Chapter 31 – Chemical Equilibrium
Section A – Reversible reactions
The Haber process is used to produce ammonia for use in ______________:
N2+ 3H2
2NH3
The arrows symbolise that the reaction is reversible
Reversible reaction:
Although the reaction is dynamic (_______________
______________) at a certain point the concentrations of
each reactant and product will remain ____________(overall
they are not changing!) and at this point we say the reaction
is in dynamic equilibrium.
Equilibrium
Section B – Equilibrium constant Kc
For the following reaction:
aA + bB
cC + dD
Kc =
 Square brackets mean concentration in
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Chapter 31 – Chemical Equilibrium
It tells us the ____________________ of the equilibrium of a reaction (whether the reaction mixture
is made up of mostly reactants or mostly products)
A large value indicates that the equilibrium lies on the ___________ hand side – there is a higher
concentration of _____________ than ______________.
A small value indicates that the equilibrium lies on the ___________ hand side – there is a higher
concentration of _____________ than ______________.
Section C – Calculations involving Kc
Example 1: 180g of ethanoic acid and 138g of ethanol were reacted together in the presence of
sulphuric acid. At equilibrium there is 46g of ethanol in the reaction. Calculate Kc for the reaction
Answer:
Write out the formula for Kc for this reaction:
Now the concentrations at each reactant and product at equilibrium must be calculated:
Kc = [CH3COOC2H5 ][H20]
[CH3COOH ][C2H5OH]
Kc =
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Chapter 31 – Chemical Equilibrium
Example 2 - The value of Kc is 4.0 at 373K. What mass of ethyl ethanoate would be present in the
equilibrium mixture if 15g of ethanoic acid and 11.5g of ethanol were mixed and equilibrium
established at this temperature?
Answer:
Write out the formula for Kc for this reaction:
Now the concentrations at each reactant and product at equilibrium must be calculated, using x for
any unknown value..
•
Now sub back values into the formula for Kc and figure out any unknown
4 = [CH COOC H ][H 0]
3
2 5 2
[CH COOH ][C H OH]
3
2 5
•
Find equilibrium concentration in mass:
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Chapter 31 – Chemical Equilibrium
Section D – Le Chatelier’s Principle
Le Chatelier’s Principle
Possible stresses and effects on a system at equilibrium:
1. Concentration changes
H2 + I2
2HI
Stress: Increase the concentration of reactants
Effect: System will oppose the stress by favouring the ________ _____reaction.
A new equilibrium is established and the concentration of ______ is
higher at equilibrium ( higher yield), although overall the value of Kc
remains the same.
Stress: Increase the concentration of reactants
Effect: System will oppose the stress by favouring the ________ _____reaction.
A new equilibrium is established and the concentration of ___________is higher
at equilibrium ( higher yield), although overall the value of Kc remains the same.
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Chapter 31 – Chemical Equilibrium
2. Temperature changes
H2 + I2
2HI
∆H = -12.6Kjmol-1
This reaction is exothermic and so gives out ____________.
Stress: Increase the temperature of the reaction
Effect: The System will oppose the stress by favouring the ________ _____
reaction.
A new equilibrium is established and the concentration of __________
is higher at equilibrium ( higher yield) than before.
Stress: Decrease the temperature of the reaction
Effect: The System will oppose the stress by favouring the ________ _____
reaction.
A new equilibrium is established and the concentration of __________
is higher at equilibrium ( higher yield) than before.
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Chapter 31 – Chemical Equilibrium
3. Pressure changes
2NO2(g)

N2O4 (g)
Pressure changes will only effect reactions when ALL reactants and ALL products are ____________.
In General: The higher the pressure the more the reaction moves in the direction with less gas
molecules.
Stress : Increase the pressure
Effect: The System will oppose the stress by favouring the ________ _____ reaction.
A new equilibrium is established and the concentration of __________ is higher at equilibrium (
higher yield) than before.
