word equilibrium is commonly used in science
“Forces are in equilibrium”
Forces are equal
“Objects have reached thermal equilibrium”
Temperatures are equal
Chemical equilibrium occurs when a forward
reaction and its reverse reaction proceed at the
same rate in a closed system
To some extent, all reactions reversible (↔)
Some reactions have very little reversibility
Referred to as “product favored”
Other reactions have lots of reversibility
Referred to as “reactant favored”
WHAT IS EQUILIBRIUM?
Different types of arrows are used in chemical equations associated with equilibrium:
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
DEPICTING CHEMICAL EQUILIBRIUM
For the general reaction:
A + B C
You can view the reaction as occurring in
three steps:
Initial mixing
Kinetic region
Equilibrium region
BASIC STEPS TO REACHING CHEMICAL
EQUILIBRIUM
When A and B are first brought together, there is no C present
The reaction proceeds as:
A + B C
This equation only represents the very start of the reaction
Things change as soon as some C is produced!
INITIAL MIXING STEP
As soon as some C has been produced, the reverse reaction is possible
A + B C
Overall, we still see an increase in the net concentration of C
As we approach equilibrium, the rate of the forward reaction becomes slower
KINETIC REGION
A point is finally reached where the
forward and reverse reactions occur at
the same rate
A + B C
There is no net change in the
concentration of any of the species
EQUILIBRIUM REGION
ILLUSTRATING CHEMICAL EQUILIBRIUM C
on
ce
ntr
ati
on
Time
Kinetic Equilibrium
Region Region
Products
Reactants
• An equilibrium exists when no further change in
concentration occurs
• Note that the concentrations of products and
reactants do not have to be equal!
• The equilibrium
concentrations of the
reactants and
products are the
same regardless of
whether or not you
start with only the
reactants or only the
products!
No products
No reactants
ACHIEVING CHEMICAL EQUILIBRIUM
3H2(g) + N2(g) ↔ 2NH3
3H2(g) + N2(g) ↔ 2NH3
Equilibrium should not be viewed as a
static condition
While concentrations do not change, products
and reactants continue to interconvert at equal
rates
Rates are NOT zero even though you see no
visible changes
Therefore, chemical equilibrium is
dynamic
HOW TO VIEW CHEMICAL EQUILIBRIUM
A SUMMARY OF CHEMICAL EQUILIBRIUM
The forward and reverse reaction rates are equal
Macroscopically, the system (reactants and products)
appears to not be doing anything
Microscopically, the system (reactants and products)
is dynamic
The net change in concentrations of reactants and
products will remain unchanged
The equilibrium concentrations of reacts and products will not
usually be equal
HOW TO DEPICT THE
LEVEL OF COMPLETION
A CHEMICAL REACTION
REACHES
INTRODUCING THE EQUILIBRIUM EXPRESSION
AND THE EQUILIBRIUM CONSTANT
THE LAW OF MASS ACTION
Norwegian chemists Guldberg and Waage
proposed in 1864 that the rate of a chemical
reaction is directly proportional to the products of
the reactants
Called the Law of Mass Action
The Law of Mass Action is represented by an
equilibrium expression
To generalize this expression, consider the reaction :
The equilibrium expression for this reaction would be :
Keq is called the equilibrium constant
Defined as the ratio of the concentrations of the products compared to the concentrations of the reactants
It is a number with NO UNITS!
THE EQUILIBRIUM EXPRESSION
Keq = [C]c[D]d
[A]a[B]b
aA + bB cC + dD
SOME NOTES ON THE EQUILIBRIUM
EXPRESSION
Products always goes in numerator of
expression whereas reactants go in
denominator
Values are raised to the coefficient in the chemical
equation
ONLY aqueous solutions or gases go in the
equilibrium expression
The concentrations of solids and liquids are
essentially constant
Therefore, NO pure substances - solids or liquids –
appear in equilibrium expression
If K << 1 (K< 0.10), the
reaction doesn’t go very
far to completion
Reactant-favored
At equilibrium, [products] <<
[reactants]
If K = 1, there are
substantial amounts
of both product and
reactant at
equilibrium
MAGNITUDE OF K
WHAT DOES IT TELL ME ABOUT A CHEMICAL
REACTION?
