KENT TRIPLE SCIENCE NETWORK
From Kent Science Resource
Centre
Chemical Equilibria
Difficulties and Misconceptions
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Students believe equilibria are static, not dynamic (which is
why the concentrations stay the same)
An equilibrium reaction comprises two separate reactions
Le Chatelier’s Principle is used as an explanation rather than
a predictive tool
Rate and Equilibria are often confused
A suggested approach:
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Present a wider range of reactions to students
Start with simple reversible reactions
Use models to show the dynamic nature of an equilibrium
Teach using equilibria laws
Use diagnostic questions to determine students
understanding
Getting Started
Start with a simple reversible reaction.
Mix together fumes of ammonia and
hydrogen chloride.
Now WARM a small amount of solid
ammonium chloride in a cold test tube.
Ask pupils to work out what is happening.
This is often (wrongly) referred to as
simple sublimation. It is actually
dissociation.
Reversible reactions
A reversible reaction is a chemical reaction that can
go both ways.
Ammonium chloride
NH4Cl(s)
Ammonia + Hydrogen chloride
NH3(g) +
HCl (g)
Reversible reactions
They often result in a dynamic equilibrium mixture
in which the forward and backward reactions occur
at the same rate.
CH3COOH (aq)
H+(aq) + CH3COO-(aq)
Don’t share this reaction at an early stage.
However an understanding of these principles are
needed when teaching strong and weak acids
Your First Equilibrium
Add some iodine in potassium iodide solution into a test tube.
Pour an equal volume of cyclohexane into the test tube.
Stand it in a test tube rack and wait!
It will take a long time to reach equilibrium
Extension: Groups of students do this with different quantities
of the two liquids. How do the final colours compare?
Modelling equilibria
What model or analogy do you use?
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Paper toss
Water transfer
The steady state bottle
Encourage your students to develop their own model!
Paper Toss Model
Water Transfer Model
Two tanks. One empty, one containing water.
Two beakers, one large, one small
Transfer water from the right hand tank using the large beaker
Transfer water back from the left hand tank using the small beaker
Repeat the process over and over again
What happens eventually to the levels of water?
How good is this model? What weaknesses?
The Steady State Bottle
Characteristics of a dynamic equilbrium
1.
2.
3.
4.
5.
Forward and backward rates are the same
The equilibrium can be approached from either direction
Can only happen in a closed system
The position of the equilibrium can vary
Macroscopic properties are constant, microscopic
properties are changing continually
#1 & #2 can be shown using the models introduced so far.
#5 can be explained with the models. Overall concentrations
stay the same but molecules are continually changing.
Closed Systems
Chemical equilibria can only be established in a closed system.
Consider the thermal decomposition of calcium carbonate
CaCO3(s)
CaO(s) + CO2(g)
What would happen in a sealed container?
Sketch a graph of amounts of reactants (and products) against
time
Are these similar to the graphs created in the paper throwing
activity?
Position of Equilbrium
• A difficult idea for students.
• Essentially they must appreciate that at equilibrium there
does not have to be equal amounts of chemicals on each
side.
• Use an analogy again. For example, consider a car park at
equilibrium at both busy and quiet times of the day.
• With the water transfer model, what would affect the
position of the equilibrium?
Examples of Chemical Equilibria
CO2(g) + H2O(l)
H2CO3(aq)
This occurs in carbonated drinks when the equilibrium is disturbed by
opening the can or bottle.
Cl2(g) + NaOH(aq)
NaClO(aq) + Cl-(aq) + H+(aq)
NaClO is household bleach. If other cleaning products containing chloride
ions are added, the reverse reaction will lead to chlorine being produced.
le Chatelier's principle
The position of equilibrium shifts
to try to cancel out any changes
you make
A tool to help us predict what will
happen but it is not an
explanation
Changing concentration
The position of equilibrium shifts to try to cancel out
any changes you make:
A+B
C+D
Increasing the concentration of A means more C
and D are produced to counteract the change
Think through this process in terms of the effect on
the rate of forward and backward reactions.
Changing temperature
The position of equilibrium shifts to try to cancel
out any changes you make:
A+B
C + D + heat
Heating the mixture means the equilibrium moves
to the left to counteract the change.
Think this through in terms of the effect on the
forward and backward rates
Changing pressure
The position of equilibrium shifts to try to cancel
out any changes you make:
A(g) + B(g)
C(g)
Compressing the mixture means the equilibrium
moves to the right to counteract the change.
Think this through in terms of the effect on forward
and backward rates of reactions
A visual equilibrium mixture
This is a good equilbrium mixture to show to able Triple Science
students.
