Students’ ideas about thermodynamics

Post-16 students also learn the First Law of Thermodynamics, which states that “The energy of an isolated system is constant” (Atkins, 1986, p 40) and are taught to apply this in calculations of enthalpy changes. Students’ ideas about these aspects of chemistry have received relatively little attention from researchers.

Energy is released when chemical bonds form

Ross (1993) notes that many students think energy is released when chemical bonds break. He believes this misconception is a barrier to learning that begins when students develop a strong association between fuels and energy, learning the phrase “fuels contain energy” by rote. Development of the idea continues when students associate “fuel is an energy store” with chemical bonds. For example, they will learn that each methane molecule involves forming four covalent bonds between carbon and hydrogen. It is easier to imagine that the energy associated with burning methane is generated when these bonds break, rather than is “leftover” when new bonds form. Students’ ideas about burning were discussed earlier.

These reveal that many 15 year olds do not know where the heat produced in burning comes from. Chemical bonding provides them with an answer. Ross (1993) suggests that to assist students, teachers should present the reactions between fuels and oxygen as a “fuel - oxygen system” and help them to develop ideas about the relative strengths of covalent bonds in different molecules.

Support for the persistence of these ideas among post-16 chemists comes from Barker’s (1995) longitudinal study. Students were asked to explain where the energy comes from when methane burns. Initially, only 6% of students (aged 16) said that the energy was from bond formation. Other incorrect or descriptive answers included; energy is stored in methane (13%); from burning the methane (14%); from the flame (7%) or simply “from the methane” (6%). Fifteen months later, about 50% said the energy came from bond formation. Alongside this, though, the proportion thinking that energy was stored in the methane also increased, to about 19%. All the other incorrect responses showed a marked decline. Additional evidence indicated some students recalled “fuels are energy stores” from their pre-16 courses and found this difficult to replace with chemically accurate thinking.

In a second question, Barker asked students to select the energy level diagram they thought best represented the exothermic reaction between sodium and chlorine. Three diagrams of exothermic reactions were given - one highly exothermic reaction, another a giving out very little energy and the third mid-way between the top and bottom. The highly exothermic reaction was the “best fit” response, but no supporting data were given, so in analysing responses either of the two relatively more exothermic diagrams were accepted as correct. Initially, only around 12% selected an appropriate diagram supported with an acceptable reason, while about 30% chose an appropriate diagram but gave incorrect or simple descriptive statements including “the reaction is exothermic”. About 14% misunderstood the term “exothermic”, so selected the diagram with the very small energy difference, explaining “the reaction doesn’t give out much energy” or “the reaction needs lots of energy to start”. About 5% connected the stoichiometry of the equation for the reaction to the arrow lengths, so selected the mid-point diagram arguing that this represented a 2:1 ratio. Fifteen months later, marked changes were apparent. About 28% gave an expected response together with a correct explanation. A further 40% chose a correct diagram without explanation. The proportions giving the other responses remained almost unchanged.

Energy is conserved in chemical reactions

Brook and Driver (1984) found that less than one in twenty 15 year-olds used ideas about the conservation of energy in written responses. When asked more directly about this principle, two-thirds of the students said, “Energy is used up or lost”. The authors concluded.

“…including an explicit statement of the principle of conservation of energy in the question stem does not have much effect on the pattern of responses.” (Brook and Driver, 1984, p 12).

Finegold and Trumper (1989) found similar difficulties in their study of 14 - 17 year olds. They report that 80% of their 14 and 15 year olds did not conserve energy in responding to basic questions. Energy being “used up” was commonplace. Ross (1993) notes that students acquire this idea from everyday experience of batteries going flat, petrol tanks needing refilling and electricity being “used up” in providing heat and light. Some students in Finegold and Trumper’s study described energy as being “caused” by something, for example:-

“Student: I think something is supplying, that causes energy…

Teacher: I don’t understand.

Student: For all energies there is something that activates them, that gives the strength” (p 106). 

This student seems to suggest that energy is made by something. The authors do not give the exact proportion of students with this view, but say the response is used “frequently” (p 103).

Entropy increases to a maximum in chemical reactions

The essential principle of the Second Law of Thermodynamics is that disorder, or entropy, increases when a chemical reaction occurs. An alternative statement is that “heat will not flow spontaneously from a colder to a warmer body” (Freemantle, 1987, p 177). Duit and Kesidou (1988) studied 13 - 16 year olds’ understanding of this statement of the Second Law. They report interviews with fourteen German students aged 16 years. A significant finding was that:-

“Most students have intuitively the correct idea that temperature differences tend to equalise and that the processes will not totally run back after equalisation.” (p 193).

The principle embodied in the Second Law does not seem to run against students’ everyday experiences, so perhaps this idea is less problematic. The First Law is more problematic because the energy transfers included in a system are frequently invisible. For example, a toy car when wound up will only run for a limited period of time and to a child the energy seems to have simply “run out” or has been “used up”. That the energy has done work in making the car move against the environment is not obvious. In contrast, students are more likely to think that heat can only go in one direction, since again this fits with their every day experience.