MAGNITUDE OF K
WHAT DOES IT TELL ME ABOUT A CHEMICAL
REACTION?
If K >> 1 (K > 10), the
reaction goes to
completion
Product-favored
At equilibrium,
[products] >>
[reactants]
Equilibrium constant for a reaction is the
same REGARDLESS of initial reactant
concentrations and equilibrium concentrations
RATIO IS CONSTANT!
ONE MORE THING…
3H2(g)+N2(g) 2NH3(g) The equilibrium constant, Keq, for the
reaction above is determined by:
Keq = [ NH3 ] 2
[ H2 ] 3 [ N2 ]
At 472C, Keq = 0.105
EXAMPLE
2NH3(g) 3H2(g)+N2(g) • Reversing a reaction will result in the new
equilibrium constant, :
Keq new =1 / Keq old
Keq new = [ H2 ] 3 [ N2 ]
[ NH3 ] 2
= (1 / 0.105) or 9.52
WHAT IF I REVERSE THE REACTION?
NH3(g) 1.5H2(g)+0.5N2(g) • Changing the number of moles of reactants and
products will exponentially change the equilibrium
constant:
Keq new= (Keq old)^n
Keq new = [ H2 ]1.5 [ N2 ]
0.5
[ NH3 ]
= (9.52)^0.5 or 3.09
WHAT IF I CHANGE THE # OF MOLES IN
THE REACTION?
GENERAL SUMMARY
If the equation is multiplied by a factor,
the equilibrium constant is raised to the
same factor
WHAT IF THE REACTION IS A MULTI -
STEP PROCESS?
When chemical
equations are added,
their equilibrium
constants are
multiplied together to
get the overall
equilibrium constant
Different reactions have different symbols for
K
Kc The most used, general equilibria constant with molar concentrations [M]
in the expression
THE DIFFERENT TYPES OF K
Kc = [C]c[D]d
[A]a[B]b
For equilibria that involves gases, partial pressures
can be used instead of concentrations!
aA (g) + bB (g) eE (g) + fF (g)
Kp =
Kp is used when the partial pressures are expressed
in units of atmospheres (atm)
EQUILIBRIA INVOLVING GASES
pEe pF
f
pAa pB
b
TWO DIFFERENT K’S
For the reaction:
2SO3(g) 2SO2(g) + O2(g)
We can write two equilibrium expressions!
In general, Kp ≠ Kc
However, partial pressures are proportional to concentration at a constant temperature via:
PV = nRT
Where:
R is the gas law constant:
0.0821 atm·L/mol·K
T is the temperature, K
RELATIONSHIP BETWEEN
CONCENTRATION AND PRESSURE
CONVERTING BETWEEN KC AND KP
The Δn is the change in the number of
moles of gasous products minus the
number of moles of gaseous reactants
For the following equilibrium, Kc = 1.10 x 107 at 700. oC.
What is the Kp?
PRACTICE!
atm L
mol K
2H2 (g) + S2 (g) 2H2S (g)
Kp = Kc (RT)Dng
T = 700 + 273 = 973 K
R = 0.0821
Dng = ( 2 ) - ( 2 + 1) = -1
Equilibria that involve more than one phase is called
heterogeneous equilibria
Example:
CaCO3 (s) ↔ CaO (s) + CO2 (g)
Equilibrium expressions for these types of systems do not
include the concentrations of the pure solids (or liquids)
because their concentrations do not vary!
Kc = [CO2]
Kp = PCO2
WHAT DO YOU DO IF THE REACTION HAS
MORE THAN ONE PHASE?
Chemical reactions tend to go to equilibrium
provided that the reaction takes place at a
significant rate
There is no relationship between the magnitude of
the equilibrium constant and the rate of a reaction!
