3 Difficulties and Misconceptions Students believe equilibria are static, not dynamic (which is why the concentrations stay the same)An equilibrium reaction comprises two separate reactionsLe Chatelier’s Principle is used as an explanation rather than a predictive toolRate and Equilibria are often confused
4 A suggested approach: Present a wider range of reactions to students Start with simple reversible reactionsUse models to show the dynamic nature of an equilibriumTeach using equilibria lawsUse diagnostic questions to determine students understanding
5 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.
6 Reversible reactionsA reversible reaction is a chemical reaction that can go both ways.Ammonium chloride Ammonia + Hydrogen chlorideNH4Cl(s) NH3(g) HCl (g)
7 CH3COOH (aq) H+(aq) + CH3COO-(aq) 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
8 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?
9 Encourage your students to develop their own model! Modelling equilibriaWhat model or analogy do you use?Paper tossWater transferThe steady state bottleEncourage your students to develop their own model!Princess into toads activity:Cards with Princesses on one side and toads on the other. Students ‘magically’ convert princesses into toads. If approximately 60% conversion when at equilibrium then changes will occur but for different princesses changing etc. Toads take up less space this can be discussed when evaluating the model.
11 Water Transfer Model Two 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 againWhat happens eventually to the levels of water?How good is this model? What weaknesses?
13 Characteristics of a dynamic equilbrium Forward and backward rates are the sameThe equilibrium can be approached from either directionCan only happen in a closed systemThe position of the equilibrium can varyMacroscopic 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.
14 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?
15 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?
16 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.
17 le Chatelier's principle The position of equilibrium shifts to try to cancel out any changes you makeA tool to help us predict what will happen but it is not an explanation
18 Changing concentration The position of equilibrium shifts to try to cancel out any changes you make:A + B C + DIncreasing the concentration of A means more C and D are produced to counteract the changeThink through this process in terms of the effect on the rate of forward and backward reactions.
19 Changing temperatureThe position of equilibrium shifts to try to cancel out any changes you make:A + B C + D + heatHeating 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
20 Changing pressureThe 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
21 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 blueExperimental details are at:
23 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 anyway German scientists raced to find a way to use the nitrogen in air to make ammonia N2(g) + 3H2(g) 2NH3(g)Fritz Haber
24 Haber Process – optimum conditions N2(g) + 3H2(g) NH3(g) ΔH = -92KJmol-1PressureHigh 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 chosenTemperatureThe forward reaction is exothermic so low temperature favours the forward reaction. However at low temperature the rate is too slow. A compromise of around C is chosenCatalystA catalyst of iron is used
25 Flow chart for manufacture of Ammonia Sulphur removalShift reactorsPrimary reformerSecondary reformerCO2 absorbersMethanatorCO2AirSteamNatural GasSome H2CompressionAmmonia conversionHeat removal and product condensationPurge gasAmmonia productSynthesis LoopFlow chart for manufacture of AmmoniaTSSP Gases, Ammonia & EquilibriaJanuary 2012The flow chart will be used for a multi-stage task. You do not need to talk through it in detail – the task is to explore it in more depth. It probably is worth pointing out the bit which makes the ammonia, and reflecting that this is just one part of a much larger and more involved process. Like with a circuit diagram, there is little visual correlation between the flow chart and the actual chemical plant, although a skilled engineer would be able to make the relevant connections.Teachers will be familiar with the basic reaction of nitrogen with hydrogen to make ammonia, and may know some of the history of the process developed by Fritz Haber. What they may not realise is that the raw materials are natural gas, air and steam, or that much of the overall process is taken up with getting feedstocks to a sufficiently pure state so that catalysts are not destroyed, or unwanted side-reactions do not happen.Resist questions about the process at this point – there will be a chance to raise questions during the tasks.You could discuss briefly the importance of ammonia, which is mainly for making fertilisers and as the feedstock for nitric acid manufacture. It is a very important chemical made in large quantities. Annual world production is around 140 million tonnes. Pupils, not being able to buy ammonia at the shops, may not appreciate its importance, so ideas for how to help them could be discussed.
26 A Suggested Activity....Put the A3 ammonia flow chart on a piece of flipchart paper.Study the diagram and label cardsDiscuss where the label cards should be positionedPlace the label cards down and use a marker pen if necessary to connect them to the most relevant point on the diagram.
27 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.Estimated time for task: Up to 10 minutes, including follow-up discussion.This task requires the teachers to interpret their diagram, using the information about each process from the label cards. They may not be able to complete the task fully – task 3 gives them the opportunity to raise questions.
28 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 processChoose the best three and write them on post-it notes.How would you use this exercise, or one like it, with triple science pupils?Estimated time for slide: Up to 10 minutes, including follow-up discussion.This is a discussion task. The first question is a hypothetical one – specifications do not require pupils to know such a diagram, but it is nevertheless a valid question, as it is about learning.Suitable stimulus questions could be:Why would argon build up in the process and need purging?Why is some hydrogen mixed with the natural gas at the start?Why do the gases need compressing before the ammonia synthesis loop?After the discussion on this task has finished, ask the teachers for their thoughts about their understanding of the synthesis of ammonia, and how effective the exercises they have just done have been in getting them to engage with studying the process. Discuss with them whether they could devise a similar activity themselves for an industrial process relevant to their specification. What would they do the same and differently?
30 Additional Activities.... Fritz Haber ‘friend or foe’ – Socratic discussion activityHaber process mystery
31 Classical Greek philosopher SocratesClassical Greek philosopher470(?) – 399 B.C.Socrates believed the answers to all human questions reside within us and that through disciplined conversation we can discover ultimate truth.
32 What is a Socratic Circle? A constructivist strategy in which participants engage in a conversation to collectively seek a deeper understanding of complex ideas.
35 Socratic DiscussionIn 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 circleLSS 2.3 Observing from different points of viewLSS 6.2 Reflecting on PresentationsTalk mats to support discussionOuter circle will have cards to focus their observation and feedbackAllow about 10 minutes for the discussion dependent on how the discussion flows – ensure time for the outer circle to feedback
36 Seminar ReflectionWhat 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?
37 Benefits of Socratic Circles Advances critical readingSpurs critical thinkingImproves discussion and listening skillsIncreases vocabularyProvides student ownership, voice, and empowermentAllows students to synthesize both the knowledge-base and the skills-base of the curriculum
38 Drawbacks of Socratic Circles Time consumingDiscussion is often left without complete “closure”Discussion may arrive at a conclusion with which the teacher is unfamiliarAppears “unstructured” to the uninformed observer
39 The Haber Process Mystery All the fish have died in Lake Scienco. Use the cards to work out why.
40 The Haber Process Mystery Get students to group the cards then justify their reasons for grouping themDisplay 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 bitYou can then use the plenary slide to assess what they have learnt
41 Why was Haber’s method of making ammonia a difficult but important discovery?
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