In my previous blog post I described some problems I encountered when beginning my instruction on energy this year. From the misconceptions fostered by the biology textbooks using the phrase “high-energy phosphate bond” to idea that energy comes in different forms, the Modeling community recognizes the challenges of teaching the energy concept and has developed a way of talking about energy designed to help students construct a consistent and cohesive model.
Three principles guide the model construction for energy:
- Energy is not a substance, but it is substance-like and can be stored in physical system.
- Energy can be transferred into and out of a system and can cause change.
- Energy does not change its identity after being transferred.
We use metaphors to help clarify what is meant by “substance-like”. We examine how we talk about information. Information is not a substance, just as energy is not. However, information can behave like a substance. Information can be stored in books, on CD’s, on computers – it can also be transferred from one place to another via Bluetooth, the Internet, or through cables. Most importantly, for the purpose of our discussion, when information is transferred from one place to another the information itself does not change. This is an important parallel to draw between energy and the information metaphor. When energy is transferred from one place to another, it remains energy. Nothing about the energy itself has changed.
The second metaphor we present is the money metaphor. We talk about different ways we can store money. Students provide examples of different types of accounts at banks. We talk about how each type of account – savings, checking, money market, etc. – stores one thing. They store money. Is $100.00 in a savings account worth a different amount if it is transferred into a checking account? No – money can be moved between accounts and it still retains its original identity, its value remains the same. The same is true for energy.
The Modeling Curriculum uses the concept of accounts discussed in the money metaphor to begin to build the model of energy storage and transfer used in both the Physics and Chemistry Modeling curriculums. We establish different types of “accounts’” to help students keep track of energy as it is transferred. The discussion of energy accounts immediately following the examination of the money metaphor helps students start to see that the act of transferring energy does not change the identity of the energy. The Chemistry curriculum is concerned with three accounts:
The Thermal Energy (Eth) Account
Energy in this account is the energy stored by the movement of particles. The quantity of energy is related to the mass and velocity of the particles in the system. This energy can be described by the temperature of the system. Warmer temperatures equate to more energy in the thermal account.
The Phase Energy (Eph) Account
Energy in this account is the energy stored in the system due to the arrangement of the particles and the interactions between those particles. Greater attraction decreases the energy of the system. In other words, as the particles are more tightly bound – more attracted to one another – the energy of the system decreases. Solids possess the least amount of energy in this account, followed by liquids. Gases have the most.
The Chemical Potential Energy (Ech) Account
Energy in this account is the energy due to attractions within molecules.
Once we have built the model for energy storage we introduce the methods of energy transfer. Traditional texts will name these methods work, heat, and radiation. We will refer to them as working (W), heating (Q), and radiating (R). While this difference may seem subtle, it is actually a very powerful and purposeful change. Using the terms in gerund form emphasizes they these are actions – they are processes and not things separate from energy. During the discussion of energy transfer we highlight the fact that when energy is transferred it affects both the system and surrounds. Energy doesn’t just appear or disappear. It comes from somewhere and when it leaves, it goes somewhere.
This is the process where energy is transferred between macroscopic objects that exert forces on one another. This method is discussed more often in the physics curriculum.
This is the process by which energy is transferred through collisions of microscopic objects. In this process energy is always transferred from the hotter object to the cooler. This is the process we use most often in the Chemistry curriculum.
This is the process where energy is transferred by releasing or absorbing photons.
The modeling curriculum then synthesizes these ideas into a tool we use to illustrate energy changes in both physical and chemical processes.
The Energy Bar Charts
My students call these LOL charts. Can you see why?
The energy bar chart is the tool we use to help students describe what is happening to energy in a system under different conditions.
Lets consider an example:
Conditions: A cup of hot coffee is allowed to cool on a table.
First, we establish what the system is. The system in this case is the cup of coffee. We would write the phrase “cup of coffee” in the circle at the center of the chart. The circle represents our system.
Second, we determine if a chemical change will occur. We start with coffee and end with coffee so no chemical change takes place. We can ignore the Ech account for now.
Next, we assign values to the energy in the phase account. We know that solids have the least amount of energy here due to the strong interactions between the particles. Liquids have slightly more energy than solids and gases have a lot more energy then either other phase. We can establish a convention to help discussion run more smoothly. Solids can be assigned one bar of energy and liquids will be given two. To illustrate gases having significantly more energy we can assign them four bars. In our problem, we start and end with coffee, which is a liquid. Applying these conventions, our diagram now looks like this:
Lastly, we look at energy stored in the thermal account. The coffee begins hot and cools down. We need to show this in our picture. We can begin by assigning four bars of energy to the thermal account when the coffee is hot and showing a decrease in the amount of energy in the final picture. I usually use two bars to represent room temperature, so lets assign two bars to the thermal account in the final picture. The picture now looks like this:
Okay, so we begin with a total of six bars in the initial picture but end with only four bars at the final. Where did those two bars go? They were released to the surroundings. What method of transfer is used? Well, the only change taking place is the cooling of the system. The particles are slowing down because some of the energy is being transferred to the surroundings. This is energy transfer via heating. We can label the bars leaving with the letter “Q” to indicate we know the energy is leaving via heating. We can illustrate this by drawing the two bars leaving the system – leaving the circle in the center.
By looking at the diagram I can glean a lot of information about what my students think is going on in this situation. I see they know there is no chemical change in this example. I can also immediately see they understood no phase change took place and that the process is exothermic. The diagram also indicates the system starts hot and cools down. All of this information is conveyed without writing a single word.
Here is a video of one of my students sharing her ideas on the same example:
This is just one example of how energy bar charts can be used to illustrate energy changes. Consider the example which includes a phase change:
Conditions: A tray of ice cubes at -8.0oC is left on the counter. It melts and warms to 20.0oC. What would your EBC look like for this situation?
(I’ll post the solution in the comments)
The role of energy in chemistry is a complex and difficult topic to teach. This is just a small sampling of how the Chemistry Modeling Curriculum addresses some of the difficulties chemistry teachers face when trying to teach this abstract concept. My next post will look at how EBCs are used when chemical change occurs.