Download Chemical Equilibrium

11
Topic
Chemical Equilibrium
Unit 39 An introduction to chemical equilibrium
Unit 40 Factors affecting chemical equilibrium
systems
Key
C o ncepts
An introduction to chemical
equilibrium
• Irreversible and reversible reactions
• Characteristics of a system in dynamic
equilibrium
• Calculations involving equilibrium
constant, Kc
Chemical Equilibrium
Factors affecting chemical
equilibrium systems
• Le Chatelier’s principle
• Effect of concentration changes
• Effect of pressure changes
• Effect of temperature changes
• Applying principles of reaction rates
and equilibria to industrial processes
Topic 11 Chemical Equilibrium
Unit 39
An introduction to chemical equilibrium
Unit 39 An introduction to chemical equilibrium
39.1 – 39.13
Summary
39.1
Irreversible and reversible reactions
39.2
Equilibrium
39.3
Chemical equilibrium for a reversible reaction
39.4
The importance of a closed system
39.5
39.6
1 A dynamic equilibrium is reached when the forward and backward reactions occur
at the same rate.
2 Characteristics of a system in dynamic equilibrium:
• Equilibrium can only be established in a closed system with constant temperature
and pressure.
Equilibrium established from either direction of a reaction
• Once equilibrium is reached, the composition of the system remains constant.
Effect of changing conditions on chemical equilibrium
systems
• The reaction has not stopped, rate of forward reaction = rate of backward
reaction.
39.7
Characteristics of a system in dynamic equilibrium
39.8
The equilibrium constant
39.9
The equilibrium law
39.10 Calculating equilibrium constants
39.11 What does the equilibrium constant tell us?
39.12 Equilibrium systems involving components in more than one
state
39.13 Determining the equilibrium constant for an esterification
reaction experimentally
• Reactants and products will both be present in equilibrium mixture.
• Equilibrium can be reached from either direction.
• If the conditions (temperature, pressure and concentration) are changed, the
equilibrium established may be affected.
3 For a reversible reaction:
aA + bB
equilibrium constant Kc =
cC + dD
[C]ceqm [D]deqm
[A]aeqm [B]beqm
4 At a given temperature, the equilibrium constant, Kc, for a reaction always has the
same value.
5 The expression for the reaction quotient, Qc, is defined in the same way as the
equilibrium constant except that the concentrations of reactants and products can
be taken at any moment of the reaction (not necessarily the concentrations at
equilibrium). Qc is not a constant.
• If Qc = Kc, the system is at equilibrium.
• If Qc < Kc, a net forward reaction occurs until equilibrium is reached.
• If Qc > Kc, a net backward reaction occurs until equilibrium is reached.
Exam tips
♦ Questions often give the percentage dissociation of a substance and
ask students to calculate the equilibrium constant.
♦ When calculating Kc, remember that the concentration of a solid is
constant.
e.g.
NH4HS(s)
NH3(g) + H2S(g)
Kc = [NH3(g)][H2S(g)]
♦ DO NOT confuse the terms ‘reaction quotient’ and ‘equilibrium
constant’.
Topic 11 Chemical Equilibrium
Unit 39 An introduction to chemical equilibrium
♦ Questions may ask students to decide the direction in which the reaction
will proceed to achieve equilibrium based on the value of Qc.
e.g.
3
–1
The equilibrium constant, Kc, for the following reaction is 0.200 dm mol
at 873 K.
CO(g) + Cl2(g)
COCl2(g)
A mixture of 2.00 moles of CO(g), 1.00 mole of Cl2(g) and 0.500 mole
3
of COCl2(g) is introduced into an evacuated vessel of 5.00 dm kept at
873 K.
=
)(
)
–1
= 1.25 dm mol
Hydrogen is manufactured by steam reforming of natural gas, which involves the
following reaction:
Ni(s)
CO(g) + 3H2(g)
In a simulation study of the manufacturing process, CH4(g) and H 2O(g) are
allowed to react in the presence of nickel as catalyst in a closed container kept at
a constant temperature at 1 000 K. The initial concentrations of CH4(g) and H2O(g)
are 0.0120 mol dm–3. When equilibrium is attained, the concentration of CH4(g) is
0.00694 mol dm–3.
a) Calculate the equilibrium constant, Kc, for the reaction at 1 000 K.
