kent triple science network from kent science resource centre
TRANSCRIPT
KENT TRIPLE SCIENCE NETWORK
From Kent Science Resource
Centre
Chemical Equilibria
Difficulties and Misconceptions• 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:• 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 StartedStart 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 Ammonia + Hydrogen chloride
NH4Cl(s) NH3(g) + HCl (g)
Reversible reactionsThey 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?
• Paper toss• Water transfer• The steady state bottle
Encourage your students to develop their own model!
Paper Toss Model
Water Transfer ModelTwo tanks. One empty, one containing water.Two beakers, one large, one smallTransfer water from the right hand tank using the large beakerTransfer water back from the left hand tank using the small beakerRepeat 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 equilbrium1. Forward and backward rates are the same2. The equilibrium can be approached from either direction3. Can only happen in a closed system4. The position of the equilibrium can vary5. 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 SystemsChemical 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 EquilibriaCO2(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 mixtureThis 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) [CoCl⇌ 4]2-(aq)+ 6H2O(l)
pink blue
Experimental details are at:http://www.nuffieldfoundation.org/practical-chemistry/equilibrium-between-two-col
oured-cobalt-species-aqueous-solution
Ammonia & The Haber Process
The Haber processAmmonia (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 anywayGerman scientists raced to find a way to use the nitrogen in air to make ammonia
N2(g) + 3H2(g) 2NH3(g)Fritz Haber 1868-1934
Haber Process – optimum conditionsN2(g) + 3H2(g) 2NH3(g) ΔH = -92KJmol-1
PressureHigh 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
TemperatureThe 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
CatalystA catalyst of iron is used
January 2012TSSP Gases, Ammonia & Equilibria Sulphur
removal
Shift reactors
Primary reformer
Secondary reformer
CO2 absorbers
Methanator
CO2
AirSteamNatural Gas
Some H2
Compression
Ammonia conversion
Heat removal and product condensation
Purge gas
Ammonia product
Synthesis Loop
Flow chart for manufacture of Ammonia
A Suggested Activity....
• Put the A3 ammonia flow chart on a piece of flipchart paper.
• Study the diagram and label cards
• Discuss where the label cards should be positioned
• 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
• The rectangles on the flow chart are processes.
• Write on your diagram to show what substances are entering and leaving each process. Include the impurities you know about.
• Add the temperatures you know at each point where substances leave a process.
• From this work out whether the heat exchangers are adding or removing heat.
Reflecting on the process
• Look at your completed diagram.
• What would help make the diagram easier to understand?
• What questions do you have about the process
– Choose the best three and write them on post-it notes.
• How would you use this exercise, or one like it, with triple science pupils?
Uses of Ammonia
others
10%Nitric acid
10%Nylon
5%
Fertilisers
75%
Additional Activities....
• Fritz Haber ‘friend or foe’ – Socratic discussion activity• Haber process mystery
SocratesClassical 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 The Inner and Outer CirclesCircles
Inner Inner CirclCirclee
Outer Outer CircleCircle
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 knowledge-
base 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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