Summary of key difficulties

1. Fuels are energy “stores”

This idea is common, even among post-16 students. Sensory perception leads students to this idea, because the oxygen involved in a combustion reaction is invisible. Also, in teaching about fuels and food reference is often made to fuels “containing” energy, or food “giving” energy without reference to the chemical reactions involved. The idea contributes to students finding calculation of energy changes in reactions problematic.

2. Energy can be created and used up

Allied to the above is the idea that energy can be created and used up. Energy appears to be created from a burning reaction. Energy “runs out” when a fuel supply is exhausted; a non-rechargeable battery will also “run out” of energy eventually. The language we use to describe these events leads students to the perception that energy is like a substance that can be made and used up. This prohibits their learning that energy is conserved and dissipated when released in chemical reactions.

3. Energy is released when chemical bonds break

Taking the idea that fuels are energy “stores” further leads students learning about chemical bonds to think that bond breaking releases energy. This is similar to thinking that cracking an egg releases the egg’s contents. Some students will also reason that although some energy is needed to break a bond, more is released when it is broken.

Suggested activities

1. Improve students’ understanding of energy conservation

Boohan and Ogborn (1996) developed a useful “picture language” representing a wide range of energy changes. This can be used to introduce key ideas about energy transfer including that energy is conserved and never destroyed. The language can be used with 11- 16 year olds as it stands, but some modification is needed to develop chemical situations. The principle must be to encourage students to think of energy as being available in “useful” and “non-useful” forms. As such, a fuel-oxygen system may be described as a “useful” form of energy because the energy can be transferred to do “work” in some way.

2. Use consistent language referring to “fuel-oxygen systems”

In teaching about energy we must refer to “X-oxygen systems” not just “X” – where this may represent a fuel or other reactant. This will help to prevent students thinking that just the fuel or other chemical is an energy “source”. By doing this, students can be led towards the understanding that chemical bonds are involved in “storing” and “releasing” energy.

3. Introduce entropy at an early stage

Introducing entropy early on will help students to understand how energy is conserved and why we can use some forms of energy and not others. Spread out energy is “non-useful”, although the amount of energy present in any change is constant. The qualitative ideas associated with entropy are not difficult and make a lot of sense when coupled with energy conservation. An approach for introducing entropy qualitatively is suggested by the Salters Advanced Chemistry course (Burton et al, 1994). This adopts the idea of “number of ways” in which particles can be arranged, leading to the fact that the most likely event will be the one which occurs. This can be related to every day events, such as winning the big prize in a national lottery, a certain football team winning the major league, the sun rising tomorrow, and so on. Students can be invited to think of the “odds” of the most likely event happening. For example, the odds on winning the UK national lottery big prize are approximately 14 million:1, so the most likely event on buying a ticket is that the buyer will not win! Similar reasoning can be applied to chemical events – for example, the odds on two chemicals mixing is very high if both have similar types of molecules. There is only one possible arrangement that ethanol and water could be completely separated if the two liquids are poured into one container, but many ways these could be mixed. The prediction would therefore be that the chemicals would mix. The message students need is that the most likely event to happen in real life is the one with the most possibilities that it can occur. For energy, the most likely event is that energy will spread out, not stay in one place. Therefore, this is what is most likely to happen.

4. Use molecular models to improve understanding about chemical bonds

Students need help to focus on energy being required to break chemical bonds. One approach to help with this is “molecular murder”. In introducing thermodynamics many students carry out an experiment which involves burning liquid fuels, heating water and calculating erroneous energy changes. To maximise the benefit from this experiment, related fuels, such as the alcohols, should be used, rather than comparing say, hexane and ethanol. Using a sequence allows students to play “molecular murder” effectively. To do this, students are divided into groups. Each group is given a different fuel, although some duplication across a large class may be needed. Either before or after the practical experiment, students are asked to name their fuel for themselves and to make a model of one molecule. They then work out what happens when the fuel molecule burns. Models of oxygen molecules will be made. They then realise that to make anything happen the fuel molecule and oxygen molecules need to be torn apart. This sounds rather grim, but I encourage students to put as much energy into “murdering their special molecule”, that is ripping the model to pieces as possible. This makes the point that energy is needed to break bonds. We say that in fact all bonds of the same type require (within limits) the same amount of energy to break. This makes sense. When all the atoms are separated, new bonds can form and so the natural question to ask is, “If we put energy into breaking bonds, what must happen when they form?” Although we cannot “see” the energy released in building new models, students grasp the idea of bond formation being a reverse process, so realise that this involves energy release. The precise calculations can be carried out using a simple spreadsheet, which reinforces the point that combustion is always exothermic.

We also need to revisit teaching ionic bond formation. In teaching thermodynamics, specifically Hess’ Law, we focus almost entirely on covalent molecules, and in particular, fuel-oxygen systems. In teaching ionic bonding, we use Born-Haber cycles, but do not explicitly make the link to Hess’ Law. These are presented to students as two distinct systems. To help reinforce the point that bond making is exothermic, we need to approach the teaching of these bond types and the application of thermodynamics ideas in a much more consistent way than is traditionally done at present.