(4 marks)
b)Give TWO advantages of using H2(g) as a source of energy.
(2 marks)
Answer
a) (0.00506 mol dm–3)(0.0152 mol dm–3)3
(0.00694 mol dm–3)(0.00694 mol dm–3)
=
= 3.69 x 10
–4
2
–6
mol dm ∴ the equilibrium constant, Kc, for the reaction at 1 000 K is
3.69 x 10–4 mol2 dm–6.
CH4(g) + H2O(g)
CO(g) + 3H2(g)
Initial concentration
(mol dm–3)
0.0120
0.0120
0
0
Equilibrium
concentration
(mol dm–3)
0.00694
0.00694
0.00506
0.0152
(1)
(1)
Hydrogen has a small molar mass. The ratio of energy output per unit mass of H2(g)
is higher than that of other fuels.
(1)
➤Instead of the concentrations of the species involved, questions may give
the number of moles of each species. So, remember to use concentrations
when calculating the equilibrium constant, Kc.
Example
CH4(g) + H2O(g)
Remarks*
Remarks
Qc > Kc ∴ reaction will proceed to the left to achieve equilibrium.
(1)
➤Given the equilibrium constant, Kc, students should be able to calculate the
equilibrium concentrations, and vice versa.
0.500
mol dm–3
5.00
2.00
1.00
mol dm–3
mol dm–3
5.00
5.00
3
[CO(g)][H2(g)]
[CH4(g)][H2O(g)]
[COCl2(g)]
[CO(g)][Cl2(g)]
(
Kc=
b)The combustion of H2(g) gives only water, which will not cause pollution to our
environment.
(1)
Calculate the reaction quotient, Qc, of the system at the start of the
reaction. Then decide the direction in which the reaction will proceed
to achieve equilibrium.
Qc =
3
(1)
Topic 11 Chemical Equilibrium
Unit 40
Factors affecting chemical equilibrium systems
Unit 40 Factors affecting chemical equilibrium systems
40.1 – 40.9
Summary
40.1
Position of equilibrium
40.2
Effect of changing conditions on systems in equilibrium
40.3
The Haber process
40.4
The effect of concentration changes on chemical equilibrium
system
40.5
1 Le Chatelier’s principle states that if the condition of a system in equilibrium is
changed, the position of equilibrium will shift so as to reduce that change.
2 The following is a summary of effect of concentration changes on a chemical
equilibrium system.
Predicting the shift in equilibrium position using the reaction
quotient, Qc
Action on
equilibrium
system
Rates of forward
and backward
reactions
Direction of
net reaction
Attainment
Shift of
of
position of Value of Kc
equilibrium equilibrium
Increasing the
concentration
of a reactant
rate of forward
reaction increases
a net forward
reaction occurs
faster
to the right
Decreasing the
concentration
of a reactant
rate of forward
a net backward
reaction decreases reaction occurs
slower
to the left
40.6
The effect of pressure changes on chemical equilibrium
systems
40.7
The effect of temperature changes on chemical equilibrium
systems
40.8
Applying principles of reaction rates and equilibria to industrial
processes
Increasing the
concentration
of a product
rate of backward a net backward
reaction increases reaction occurs
faster
to the left
40.9
Linking equilibria together
Decreasing the
concentration
of a product
rate of backward
a net forward
reaction decreases reaction occurs
slower
to the right
no change
3 The following is a summary of effect of pressure changes on a chemical equilibrium
system.
aA(g) + bB(g)
cC(g) + dD(g)
Pressure (volume)
change on
equilibrium
system
Rates of forward
and backward
reactions
a + b >
c + d
rates of both
reactions increase,
but the forward
reaction rate
increases more
a net
forward
reaction
occurs
a + b =
c + d
same increase
in rates of both
reactions
no net
reaction
a + b <
c + d
rates of both
reactions increase,
but the backward
reaction rate
increases more
a net
backward
reaction
occurs
to the left
a + b >
c + d
rates of both
reactions decrease,
but the forward
reaction rate
decreases more
a net
backward
reaction
occurs
to the left
a + b =
c + d
same decrease
in rates of both
reactions
no net
reaction
a + b <
c + d
rates of both
reactions decrease,
but the backward
reaction rate
decreases more
a net
forward
reaction
occurs
Increase
in
pressure
(decrease
in
volume)
Decrease
in
pressure
(increase
in
volume)
Direction Attainment
Shift of
of net
of
position of Value of Kc
reaction equilibrium equilibrium
to the right
faster
slower
no change
no change
to the right
no change
no change
Topic 11 Chemical Equilibrium
10
Unit 40 Factors affecting chemical equilibrium systems
4 The following is a summary of effect of temperature changes on a chemical equilibrium
system.
