Lifelong Learning Programme

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Teaching Resources

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Niki Rapti
5th Geniko Lykeio Petroupolis
teacher involved
Phet (Reactants/products/leftovers)
Topics related to the resource
Chemical reactions, law of Conservation of Particles, limiting reactants in molecular approach (particle level)
Examples of learning objectives
The simulation is designed to give students
• a conceptual understanding of limiting reactants, rather than practice at solving algorithmic problems that require mass/mole conversions.
• reinforcement of concepts related to chemical equations
It relates the real-world example of making sandwiches to chemical reactions by representing particles as space-filling models. The simulation does not include ionic compounds.
The students are intended to
• Describe what limiting reactant means using examples of sandwiches and chemicals at a particle level.
• Identify the limiting reactant in a chemical reaction given the initial amounts of two reactants and the chemical equation
• Use their own words to explain the Law of Conservation of Particles (atoms/molecules) by using examples of sandwiches and chemical reaction.
• Predict the amounts of products and leftovers (excess reactants, reactants that do not react) after reaction using the concept of limiting reactant.
• Predict the initial amounts of reactants given the amount of products and leftovers using the concept of limiting reactant.
• Translate from symbolic (chemical formula) to molecular (pictorial) representations of matter.
• Explain how subscripts and coefficients are used to solve limiting reactant problems.
Practical information regarding the use
of the site/simulation...
The simulation is very user friendly. The only guidance needed is in the sequence of the three possible choices: sandwich shop/real reaction/game
Information about the class
About 20 students (aged 16-17), 2nd grade of upper secondary school (in the science track) in the very first lessons. Students have already been taught (in the 1st grade of upper secondary school) the principles of chemical reactions and stoichiometry but did not have satisfactory practice and have not yet known chemical equilibrium.
Suggestion for use
• A lab equipped with 10 computers is used. A link to the simulation is saved to the desktops of the lab computers, so that students had no need to access the internet.
Students are told to work in pairs and to use the activity sheet for guidance and for homework. During the activity, the teacher goes about the room to answer student questions.
The model A.«prediction» - B.«experiment by means of simulation» - C «comparison of results to predictions- explanation», and D «conclusions-discussion- conceptual understanding in molecular level» is followed in 2 parts:
1. Making sandwiches: it is used as a simulation of a two-reactant synthesis reaction with a simple mole ratio 2:1:1. First they complete the predictions, then they familiarize themselves with the simulation and check their answers. Finally they explain their errors. One reactant will be limiting, while the other in excess.
2. Real Chemical Reactions (synthesis of water and ammonia and combustion of a hydrocarbon like methane)
• Finally the best part:
3. Game. A challenge is given to every pair to obtain the best score in the lab group. They can take it for homework and note their initial score and their best score.

• Due next time, students have to fill in part D, to play the game for practice and to fill the post-lab homework sheet. It contains problems that require mass/mole conversions and distinction between atoms of every species (constant) and molecules of reactants and products (different).
The test questions are taking into consideration the chemical education research literature and are designed to elicit student misconceptions and difficulties to transfer from microcosmos (simulation) to macrocosmos (moles/ masses). Also this is not included in the simulation. The contribution of discussion with an open-response version of the post-test is recommended.
It is remarkable that the simulation in molecular approach does not allow the production or reaction of fractions and allows null product.
Insights into student
use / thinking
• Students enjoy the simulation as a game and understand very well their wrong predictions. Molecular models are very helpful in understanding chemical equations(chemical symbols)
• Students usually think that the reactant with the least relative amount is the limiting one. It is helpful to watch the histograms of consumption/production
• Students usually confuse the coefficients in chemical equations with the subscripts in chemical formula. This simulation makes clear that the rule in real reaction is the ratio (coefficients) and the conservation of atoms in contrast with the production of new different (in quality and quantity) molecules.
• The wrong predictions are mainly made when there is no product. The simulation helps students to understand it but from the teacher viewpoint, I believe that there is no reason to explain to students something that could never happen! That is, to be in the condition of an amount of atoms that cannot form at least 1 molecule!. In addition, a misconception can be caused when there are both reactants in leftovers (this happens in reversible equations). We use to say that the limiting reactant is finished in one-way reactions…
Teacher’s conclusions
The experience of the application was very positive :
1. Very good understanding of the conservation of atoms and production of different molecules according to the mole ratio of the chemical equation.
2. Very good understanding of the concept of the limiting reagent. The difficulties of students with analogies are alleviated due to the simulation
There are some points that need attention and that could be optimized as far as the understanding of massive production and chemical equilibrium is concerned. Two consideration are the following:
1. Do not allow to have no product (≠0) in order to use fractions in moles ratio.
2. Give the choice of reversible equations in chemical equilibrium in order to help students understand the concept of reaction yield in combination with limiting agent. For example it would be very intelligible for students in the sandwich shop to understand that if we make 100 sandwiches and we sell 60 then the leftovers are the reactants for 40 sandwiches (of course there is need to reiterate that it is dynamic equilibrium, not static)

National Reports on successful experiences to promote lifelong learning for chemistry The national reports on chemistry successful experiences to promote lifelong learning for chemistry are now available on the related section of the project portal. The reports presents examples of successful experiences in the partner countries and the results of testing of ICT resources with science teachers.