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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