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BIG SCIENCE IDEAS/CONCEPTS
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A.
In a chemical reaction (one or more) new substances are produced.
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B. Chemical substances can be
represented by formulae.
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C. Equations describe the reactants and
products in a chemical reaction.
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D. There are patterns to many chemical
reactions.
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E. Organic chemicals contain carbon.
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What you intend the students
to learn about this idea.
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- A
chemical reaction involves an input (reactants) and an output (products - which have different chemical properties).
- Chemical
reactions are all around us.
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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.
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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:
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The
equation represents the proportion of reactants needed and of the products
produced.
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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.
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- 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.
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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').
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Why it is important for students to
know this.
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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.
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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.
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Equations
form the language used to explain chemical reactions.
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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).
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There are
lots of organic chemicals all around us.
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What
else you might know about this idea (that you don't intend students to
know yet).
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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.
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States of
matter are usually indicated.
Complex
formulae are left out as they are too confusing for students (eg toluene).
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Chemical
equations involving half reactions, redox reactions and ionic transfer are
complex and can be too confusing at this level.
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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.
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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.
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Difficulties
/ limitations connected with teaching this idea.
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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.
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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:
eg 
This works
without understanding but can lead to problems (eg Mg2O2).
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Too much
emphasis on the microscopic behaviour can detract from development of
knowledge and understanding of the macroscopic behaviour of substances.
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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.
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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.
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Knowledge
about students' thinking that influences your teaching of this idea.
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Teachers
can get a 'feeling' of general interest level of the class by the links
the students are making to other ideas and experiences.
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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.
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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.
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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.
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Other factors that influence your
teaching of this idea.
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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.
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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.
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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.
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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.
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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.
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Teaching procedures (and particular
reasons for using these to engage with this idea).
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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.
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Chalk and
talk (often
effective for those who grasp ideas easily).
Making
models
Make models of molecules and ionic
substances using 'Playdoh' provides a sensory and visual aid to understanding
formulae.
'Dirty
tricks'
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].
Linking
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.
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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.
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- 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.
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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).
Soap making
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Specific
ways of ascertaining students' understanding or confusion around this
idea.
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'Dirty
tricks'
The teacher deliberately makes mistakes and waits for students to notice (eg
write CaOH2 instead of Ca(OH)2
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Write
the formulae first, then get students to balance the equation - this shows
whether
or not conservation of mass is obvious to students.
POE
(Predict-Observe-Explain)
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.
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See POE in
box above
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