|Asia-Pacific Forum on Science
Learning and Teaching, Volume 4, Issue 2, Article 2 (Dec., 2003)
Pamela MULHALL, Amanda BERRY and John LOUGHRAN
Frameworks for representing science teachers' pedagogical content knowledge
CoRe for Chemical Reactions (Grade Level: 10 mixed ability)
BIG SCIENCE IDEAS/CONCEPTS
A. In a chemical reaction (one or more) new substances are produced.
B. Chemical substances can be represented by formulae.
C. Equations describe the reactants and products in a chemical reaction.
D. There are patterns to many chemical reactions.
E. Organic chemicals contain carbon.
What you intend the students to learn about this idea.
- A chemical reaction involves an input (reactants) and an output (products - which have different chemical properties).
- Chemical reactions are all around us.
The formula of a substance reflects what it 'looks like' at the atomic level.
A particular chemical always has the same formula regardless of where it comes from. The way of writing formulae is universal amongst chemists.
'Rules of the game'
Some elements are represented as if they are single atoms (eg metals such as zinc, which is Zn) and others are represented as molecules (eg oxygen, which is O2).
Compounds are represented by the molecules they contain (eg water, which is H2O ) or by the combination of charged atoms ('ions') they contain (eg salt, which is Na+Cl-).
All substances are made up of zillions of atoms. When you breathe in oxygen, you are breathing in zillions of 02 molecules.
Equations are a form of chemical communication - for a particular reaction, the same equation applies in all parts of the world.
The learning of more able students may be extended to include:
- The equation represents the proportion of reactants needed and of the products produced.
- When writing equations:
1 It is necessary to use correct formulae for reactants and products.
2 Equations need to be balanced because mass is conserved.
- Classifying reactions enables one to predict products.
- As with much of chemistry, this predictability is not perfect: although you can write an equation, the reaction does not always happen at all, or happen according to prediction.
An understanding of (and ability to correctly use) useful terms like acid, base, salt, combustion, precipitation etc is important.
Students should understand the 'well behaved' reaction types, eg acid-base, acid-carbonate, acid-metal, precipitation, combustion, synthesis/ decomposition.
Organic chemicals contain carbon. Carbon atoms can form 4 bonds with other atoms. This means that they can form an infinite array of compounds. Many of these compounds contain long chains of carbon atoms to which hydrogen and other atoms are linked.
Most of the chemicals in the world around us (and inside us) are organic.
Organic chemicals can react to make molecules which we can use (eg glucose, carbon dioxide - the latter is not always considered 'organic').
Why it is important for students to know this.
It enables an understanding of real life situations (eg as reported in newspapers) and personal experiences:
eg environmental issues such as pollution, corrosion, analysis of bottled water, cooking a cake, lighting a match, effervescent powder, BBQ
It helps develop an understanding of consumer chemistry, eg ammonia in floor cleaners.
Formulae are part of the language of chemistry. The ability to communicate the structure of a substance through writing its formula is a vital precursor for further studies.
To understand chemical reactions in Year 11 students need to understand the order of magnitude of numbers of particles involved.
Equations form the language used to explain chemical reactions.
Categorising reaction types provides students with reasonable predicting measures about products.
If an experiment does not proceed as anticipated, students need to appreciate that they should consider the possibility that there is something that they haven't taken into account (rather than assume something is wrong with their observations).
There are lots of organic chemicals all around us.
What else you might know about this idea (that you don't intend students to know yet).
Not all reactions are complete (eg some biological reactions & some industrial processes) - this is not addressed unless it is raised by a student.
The chemical equilibrium constant is a guide to the extent to which a reaction proceeds.
Chemical reactions involve the breaking of existing chemical bonds and the formation of new ones.
States of matter are usually indicated.
Complex formulae are left out as they are too confusing for students (eg toluene).
Chemical equations involving half reactions, redox reactions and ionic transfer are complex and can be too confusing at this level.
Although most reactions are predictable, there are rare exceptions, some of which are the consequence of important rules met in Senior Chemistry (eg why Zn reacts with dilute acid, but Cu does not — a Redox reaction).
While combustion at this level refers to combining with oxygen, in fact something can be burned in anything which can oxidise it, eg chlorine, fluorine.
There are more organic chemicals than all the others put together. At this level we hardly scratch the surface beyond seeing that organic chemicals react too.
Difficulties / limitations connected with teaching this idea.
The explanations of what is occurring are quite abstract. This is compounded by the fact that the scale is so small in comparison to the macroscopic level at which the students are working. Thus it is difficult for students to make links between the macro- and micro-scopic levels of behaviour of chemicals.
If students cannot write the formulae of substances then further work (eg on equations) is difficult.
One can provide a technique for struggling students of swapping valencies and writing them as subscripts:
This works without understanding but can lead to problems (eg Mg2O2).
Too much emphasis on the microscopic behaviour can detract from development of knowledge and understanding of the macroscopic behaviour of substances.
This topic is so broad, one can only deal with a few types, on the basis of safety and those which are available in the school lab.
It is difficult to generate many examples that are related to students' everyday experiences because what happens in the lab is oversimplified and specialised.
Students have to accept 'in good faith' the teacher's explanation of the details of a reaction.
Teachers' concern for management and safety often creates a dilemma for the construction of good learning episodes.
It is always difficult to know whether or not one should teach functional groups, which are the basis of lots of organic reactions.
It is difficult to provide suitable experimental work because most organic chemicals are either flammable, toxic or both!
