Equilibria and ammonia
Students mainly experience chemical reactions that appear to go to completion. When they meet a reaction that does not go to completion but which has a reverse reaction occuring they find the concept difficult to understand.
One major misconception students have about equilibrium is that they think equilibrium positions are fixed and once achieved there is no movement of particles between the two 'sides' i.e. they believe that equilibria are static not dynamic.
Le Chateliers Principle is a way of predicting changes to an equilibrium position under some circumstances but is generally wrongly applied due to the misunderstanding about the equilibrium position.
Rate and equilibria are often confused because students think that the rate of one reaction may change while the other slows or remains constant. They have not grasped that rate applies to the system as a whole.
It is important to use a wide range of reversible reactions to help get these ideas across to the students. A variety of simulations and modelling activites are available to support students which are explained in the following list.
Whilst this list provides a source of information and ideas for experimental work, it is important to note that recommendations can date very quickly. Do NOT follow suggestions which conflict with current advice from CLEAPSS, SSERC or recent safety guides. eLibrary users are responsible for ensuring that any activity, including practical work, which they carry out is consistent with current regulations related to Health and Safety and that they carry an appropriate risk assessment. Further information is provided in our Health and Safety guidance
Practical Chemistry Experiment
This experiment is a useful starting point to introduce the idea of equilibrium to students. It can be done as a teacher demonstration or students can carry it out themselves. The activity demonstrates how Iodine behaves when it dissolves in two solvents that do not mix.
An iodine crystal is dissolved in potassium iodide to produce an orange coloured solution. Another iodine crystal is dissolved in cyclohexane to produce a purple/pink coloured solution.
Add cyclohexane to the orange potassium iodide solution and potassium iodide to the purple/pink cyclohexane solution, shake and leave for a few minutes. You should observe that an equilibrium us set up with the iodine distributing itself evenly between the two solvents (the colour distribution should be the same if the same sized piece of iodine is used).
The important point for students to grasp in this experiment is that iodine molecules can move between the two solvents, eventually producing an equilibrium which is the same regardless of the direction from which it is approached.
Using Video to Illustrate Dynamic Equilibria *suitable for home teaching*
This is an important activity as it demonstrates what is happening in a system that has reached a dynamic equilibrium. It helps to overcome the common misconception that there are no movement of particles once the system is at equilibrium.
An alternative activity is the use of double sided cards (Frogs and Princesses). In this activity each group of students have 24 double sided cards which are placed on two A3 sheets (this represents the reaction boundary). Students start with princesses facing up and turn them over until the frogs side is showing. This models a reaction that goes to completion. They repeat the activity, this time stopping when about 66.6% complete. At this point one student represents the forward reaction while another represents the backward reaction, and they continue turning over the double sided cards while monitoring the overall number of frogs and princesses showing face up. This should help get across the idea of a dynamic equilibrium. This activity can be extended to discuss what happens when the boundaries are halved or doubled, or the number of reactants are increased.
A alternative model is the use of escalators - this can be used to model a dynamic equilibrium and the fact that the equilibrium can be at any position (not necessarily in the middle). Imagine someone was walking up the down escalator and reached the middle but then appeared to have stopped. They would actually still have to be moving to remain at that position, which represents a dynamic equilibrium i.e. still moving even though it appears to be stopped. This model can also be used to explain that the eqilibrium position is not always in the middle but can be anywhere in between.
Reactions that have equilbrium positions far to the left often need catalysts to help them proceed. Those that are far to the right are almost to completion.
Salters Equilibria Activities and Information
This website is really good and demonstrates a range of equilibrium reactions. It provides video clips of the reactions taking place and shows what happenns when the conditions that affect the position of the equilibrium are changed. There are supporting PowerPoints for each activity along with detailed teachers notes.
These videos clips can be used after the modelling activities (described in the previous resource in the list above) to check students understanding of Le Chatelier's Principle and how it can be applied.
RSC Equilibria Teachers notes
This resource has a number of equilbrium reactions and provides a good set of activities which can be used with students teach the topic. It provides technical information and teachers notes (and importantl,y answers to student activties). There is also a student pack with worksheets.
Activity 7 is particularly useful as a consolidation exercise (pages 25-28) to check students' understanding of equilibria and the application of Le Chateliers Principle. The dynamic equilibrium used is the iron/thiosulphate which can be found in the Salters video clips described above. You can show the video or carry out the demonstration to introduce the activity and ask students individually to complete the True/False/Unsure worksheet based on statements about the equilibrium. Once the worksheet has been completed individually you could ask them to work in groups to discuss their answers and then take feedback before going through the correct answers (or take the completed sheets in for marking).
This activity should be used after students have had the opportunity to see and understand a variety of equilibrium reactions.
Modelling with the students what happens when a reaction has reached equilibrium is essential to support students understanding.
Science and Technology in Society 2
This provides some useful background information about Fritz Haber and can be used to discuss the implications of the Haber Process. It is an opportunity to conduct an activity which addesses the spiritual, moral, social and cultural aspect of the national curriculum.
Students can work in groups, provided with information about Fritz Haber from which to make notes to use in a class discussion. Each group could be allocated a particular point of view to use when reading the materials and making notes i.e. a different coloured De Bono's hat. Once the groups have made their notes they nominate a spokesperson to take part in a small discussion (with representatives from other groups). This is watched by the whole class, who observe different aspects of the discussion and provide feedback. This activity is a modification of a Socratic discussion which is popular in school English departments.
Haber Process Conditions
A useful simulation which allows the reaction conditions to be changed and the observed effects of these changes. This will help students understand the compromise needed in the conditions used for the Haber Process.
Students find it difficult to appreciate that a temperature of 400 -450oC is relatively low for an industrial scale reaction.
Pressures above 300 atmospheres would cause problems of safety (and cost for pressurised containers).
Catalysts need to be used to speed up the rate at which the equilibrium is reached but does not affect the yield of the reaction. A concept difficult for students to take on board is the fact that catalysts speed up the forward and backward reaction equally.
Careers Examples and Options
Students might be interested to note that a career in chemical engineering or firework design and manufacture (page 8) would make use of the knowledge gained from understanding about equilibriaum and reaction rates.
In chemical engineering it is important to ensure the correct conditions for any given reaction to get the maximum yield at minimal cost, as in the Haber Process.
In firework design it is essential to get the correct mixture for effective and safe manufacture of fireworks!
