To help students learn more about electrochemistry by helping them increase their understanding of electron transfer and its role in chemical changes.
This lesson is the second of a three-part series on energy transformation. All three lessons have the general purpose of increasing students' understanding of energy transfer, its role in chemical change, and the factors that can influence this change. Then each lesson in addition has a specific purpose. The Transfer of Energy 1 or 2 can be done in any order; however, it is suggested that The Transfer of Energy 1 and 2 be done before The Transfer of Energy 3.
The Transfer of Energy 1: Thermochemistry is intended to increase students' understanding of heat and chemical reactions.
The Transfer of Energy 2: Electrochemistry is intended to increase students' understanding of electron transfer and its role in chemical changes.
The Transfer of Energy 3: Rust and Corrosion reinforces students' understanding of thermochemistry and electrochemistry by exposure to a process that they observe in life. This lesson allows students to understand how energy transfers in the chemical change of rust and corrosion and to understand the factors that influence this process.
By the end of elementary school, students should know several points about energy transformation. Students should know that when warmer objects are put with cooler ones (at a distance or next to each other), the warmer objects transfer internal energy (emitted as heat) to the cooler ones until they all reach the same temperature. They should understand things that give off heat can also give off other sorts of energy, including light. And, that heat is produced any time one thing rubs against something else and by mechanical and electrical machines. Students should also know that some materials transmit heat energy much better than others (materials that are poor conductors can reduce the transmission of heat from the object).
This prerequisite knowledge helps middle-school students learn the following four points about energy transformation:
- Energy cannot be created or destroyed, but only changed from one form into another.
- Most of what goes on in the universe—from exploding stars and biological growth to the operation of machines and the motion of people—involves some form of energy being transformed into another. Energy in the form of heat is almost always one of the products of an energy transformation.
- Heat can be transferred through materials by the collisions of atoms or across space by radiation. If the material is fluid, currents will be set up in it that aid the transfer of heat.
- Energy appears in different forms. Heat energy is in the disorderly motion of molecules; chemical energy is in the arrangement of atoms; mechanical energy is in moving bodies or in elastically distorted shapes; gravitational energy is in the separation of mutually attracting masses.
At this early stage, there may be some confusion in students' minds between energy and energy sources. Focusing on energy transformations may get around this somewhat. Food, gasoline, and batteries obviously get used up. But the energy they contain does not disappear; it is changed into other forms of energy through physical or chemical processes.
Research most applicable to electrochemistry shows the following information on heat, temperature, and chemical changes:
Students' meanings for "energy" both before and even after traditional instruction are considerably different from its scientific definition. For example, some students believe energy is associated only with humans or movement, is a fuel-like quantity that is used up, or is something that makes things happen and is expended in the process. Rarely, does a student think energy is measurable and quantifiable. Students of all ages hold these meanings for energy.
Energy forms and energy transformation
Middle- and high-school students tend to hold the view that energy transformations involve only one form of energy at a time. Although they develop some ability to identify different forms of energy, in most cases their descriptions of energy change focus only on forms that have perceivable effects. The transformation of motion to heat seems to be difficult for students to accept, especially in cases with no obvious temperature increase. Finally, some students fail to understand that some forms of energy, such as light, sound, and chemical energy, can be used to make things happen.
Middle- and high-school student thinking about chemical change tends to be dominated by the obvious features of the change that they can visualize. For example, some students think that when something is burned in a closed container, it will weigh more because they see the smoke that was produced. Conversely, some students do not view chemical changes as interactions, especially when the energy transfers in a reaction are invisible. Students also fail to realize that substances can be formed by the recombination of atoms in the original substances. Rather, they see chemical change as the result of a separate change in the original substance, or changes, each one separate, in several original substances. For example, some students see the smoke formed when wood burns as having been driven out of the wood by the flame.
As indicated in the Context section, research shows that students have many misconceptions about heat, temperature, and chemical changes. For example, students rarely think energy is measurable and quantifiable and they tend to associate energy only with living things. Research also shows that students have a difficult time understanding that some forms of energy, such as light, can be used to make things happen and students do not view chemical changes as interactions, especially when the energy transfers in a reaction are invisible.
As a starting point to help overcome some of these misconceptions, ask students these questions.
Note 1: You may want to ask some more simplistic questions before these questions.
Note 2: In these questions, it would probably be helpful to point out "ideas" that students tend to have problems understanding. For example, in the first question, you could point out this type of energy is related to a nonliving thing and/or that electrical energy causes "things to happen."
- In a light bulb, for example, how does electrical energy become light?
(Electrical energy excites the atoms in the filament, which in turn radiate excess energy as light.)
- Compared to the amount of electrical energy that goes into the light bulb, how much is actually emitted as light? More than, less than, or equal to the initial quantity?
(Less, because while energy cannot be created or destroyed, some energy is also radiated as heat.)
Using the How Batteries Work student esheet, students will go to How Batteries Work on the How Stuff Works site. The student esheet will instruct students to answer these questions. When students have finished, discuss the answers with the class.
- What are the two terminals of a battery, and which way do electrons flow?
(The two terminals are negative and positive. Electrons flow from negative to positive.)
- Where might we see the same reactions that occur in batteries in our daily lives?
(Wherever an electrolyte—any substance which can transfer electrons—touches two different types of metals, a reaction occurs. The difference may not be great—in fact, small impurities within a metal may cause electrons to move from one section of the metal to another, with one section acting as the positive terminal, and the other as the negative terminal. This reaction leads to corrosion—the gradual destruction of a metal by chemical means. When iron is oxidized, it rusts [e.g., an outdoor nail]; when silver is oxidized, it tarnishes [e.g., real silverware]; when copper is oxidized, a green coating forms on the surface [e.g., plumbing]. Yet another example is in our own mouths, when aluminum foil comes into contact with dental fillings, a mixture of several different metals. The "tingle" one may feel is actually a small electrical current! Feel free to mention rusting as an example. Rusting involves a transfer of electrons from the iron which rusts to either another metal or even another point on the iron. This is the same type of reaction that occurs in a battery!)
Using the student esheet to guide them, students will perform the Lemon Battery experiment found at the Hila Science Camp website. Pass out the Lemon Battery student sheet to students before they go online. Instruct students to follow the directions on the sheet to make their own lemon batteries.
Students should work in pairs to make a single lemon battery. Then, the student pairs should form larger groups to test batteries comprised of more than one lemon. After students have conducted the activity, review the questions on the student sheet with the class. You can refer to the Lemon Battery teacher sheet for answers to the questions.
Ask the students these questions about the Lemon Battery experiment:
- What role does the lemon itself play in the battery?
(The lemon is the electrolyte, which transfers electrons from the nail to the penny.)
- Which of the following fruits would make good electrolytes, and which would not: bananas, limes, tomatoes?
(Bananas would make poor electrolytes due to lower acidity. Tomatoes and limes would make good electrolytes due to high acid contents.)
Ask students to write down what they felt was the main point of this lesson. Ask students to share. The purpose of this lesson is to increase students' understanding of electrochemistry by helping them understand electron transfer and its role in chemical changes better.
Follow this lesson with the third lesson in The Transfer of Energy series: The Transfer of Energy 3: Rust and Corrosion.
Experiments in Electrochemistry on the Fun Science Gallery site contains other activities that can be used to reinforce or further develop the ideas in this lesson.