Equilibrium
DP Chemistry
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Characteristics of Equilibrium
Closed system – nothing in, nothing out
 Forward and reverse reactions are
occurring at the same rate – known as
‘dynamic equilibrium’
 Macroscopic properties (e.g. Colour,
concentration, pH) remain constant
 Changes in temp, pressure, volume and
concentration can change the equilibrium
position. Catalysts do not.

Equilibrium Graph - Concentration
A
B
This reaction starts with A only, with the [A] (concentration of A) at a maximum at Time = 0 s.
As the reaction proceeds, A gets used up and [B] steadily increases until equilibrium is
reached which can be seen as no change in either concentration.
In this graph, [B] > [A] which means the forward reaction is more favoured than the reverse.
What would the graph look like if the reverse was favoured?
Equilibrium Graph –
Rate of Reaction
A
B
This reaction starts with A only, so the rate of the forward reaction is at a maximum and
slows down as [A] decreases
As the reaction proceeds and [B] steadily increases, the rate of the reverse reaction
increases until equilibrium is reached when the rates of the forward and reverse reactions
are equal.
Equilibrium constant - Kc
products
reactants
Indices are from the
coefficients in the
balanced chemical
equation
Equilibrium constant - Kc
•Units for Kc will vary depending upon
the reaction. In fact, there may be no
units.
•The value of Kc for a particular
reaction is only affected by changes
in temperature
•In a homogeneous reaction, all of
the states of matter are the same
•In a heterogeneous reaction, there
are different states of matter. Solids
do not take part in the equilibrium
constant
Equilibrium constant - Kc
When Kc >> 1
When Kc << 1
Equil goes far right
(forward rxn almost to
completion)
Equil goes far left
(forward rxn hardly
proceeds)
What happens when Kc = 1? Kc = 0?
Le Chatelier’s Principle
“An equilibrium system that
is exposed to a stress will
shift the equilibrium position
to oppose that stress”
Note: a stress can be a change in temperature, pressure, volume or
concentration. Catalysts do not affect equilibrium; they simply affect the
rate of the forward and reverse reactions.
Le Chatelier’s Principle –
Concentration
Consider the following reaction:
A + B
Adding a reactant:
•This will stress the system
•To relieve the stress, the
system can produce more
products
•Equilibrium shifts right
C + D
Adding a product:
•This will stress the system
•To relieve the stress, the
system can produce more
reactants
•Equilibrium shifts left
What happens if you remove some reactant or product?
Le Chatelier’s Principle –
Temperature (exothermic)
Consider the following exothermic reaction:
A + B
Increasing the temperature:
•This is like adding a product
•To relieve the stress, the
system can reduce the heat by
producing more reactants
•Equilibrium shifts left
C + D + heat
Decreasing the temperature:
•This is like removing a product
•To relieve the stress, the system
can produce more products
•Equilibrium shifts right
Le Chatelier’s Principle –
Temperature (endothermic)
Consider the following endothermic reaction:
heat + A + B
Increasing the temperature:
•This is like adding a reactant
•To relieve the stress, the
system can reduce the heat by
producing more products
•Equilibrium shifts right
C + D
Decreasing the temperature:
•This is like removing a reactant
•To relieve the stress, the system
can produce more reactants
•Equilibrium shifts left
Le Chatelier’s Principle –
Pressure
Consider the following reaction:
2A + B
C + D
Assume all species
are gases
Notice that there are 3 moles of gas on the left and 2 moles on the right
Increasing the pressure:
•This means there is less room for the
particles
•To relieve the stress, the system can
reduce the pressure by producing less
moles of gas
•Equilibrium shifts right
Decreasing the pressure:
•This means there is more space for
particles
•To relieve the stress, the system can
increase the pressure by producing
more moles of gas
•Equilibrium shifts left
What happens if you increase or decrease the volume? How does this
relate to pressure?
Le Chatelier’s Principle –
Catalysts
Consider the following reaction:
A + B
C + D
Adding a catalyst
•This will not affect the
equilibrium position or Kc
•Catalysts reduce the activation
energy
•This speeds up the forward and
reverse reactions equally
•Equilibrium is reached faster
Production of Ammonia –
The Haber process
In 1912, German scientist, Fritz Haber developed a
process for manufacturing ammonia from nitrogen
and hydrogen.
N2 (g) + 3H2 (g)  2NH3 (g) + 92kJ
Notice that this reaction is reversible
which can establish equilibrium. This
means that Le Chatelier’s Principle
applies to the chemistry of this process.
Also, note that the forward reaction is
exothermic
Source: www.bbc.co.uk
Ammonia and Le Chatelier
N2 (g) + 3H2 (g)  2NH3 (g) + 92kJ
Using your knowledge of Le Chatelier’s Principle,
describe what the optimum conditions (in terms of
yield and rate) for this reaction will be in relation to
the following:
Temperature
 Pressure
 Use of a catalyst

Optimum ammonia production
Rate
Yield
Cost
Source: Chemistry Contexts 2, 2006
Conditions for Haber Process
Pressure - high(250 atm) – to shift equilibrium
right and increase rate
 Temperature – moderate (4500C) – low
favours equilibrium, high favours rate. This
temperature is a trade-off
 Catalyst – use of an iron catalyst helps to
increase the rate and overcome the relatively
low temperature required.
 Removal of ammonia – shifts the equilibrium
towards the products.

Production of Sulfuric Acid –
The Contact Process
Sulfuric acid is made in 3
steps:
Steps to make sulfuric acid
1. Sulfur dioxide is made
2. Sulfur trioxide is made from sulfur dioxide
3. Sulfuric acid is made from sulfur trioxide
Contact Process Chemistry
Step 1:
Sulfur is roasted in oxygen to produce
sulfur dioxide
S(s) + O2(g)  SO2(g)
Step 2:
Sulfur dioxide is reacted with oxygen
using a Vanadium catalyst to produce
sulfur trioxide in a reversible exothermic
reaction
SO2(g) + O2(g)  SO3(g) + heat
Step 3:
Sulfur trioxide is converted to sulfuric acid
through a series of reactions
Contact Process conditions

Using Le Chatelier's principle, the equilibrium yield of sulfur trioxide
(step 2) should increase when:
- temperatures are low, since the reaction is exothermic;
- pressure is high;
- excess reactants are present.

However the rate of the reaction is high when:
- temperature is high, hence an obvious conflict exists with the
equilibrium yield;
- the pressure is high;
- a catalyst is used.
Predict the conditions that would be used in this process. Justify
your answer
Contact Process conditions
Pressure - low(1-2 atm) – the process already
favours the foward reaction. The extra pressure
is not cost effective
 Temperature – moderate (4500C) – low favours
equilibrium, high favours rate. This temperature is
a trade-off same as Haber
 Catalyst – use of an vanadium catalyst helps to
increase the rate and overcome the relatively
low temperature required.
 Excess oxygen– shifts the equilibrium towards
the products.