Example:
2H2 (g) + O2 (g) 2H2O (g)
Kc = 2.9 x 1031 = [H2O]2
[H2]2 [O2]
This reaction is very product-favored
However, the reaction will take years to reach equilibrium at room
temperature!
EQUILIBRIUM AND RATE OF REACTION
Equilibrium constants can be found by experiment
If you know the equilibrium concentrations (or partial pressures) of the reactants and products, simply plug and chug into the expression!
Let us consider the following equilibrium:
H2 (g) + I2 (g) 2HI (g)
DETERMINING EQUILIBRIUM CONSTANTS
H2 (g) + I2 (g) 2HI (g)
At 425.4oC, it is determined that the concentration of all species at equilibrium is as follows:
H2 (g) 0.00022 M
I2 (g) 0.00772 M
HI (g) 0.00956 M
Calculate the equilibrium constant, Kc, for the system
PRACTICE!
The equilibrium expression for our system is:
Kc =
So, the equilibrium constant is calculated as:
Kc = = = 54
At equilibrium, are there mostly products, reactants, or a mixture of products and reactants in the system? Justify your answer.
PRACTICE!
[HI]2
[H2] [I2]
[HI]2
[H2] [I2] (0.00956)2
(0.00022)(0.00772)
We can predict the direction of a reaction by
calculating the reaction quotient, Q
Expression uses using any set of concentrations
of substances rather than just equilibrium
concentrations
Always written like so for the general reaction:
aA + bB ↔ eE + fF
PREDICTING THE DIRECTION OF A REACTION
Q = [E]e [F]f
[A]a [B]b
By comparing Q to the Kc value, we can predict
the direction for the reaction!
Q < Kc - Net forward reaction will occur
Q = Kc - No change, at equilibrium!
Q > Kc - Net reverse reaction will occur
COMPARING Q AND K
At 472oC, Keq = .105 for the following reaction:
N2(g) + 3H2(g) 2NH3(g)
2 minutes after this reaction starts, you measure the
concentrations and find:
[N2] = .0020M
[H2] = .10M
[NH3] = .15M
Is the system at equilibrium? If not, how must the
system shift in order to reach equilibrium?
PRACTICE!
These are not
necessarily
equilibrium
concentrations
PRACTICE!
• The reaction quotient, Q, is calculated as
follows:
Q = [NH3]2 = (.15)2 = 1.1x104
[N2][H2]3 (.0020)(.10)3
PRACTICE - COMPARE “Q” TO “KEQ”
1x104 ≠ .105
So, this reaction is NOT at equilibrium
In this case, Q > Keq
1x104 > .105
So, Q must get smaller to reach equilibrium
What do you need to make “Q” smaller?
More products or more reactants?
COMPARE “Q” TO “KEQ”
If:
More reactants are needed!
Q = [NH3]2
[N2][H2]3
Making the denominator
bigger makes Q smaller
so you need more
reactants (shifts left)
Equilibrium concentrations are based on:
The specific equilibrium
The starting concentrations
Temperature
Pressure
Reaction specific conditions
Altering conditions will stress a system, resulting in an
equilibrium shift
PREDICTING SHIFTS IN EQUILIBRIA
If a system at equilibrium is disturbed by a change in temperature, pressure, or the concentration of one of the components, the system will shift its equilibrium position so as to counteract the effect of the disturbance
In other words, any stress placed on an equilibrium system will cause the system to shift to minimize the effect of the stress
A QUICK RUNDOWN OF LE CHȂTELIER’S
PRINCIPLE
• Adding a reactant or product shifts the equilibrium
away from the increase
• Removing a reactant or product shifts the
equilibrium towards the decrease
• To optimize the amount of product at equilibrium,
we need to flood the reaction vessel with reactant
and continuously remove product
– We illustrate the concept with the industrial preparation
of ammonia
LE CHATELIER’S PRINCIPLE
CHANGE IN REACTANT OR PRODUCT
CONCENTRATIIONS
N2(g) + 3H2(g) 2NH3(g)
N2(g) + 3H2(g) 2NH3(g)
EXAMPLE
Consider the Haber process:
If H2 is added while the system is at equilibrium, the system
must respond to counteract the added H2
That is, the system must consume the H2 and produce products until
a new equilibrium is established
Therefore, [H2] and [N2] will decrease and [NH3] increases.