Bonding is central to developing an understanding of organic compounds but is a difficult concept for students to grasp at this stage.
Knowledge about students' thinking that influences your teaching of this idea.
Teachers can get a 'feeling' of general interest level of the class by the links the students are making to other ideas and experiences.
Formulae are often taught in Year 9 but always require revising. This also applies to ionic and covalent compounds. To work out the starting point for teaching, find students' ability level by getting them to write formulae/ equations as this helps to understand how they're thinking about it.
Students tend to think that a formula only represents one 'lot' of that substance, eg H2O means just two H and one O.
Students usually demonstrate a superficial acceptance of Conservation of Mass. It is a difficult concept for them to grasp so exploring their thinking beyond superficial responses matters.
At this stage of their development, students are often particularly interested in environmental issues, many of which can be linked to ideas about chemical reactions.
It is often hard to convince students of the value of their observations and of experiments that don't 'work' according to the rule, and that one can learn a lot about chemistry from one's observations. They often think that an experiment is wrong if it doesn't get the results expected and therefore do not interrogate the ideas or their approach to the experiment seriously enough.
Other factors that influence your teaching of this idea.
The idea of chemical reactions is introduced in early secondary years so this would be revisiting the concept. In particular students have already seen some chemical reactions and are familiar with tests for carbon dioxide, hydrogen and oxygen.
If students don't make links that you want them to then you need to help them by asking 'leading' questions (eg to help them see patterns in acid/base reactions).
Because of safety considerations it is difficult to provide meaningful opportunities for practical work which engage the students in designing their own experiments.
Students often enjoy working out formulae using a given table of valencies.
Expecting students to remember valencies can lead to rote learning rather than understanding.
More able students often enjoy balancing equations.
Discussion of the importance of predicting correct proportions to minimise costs and unwanted environmental effects in industrial processes can be useful in generating 'a need to know' amongst students.
Students are probably familiar with terms like acid, base, salt, combustion (or 'burning')
Students enjoy practical work and like 'playing' with chemicals and apparatus. Practical work is also appealing because it is a sensory experience.
Students are not expected to remember reaction types at this level as this would lead to cognitive overload. However given the categories of reaction types they should be able to make reasonable predictions about possible products.
Reaction types covered (see first box in this column) are interesting for students but not too dangerous.
Organic chemistry is more important and relevant for students than inorganic (eg hard to justify importance of learning about ZnCl2 for most students) but is much more complicated and dangerous.
Teaching procedures (and particular reasons for using these to engage with this idea).
Practical work can be presented in the form of a problem to be solved (eg in a forensic science context, students have to identify the nature of some mystery powders).
This helps develop 'a need to know' using a real world context and helps develop knowledge about the properties and behavior of substances. It also provides an opportunity for students to practice writing formulae and equations. Most students enjoy the activity and are able to achieve a reasonable level of success in it.
Chalk and talk (often effective for those who grasp ideas easily).
Make models of molecules and ionic substances using 'Playdoh' provides a sensory and visual aid to understanding formulae.
To help promote understanding about how formulae are written, ask which is right:
1. NaOH or Na(OH) [teacher may need to point out that brackets are not needed here]
2. CaNO32 or Ca(NO3)2 [students often realise there is something wrong with the first of these].
To explain why we write Ca(OH)2, it may be useful to make links to maths - where a mathematician might write 2(x+1) the chemist would write (x+1)2.
Practical work involving a range of different reactions where students identify as many products as possible. Results are discussed as whole class leading to writing equations as words, then symbols.
Algorithmic skills can be developed in more able classes by presenting students with a page of equations of steadily increasing difficulty. The challenge is to see how many they can correctly balance.
- Forensic science
Identification of unknown ionic compounds through practical work (using flame/ precipitation/etc tests previously derived by testing known substances) and using semi-micro test tubes (for safety).
- gives students control
- provides a real-world application
- a motivation
- helps students to remember reactions and understand equations.
- POE (Predict-Observe-Explain)
To emphasize predictability is not a guarantee of what happens, and that observation is the key to chemistry, this POE is useful: get students to predict what happens when add CaCO3 (as marble chips)+ H2SO4 and then perform experiment. There appears to be no reaction (actually it bubbles a little bit and then stops because the CaSO4 formed is insoluble, forms a coating over the unreacted CaCO3 and stops the reaction). Ask students to explain their observations.
Students make models of molecules of some familiar chemicals (eg petrol, nail polish remover) with Molymod™ kits (which are so designed that they closely replicate the exact shape of the actual molecule).
It is useful to have as many actual samples of these chemicals in the lab to help emphasise that the models are of real molecules in real things. This also provides the opportunity for students to become familiar with some of the physical properties of these chemicals (eg smell, appearance etc).
Specific ways of ascertaining students' understanding or confusion around this idea.
The teacher deliberately makes mistakes and waits for students to notice (eg write CaOH2 instead of Ca(OH)2
Write the formulae first, then get students to balance the equation - this shows whether or not conservation of mass is obvious to students.
Get students to weigh a piece of Mg, then predict weight of product after it burns, measure this weight (ie observe) and explain the result. This POE offers evidence that the burning of Mg involves an 'adding on' to the Mg. It also helps to make an abstract equation real.
See POE in box above
Copyright (C) 2003 HKIEd APFSLT. Volume 4, Issue 2, Article 2 (Dec., 2003). All Rights Reserved.