This focus idea is explored through:
Contrasting student and scientific views
Student everyday experiences
Students at this stage of their schooling have been exposed to the concept of energy in a number of different contexts. From these experiences, many students believe that energy (which includes heat, light and electricity) is a type of matter. Packets of food list the energy content (in kJ), along with quantities of ingredients such as proteins, fats and sugar, further reinforcing this view. The idea that the total energy of the system before and after a chemical reaction is the same is not one that is borne out by everyday observations.
There are two problems here; the first is that students build a meaning for energy from their everyday experiences, which has limitations. Students often restrict energy to living things, movement or heat and electricity. One consequence of this restricted meaning for energy is that it cannot explain how energy is conserved in all changes. The second, and related, problem is that chemical energy is a difficult concept for many students to grasp - they may understand that heat, light and electricity are different from matter, but unseen stored chemical energy is much more difficult to understand. This means that they are likely to believe that energy is created (or used up) in chemical reactions.
Students in classrooms have heard the pop of hydrogen burning and seen the blinding flash of magnesium and this encourages them to convert energy into matter and vice versa. Indeed, it is the energy change in many reactions that holds the focus of students' attentions, not the new substances produced.
Fensham, Gunstone & White (1994)
Students often have trouble appreciating that chemical reactions are not driven by external interventions such as heating. They do not appreciate that energy is often only needed to initiate a reaction and that the reaction will proceed without further energy input and may overall produce energy. Since so many reactions they have seen involve the application of heat energy, it is not surprising that many students consider examples like the burning of paper or a candle to be energy consuming reactions (endothermic).
In any change, physical or chemical, the total amount of energy remains the same (the only measurable exception to this occurs in
nuclear reactions). This means that in reactions that appear to produce a great deal of energy, such as combustion, new energy is not created, but converted from stored chemical energy.
Chemical energy, which is both produced and consumed in different chemical reactions, is an important way of storing energy in foods, fat reserves and fuels.
In energy producing (exothermic) reactions the total energy of the
products is less than that of the
reactants - energy is released to the surroundings.
respiration in biological systems are the most obvious examples. Some exothermic reactions require some energy to get them started, but then they release more energy than was needed for initiation. A match requires initial energy, provided by the heat generated from the friction as it strikes the rough surface on the matchbox to ignite it. Once the match starts burning, it releases more energy than was required for ignition so the reaction is still exothermic. The products still have less chemical energy than the reactants.
In energy consuming (endothermic) reactions the total energy of the products is more than that of the reactants - heat is taken from the surrounding substances. The reactions involved in
photosynthesis are perhaps the most important of these. The production of aluminium is another important example of an energy consuming process. This has implications (benefits and costs) for energy use within Victoria.
Critical teaching ideas
- Energy, particularly chemical energy, is not a form of matter.
- Energy is not created during in chemical reactions; it is transformed.
- Most of society's energy needs are supplied from chemical reactions releasing stored chemical energy.
- Many chemical processes we see everyday (such as combustion), require energy to initiate them, but overall release energy, so are energy producing reactions.
- Energy consuming reactions produce many of the materials (such as metals and plastics) we use everyday.
Explore the relationships between ideas about energy producing and consuming chemical reactions in the
Concept Development Maps - (Chemical reactions, Flow of energy in
ecosystems, Flow of matter in ecosystems)
Because of widely held alternative conceptions in this area, students will need to observe and reflect on a wide range of chemical reactions which consume and produce energy. They will need to be encouraged to examine their existing views in the light of new evidence and to reflect on how these views have changed as a result of their investigations.
Open up discussion via a shared experience
An initial activity could seek to establish existing student ideas about energy changes involved in burning.
Students in small groups could be asked to draw a diagram of the burning of a candle and with arrows show the energy changes that take place. Students could record their views in a journal or book which is not marked or assessed (this helps in ensuring students state their own current views without worrying whether they are correct). They can then attempt to explain where the heat and light energy come from. Groups then share their ideas with the rest of the class. At this stage the teacher should delay judgement on ‘incorrect comments’.
Students should experience phenomena themselves or see demonstrations to further consolidate or challenge their existing ideas. Some examples are:
- Burning magnesium ribbon or steel wool, ignited with a burner or match.
- Observing the amount of energy released in a burning food (such as peanuts, bacon pieces, bread or marshmallow) by heating small amounts in a holder. For example, a peanut once ignited will burn for a long time, releasing copious amounts of energy.
- Investigating treadmills that display information on the number of kilojoules (calories) transformed by exercise.
Clarify and consolidate ideas for/by communication to others
After observing these reactions students can be encouraged to share their views in a class discussion which focuses on the idea that energy is released in these reactions after an initial application of heat to initiate the reaction. Ideas can be compared and students asked which of several alternatives seems the most plausible in the light of their investigations.
Challenge some existing ideas
The following list of reactions can be used to demonstrate firstly, that there are many chemical reactions that do not require heat to initiate them, and secondly, that there are many examples of chemical reactions that absorb heart energy. Students can measure the temperature changes in reactions such as:
- Steel wool mixed with copper sulfate solution (exothermic).
- Ammonium nitrate or ammonium chloride added to water (endothermic). (An alternative is urea and ammonium chloride - this is what is used in many ice packs).
- Plaster of Paris mixed with water (endothermic). (Setting concrete could also be investigated - this is an exothermic reaction not the result of drying out as many students imagine).
- 3% hydrogen peroxide added to yeast (endothermic).
- Baking soda mixed with vinegar or citric acid solution (exothermic).
- Barium hydroxide octahydrate mixed with ammonium thiocynate or ammonium nitrate. These are mixed as solids (this is very strong endothermic reaction with large temperature drops).
- Seed crystals added to supersaturated sodium acetate solution (exothermic - used in hand warmers).
- Steel wool wrapped around a thermometer and dipped in dilute acid such as vinegar (exothermic).
Practise using and build the perceived usefulness of a scientific model or idea
Other forms of energy that are produced as a result of chemical reactions can be investigated. For example, batteries producing electrical energy (and consuming electrical energy when recharging), engines or explosions producing kinetic energy, and glow sticks and fireflies producing light. These reactions show that chemical energy can be created from and transformed into forms of energy other than heat.
Share intellectual control and clarify and consolidate ideas for/by communication to others
Investigations into everyday and commercial applications of energy producing and consuming reactions can further assist students to consolidate their ideas. Providing choice may increase student engagement and motivation. In these investigations, students should focus on what energy goes in and what comes out. Some examples of areas students could investigate include
photosynthesis, energy changes in cooking, the energy involved in the production of metals (aluminium is a good example – students could investigate the Portland smelter) and plastics,
combustion and the energy of various fuels. Students can also consider issues of sustainability in the use of energy - most of which is currently obtained from non renewable resources which also have environmental effects.
In order for students to clarify their ideas they can be asked to communicate their findings in such a way that they have to synthesize and evaluate information they have collected. For example, groups of students could be asked to produce short PowerPoint presentations (no more than five slides, no more that 30 words on each slide) which explain the energy flow in their chosen processes.
Promote reflection on how students’ ideas have changed
Students can then be asked to look again at the burning of a candle and compare their explanations now with their ideas at the start. Asking how their ideas have changed about what is happening in terms of energy can promote reflection on what they have learned. Explaining the energy changes in lighting a match (chemicals on the head have a low activation energy - which
friction can overcome) can further consolidate their ideas. Problem setting such asking students to explain why nylon burns very easily while wool does not can encourage students to apply their new ideas.