Presentation on theme: "KENT TRIPLE SCIENCE NETWORK From Kent Science Resource Centre."— Presentation transcript:
KENT TRIPLE SCIENCE NETWORK From Kent Science Resource Centre
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 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 chlorideAmmonia + Hydrogen chloride NH 4 Cl (s) NH 3(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. CH 3 COOH (aq) H + (aq) + CH 3 COO - (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 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.Forward and backward rates are the same 2.The equilibrium can be approached from either direction 3.Can only happen in a closed system 4.The position of the equilibrium can vary 5.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 CaCO 3 (s) CaO(s) + CO 2 (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 CO 2 (g) + H 2 O(l) H 2 CO 3 (aq) This occurs in carbonated drinks when the equilibrium is disturbed by opening the can or bottle. Cl 2 (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(H 2 O) 6 ] 2+ (aq)+ 4Cl - (aq) ⇌ [CoCl 4 ] 2- (aq)+ 6H 2 O(l) pink blue Experimental details are at: coloured-cobalt-species-aqueous-solution
Ammonia & The Haber Process
The Haber process Ammonia (NH 3 )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 N 2(g) + 3H 2(g) 2NH 3(g) Fritz Haber
Haber Process – optimum conditions N 2(g) + 3H 2(g) 2NH 3(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 C is chosen Catalyst A catalyst of iron is used
January 2012 TSSP Gases, Ammonia & Equilibria Sulphur removal Shift reactors Primary reformer Secondary reformer CO 2 absorbers Methanator CO 2 AirSteamNatural Gas Some H 2 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
Additional Activities.... 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 Inner Circle Outer 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 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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