Temperature
Rates of
change on forward and
equilibrium
backward
system
reactions
Type of
forward
reaction
Direction of
net reaction
Increase in
temperature
rates of both
a net backward
exothermic
reactions
reaction occurs
increase, but
a net forward
not to the
endothermic
reaction occurs
same extent
Decrease in
temperature
rates of both
a net forward
exothermic
reactions
reaction occurs
decrease, but
a net backward
not to the
endothermic
reaction occurs
same extent
Attainment
Shift of
of
position of
equilibrium equilibrium
Value
of Kc
to left
decrease
to right
increase
to right
increase
to left
decrease
faster
slower
Answer
a) Initial
concentration
(mol dm–3)
Equilibrium
concentration
(mol dm–3)
Kc=
CH2=CH2(g) + H2O(g) 1.64
2.00
♦ Instead of pressure change, questions may ask the effect of volume
change on an equilibrium system.
1.00 – (1.64 x 5.00%) 1.64 x 5.00%
2.00
2.00
= 0.779
= 0.459
= 0.0410
[CH3CH2OH(g)]
[CH2=CH2(g)][H2O(g)]
(1)
(1)
0.0410 mol dm
(0.779 mol dm–3)(0.459 mol dm–3)
=
= 0.115 dm mol 3
(1)
–1
(1)
(1)
When the temperature is decreased, the system will respond by raising the
temperature.
(1)
As the forward reaction is exothermic, the system will undergo a net forward
reaction.
(1)
The position of equilibrium will shift to the right.
Consider the following equilibrium system:
c) Any two of the following:
CaCO3(s)
• Cool the gaseous mixture emerged from the reaction chamber to separate the
ethanol. Allow the unreacted ethene and steam to pass back to the reaction
chamber so as to increase the percentage of conversion of ethene to ethanol.(1)
• Use excess steam. The position of equilibrium will shift to the right.
• Increase the pressure of the system. The position of equilibrium will shift to the
right.
(1)
CaO(s) + CO2(g)
∆H > 0
Kc = [CO2(g)]
As the value of Kc depends only on temperature, adding CaCO3(s) to
the system will NOT affect the concentration of CO2(g).
Example
Remarks*
Remarks
Ethanol is manufactured by catalytic hydration of ethene:
0
–3
♦ The value of Kc depends only on temperature.
e.g.
CH3CH2OH(g)
1.00
2.00
1.64 x (100 – 5.00)%
2.00
b)Kc will increase.
Exam tips
11
CH2=CH2(g) + H2O(g)
CH3CH2OH(g)
➤Questions often ask the ways of increasing the yield of a product when a
chemical process is put into industrial practice.
∆H < 0
In a simulation study of the manufacturing process, 1.60 moles of CH2=CH2(g) and
1.00 mole of H2O(g) are mixed in a 2.00 dm3 container to react in the presence of a
catalyst at 573 K and 60 atmospheres. When equilibrium is attained, 5.00% of ethene
is converted into ethanol.
a) Calculate the equilibrium constant, Kc, under the above conditions.
(4 marks)
b)State, with explanation, the effect of a decrease in temperature of the system on
Kc. (3 marks)
c) In practice, the conversion of ethene to ethanol can be made much higher than
5.00%. Suggest TWO ways to increase the conversion of ethene into ethanol.
(2 marks)
e.g.
Ammonia is manufactured by the Haber process:
N2(g) + 3H2(g)
Fe(s)
2NH3(g)
∆H < 0
Suggest TWO ways to increase the yield of ammonia when the process is
put into industrial process.
– Increase the pressure of the system.
– Separate ammonia by liquefaction and pass the unreacted N2(g) and H2(g)
back into the reaction chamber.
(1)