N2 and H2 are pumped
into a chamber The pre-heated gases
are passed through a
heating coil to the
catalyst bed
The catalyst bed is
kept at 460 - 550 C
under high pressure
The product gas
stream (containing
N2, H2 and NH3) is
passed over a
cooler to a
refrigeration unit
In the
refrigeration unit,
ammonia liquefies
but not N2 or H2
ILLUSTRATION OF THE HABER PROCESS
THE HABER PROCESS
The unreacted nitrogen and hydrogen are recycled with the
new N2 and H2 feed gas
The equilibrium amount of ammonia is optimized because the
product (NH3) is continually removed and the reactants (N 2
and H2) are continually being added
LE CHATELIER’S PRINCIPLE
EFFECTS OF VOLUME AND PRESSURE
Note - Changing the pressure does not change the value of the
equilibrium constant at constant temperature
Solids and liquids are not affected by pressure changes
Changing pressure by introducing an inert gas will not shift an
equilibrium
Pressure changes only affect gases that are a portion of an
equilibrium
This is due to the relationship between volume and pressure for a
gas:
As volume is decreased, pressure increases
If pressure is increased the system will shift to counteract the
increase by producing fewer moles of gas
That is, the system shifts to remove gases and decrease pressure
An increase in pressure favors the direction that has fewer moles of
gas
In a reaction with the same number of product and reactant
moles of gas, pressure has no effect!
LE CHÂTELIER’S PRINCIPLE
EFFECTS OF VOLUME AND PRESSURE
EXAMPLE
Consider:
An increase in pressure (by decreasing the volume) favors the
formation of colorless N2O4
The instant the pressure increases, the system is not at
equilibrium and the concentration of both gases has
increased
The system moves to reduce the number moles of gas
The forward reaction is favored
A new equilibrium is established in which the mixture is
lighter because colorless N 2O4 is favored
N2O4(g) 2NO2(g)
The equilibrium constant is temperature dependent!
For an endothermic reaction, DH > 0 and heat can be
considered as a reactant
For an exothermic reaction, DH < 0 and heat can be
considered as a product
Adding heat (i.e. heating the vessel) favors away from the
increase
If DH > 0, adding heat favors the forward reaction
If DH < 0, adding heat favors the reverse reaction
Removing heat (i.e. cooling the vessel), favors towards the
decrease
If DH > 0, cooling favors the reverse reaction
If DH < 0, cooling favors the forward reaction
LE CHATELIER’S PRINCIPLE
EFFECTS OF TEMPERATURE CHANGES
Cr(H2O)6(aq) + 4Cl-(aq) CoCl42-(aq) + 6H2O(l)Co
EXAMPLE
Consider the following reaction:
DH > 0
Co(H2O)62+ is pale pink and CoCl 4
2- is blue
If a l ight purple room temperature equilibrium mixture is placed in a beaker of warm water, the mixture turns deep blue
Since DH > 0 (endothermic), adding heat favors the forward reaction, i.e. the formation of blue CoCl4
2-
If the room temperature equilibrium mixture is placed in a beaker of ice water, the mixture turns bright pink
Since DH > 0, removing heat favors the reverse reaction which is the formation of pink Co(H2O)6
2+
A catalyst lowers the activation energy barrier for the reaction
Therefore, a catalyst will decrease the
time taken to reach equilibrium
A catalyst does not
effect the composition of
the equilibrium mixture
LE CHATELIER’S PRINCIPLE
THE EFFECT OF CATALYSTS
“Any stress placed on an equilibrium system will cause the
system to shif t to minimize the effect of the stress”
LE CHATELIER’S PRINCIPLE
• How can you cause the color to change from
pink to blue?
• You can put stress on a system by adding or removing something from one side of a reaction
Co(H2O)62+ + 4Cl1- CoCl4
2- + 6H2O