Ben Rogers has got me thinking about bar models as a way of helping to explain the concepts behind calculations and to reduce cognitive load. He’s written about them here and here. He explains that it’s a form of Dual Coding, which helps support transfer of ideas to long-term memory. I also think it helped to support metacognition in this example, as it helped me to explain my thought processes.
Ben and I recently had a chat about them over a coffee and afterwards, I sketched this idea, because I was going to be introducing energy changes in reactions to my year 10s.
Friday period 5
I used bars to illustrate a central concept in my lesson today, and I feel that it really helped. I didn’t get a chance to ask my year 10s what they thought (Friday period 5 is not really the time to have a chat about this kind of thing!) but certainly from my point of view, I felt it really helped me focus in on the concepts and give the class something concrete to relate the abstract ideas to.
I have a confession. I have always struggled to get my head around the explanations for this bit of Chemistry. And I’m talking about explaining it to myself as well. See BBC Bitesize:
Now, each time I go to write the equation in step 3, I struggle to remember whether it’s “energy in – energy out” or “energy out – energy in” . It takes me a while, but I do eventually work it out. But it frustrated me that I found it so difficult to get my head around it enough that it would “stick” and feel automatic.
I am very interested in the idea of Threshold Concepts, troublesome ideas that are tricky to understand, but once mastered, can transform understanding. When I first started thinking about them, I asked my dad which concepts he found particularly difficult to understand. He was teaching himself A level Chemistry at the time, and we’d often have little chats about what he had learnt, and what he was struggling with. One of the things he mentioned was energetics. Not the simple stuff (ie the definition of exothermic), but the ideas about which way the energy is transferred, and how it balances, and whether changes are positive or negative.
I think the concept of the balance between energy in and out, along with the idea of energy changes in the system vs the surroundings and consequent magnitude/ sign of energy changes might be a Threshold Concept. I remember how counter-intuitive it felt that exothermic reactions have a negative enthalpy change, for example. Visualising the changes using, and emphasising the system/ surrounding aspects, really helps to explain it all, I think. And once it “clicks”, lots of other ideas fall into place, hence qualifying it as threshold concept.
Using bond energies to explain energy changes
I outline below how I used bar models to explain the ideas to my year 10s (…and to sort it out in my head …again!). Apologies to anyone that feels this isn’t exactly rocket science! I’m not claiming it is. It just helped me. And apolgies to Ben if I’m stretching the definition of bar models to breaking point and this isn’t really what they are…
The first point that I emphasised is that the energy diagrams I’m drawing here are for the “system”. And when the system gains energy, it comes from the surroundings. If energy is lost from the system, then it’s lost to the surroundings. I like Adam Boxer’s explanation in his SLOP booklet that the “surroundings” is wherever the thermometer is!
So after reminding the class that energy is required to break bonds, but it’s released when bonds are formed, I drew this diagram to illustrate the difference in magnitude of energy in and out. Remembering that we’re talking about the system, it has “gained” 400 kJ (I’m simplifying units for now) but it’s “lost” 500 kJ to the surroundings. I used the analogy of someone giving me £400, but then me giving them £500 back. I’d be in debt. I’d have lost money, but they’d have made £100! Similarly, the system has lost energy, but it has transferred to the surroundings, hence why an increase in temperature is measured.
I then reminded them about the energy profile diagrams we’d drawn before, which are also a way of representing energy changes in the system. So an exothermic profile shows a decrease in energy.
I then reminded them again that energy is lost to the surroundings:
Next, I illustrated all this with some actual (made up) bond energies and an actual (not made up) reaction!
I showed how the 400 kJ bar represented the energy transferred from the surroundings to break bonds. This is the energy from the surroundings that it “gained” by the system (energy in).
And the 500 kJ bar represents the energy “lost” to the surroundings when bonds form (energy out).
We can see that overall, more energy is lost than gained, but we don’t know how much energy is lost unless we calculate the difference in the values.
I personally found it really helped to focus on the bars, and the difference between them, to represent the energy changes. It helped me to visualise, and to explain, the balance between energy in and out. It was important to emphasise which aspects relate to the system, and exactly when I was talking about the surroundings.
Overall, found that the lesson flowed well, and the class seemed confident and happy to get on with the example questions afterwards. I’ll certainly be revisiting this approach.