The species on each side are different colours. This enables
students to see where the position of equilbrum lies and how it
can be changed
[Co(H2O)6]2+(aq)+ 4Cl-(aq) ⇌ [CoCl4]2-(aq)+ 6H2O(l)
pink
blue
Experimental details are at:
http://www.nuffieldfoundation.org/practical-chemistry/equilibrium-between-twocoloured-cobalt-species-aqueous-solution
Ammonia & The Haber Process
The Haber process
Ammonia (NH3)is a very important
chemical used to make fertilisers and
explosives.
Before WW 1 Germany imported
nitrogen compounds from Peru and
Chile – supplies were running out and
war would make imports impossible
anyway
German scientists raced to find a way
to use the nitrogen in air to make
ammonia
Fritz Haber 1868-1934
N2(g) + 3H2(g)
2NH3(g)
Haber Process – optimum conditions
N2(g) + 3H2(g)
2NH3(g) ΔH = -92KJmol-1
Pressure
High pressure is needed to push the equilibrium to the right. While
some plants have operated at 1000 atm, the cost of operating at
this pressure is prohibitive. 250 atm is generally chosen
Temperature
The forward reaction is exothermic so low temperature favours the
forward reaction. However at low temperature the rate is too slow.
A compromise of around 400- 4500C is chosen
Catalyst
A catalyst of iron is used
Natural Gas
Steam
TSSP Gases, Ammonia &
Equilibria
Sulphur
removal
Air
January 2012
Primary
reformer
Secondary
reformer
CO2
absorbers
Shift
reactors
Some
H2
Methanator
CO2
Synthesis
Loop
Purge gas
Compression
Flow chart for
manufacture
of Ammonia
Ammonia
conversion
Heat removal
and product
condensation
Ammonia product
A Suggested Activity....
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Put the A3 ammonia flow chart on a piece of flipchart
paper.
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Study the diagram and label cards
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Discuss where the label cards should be positioned
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Place the label cards down and use a marker pen if
necessary to connect them to the most relevant point on
the diagram.
Understanding how industrial processes
are represented
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The rectangles on the flow chart are processes.
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Write on your diagram to show what substances are entering
and leaving each process. Include the impurities you know
about.
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Add the temperatures you know at each point where
substances leave a process.
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From this work out whether the heat exchangers are adding
or removing heat.
Reflecting on the process
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Look at your completed diagram.
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What would help make the diagram easier to
understand?
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What questions do you have about the process
– Choose the best three and write them on post-it notes.
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How would you use this exercise, or one like it, with triple
science pupils?
Uses of Ammonia
Fertilisers
75%
others
Nylon
5%
Nitric acid
10%
10%
Additional Activities....
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Fritz Haber ‘friend or foe’ – Socratic discussion activity
Haber process mystery
Socrates
Classical Greek
philosopher
470(?) – 399 B.C.
Socrates believed the
answers to all human
questions reside within
us and that through
disciplined conversation
we can discover ultimate
truth.
What is a Socratic Circle?
A constructivist strategy in
which participants engage in
a conversation to collectively
seek a deeper understanding
of complex ideas.
Preparing for Discussion
Highlight the text on Fritz Haber.
Highlight in one colour his achievements and in
another colour his tragedies.
Working in small groups prepare notes to use in
the discussion
Identify the most important point in the material
provided?
The Inner and Outer Circles
Outer Circle
Inner
Circle
Socratic Discussion
In the Inner circle 4 – 6 participants will discuss the
question ‘Fritz Haber: Friend or foe to mankind?’
2-3 people will present opposing arguments
Arguments must be backed by evidence
The Outer Circle will observe from different points of
view and will provide feedback after the discussion to
the inner circle
Seminar Reflection
• What ideas were generated through conversation that
you had not previously considered?
• What’s the most unsupported claim or idea you’ve
heard?
• Which idea seems the most obscure or ambiguous?
• What’s the most controversial statement you’ve heard
today?
• How did the interactions of the group help to expand
your thinking?
• How did the feedback of the outer circle help to improve
the quality of the conversation and spur further ideas?
Benefits of Socratic Circles
Advances critical reading
Spurs critical thinking
Improves discussion and listening skills
Increases vocabulary
Provides student ownership, voice, and
empowerment
Allows students to synthesize both the knowledgebase and the skills-base of the curriculum
Drawbacks of Socratic Circles
Time consuming
Discussion is often left without complete
“closure”
Discussion may arrive at a conclusion with
which the teacher is unfamiliar
Appears “unstructured” to the uninformed
observer
The Haber Process Mystery
All the fish have died in
Lake Scienco.
Use the cards to work
out why.
The Haber Process Mystery
• Get students to group the cards then justify their
reasons for grouping them
• Display mystery slide, students have to use cards
to explain how fish died. each group generally have
different theories. There is no right or wrong
answer - their reasoning is the important bit
• You can then use the plenary slide to assess what
they have learnt
Why was Haber’s method of
making ammonia a difficult but
important discovery?
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