Stress: Decrease the pressure
Effect: The System will oppose the stress by favouring the ________ _____ reaction.
A new equilibrium is established and the concentration of __________ is higher at equilibrium (
higher yield) than before.
4. Adding a Catalyst
Stress: Adding a catalyst
Effect: ______________. Explanation: Adding a catalyst to a reaction mixture at equilibrium will
increase the rate of the forward reaction and backward reaction to the same extent.
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Chapter 31 – Chemical Equilibrium
Section E – Mandatory experiment:
To illustrate Le Chatelier’s Principle using the reaction between Iron (III) Chloride and
potassium thiocyanate
Fe3+ + CNSFe(CNS)2+ ∆H = +
Yellow
red
Procedure
1) Mix together some iron(III) chloride and some potassium thiocyanate. A __________ colour
results.
*** This shows that the equilibrium lies on the ________ hand side of the reaction
2) add in some hydrochloric acid solution ( this causes the removal of Fe3+, due to the formation of a
complex ion containing iron and chlorine). A __________ colour results
*** The equilibrium shifts to the ____________ hand side to oppose the stress.
3) Add some potassium thiocyanate solution ( contains CNS- ions) to the reaction. A
____________colour results
*** The equilibrium shifts to the ____________ hand side to oppose the stress.
4) Divide the mixture into three different test tubes a,b,c:
a) Warm test tube by placing it in hot water- . An intense __________ colour results
*** The equilibrium shifts to the ____________ hand side to oppose the stress and absorb
the extra heat added to the reaction.
b) Keep at room temperature. No colour change is observed as no stress has been applied to
the system at equilibrium.
c) Cool test tube by placing it in an ice bath - . A __________ colour results
*** The equilibrium shifts to the ____________ hand side to oppose the stress and replace
the heat that has been removed from the reaction.
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Chapter 31 – Chemical Equilibrium
Section F – Industrial application of Le Chatelier’s Principle
Case study 1 – Haber process
2NH3 ∆H = -92.4Kjmol-1
N2 + 3H2
Haber process
Temperature conditions
Contact process
Pressure conditions
For the maximum yield of
Section E – Mandatory experiments
ammonia _______ temperatures
are best.
For the maximum yield of ammonia
_______ pressures are best.
However high pressure conditions
can be expensive to maintain and
can cause dangers so a compromise
pressure of _____ is used.
However low temperatures can
cause the rate of the reaction to
be very slow so a compromise
temperature of _____ is used.
Case study 2 – The Contact process
2SO2 + 2O2
2SO3
∆H = -196Kjmol-1
Temperature conditions
Pressure conditions
For the maximum yield of sulfur
trioxide _______ temperatures
are best.
For the maximum yield of trioxide
_______ pressures are best.
However high pressure conditions
can be expensive to maintain and
can cause dangers so a compromise
pressure of _____ is used.
However low temperatures can
cause the rate of the reaction to
be very slow so a compromise
temperature of _____ is used.
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Chapter 31 – Chemical Equilibrium
Check your learning of
Chemical Equilibrium
Green = I know this already
Orange = I am not sure – need to check this
Red = I don’t know this yet – need to start learning it
Green
Orange
Red
Define Reversible reaction
Define Equilibrium
Explain Why reactions at equilibrium are considered
to be Dynamic
Equilibrium constant (Kc ) – what it tells you about a
reaction and that it is changed by temperature, but
not by catalysts
Higher level only - Calculations involving equilibrium
constants (Kc ).
Definition of Le Chatelier’s principle.
Effect (if any) of Le Chatelier’s principle on
equilibrium
position of concentration,
pressure,
temperature and
catalyst.
Mandatory experiment - To illustrate Le Chatelier’s
Principle using the reaction between Iron (III)
Chloride and potassium thiocyanate
Industrial application of Le Chatelier’s principle in the
(i)
catalytic oxidation of sulphur dioxide to
sulfur trioxide and
(ii)
in the Haber process.
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