To understand how energy transfers during the chemical changes that occur in the process of rusting and corrosion, and to understand the factors that can influence these changes.
This lesson is the third 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 everyday life. Through a practical experiment, this lesson allows students to understand how energy transfers during the chemical changes that occur in the rust and corrosion process, and to understand the factors that can influence these changes.
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 these 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.
A lot of time can be invested in having students memorize definitions—for heat, temperature, system, transformation, entropy, and the like—with little to show for it in terms of students' understanding. This lesson aims to teach students about the principles of thermochemistry and electrochemistry by providing them with a practical experiment involving both principles.
Energy is a mysterious concept, even though its various forms can be precisely defined and measured. At the simplest level, children can think of energy as something needed to make things go, run, or happen.
Three energy-related ideas may be more important than the idea of energy itself. One is energy transformation. All physical events involve transferring energy or changing one form of energy into another—radiant to electrical, chemical to mechanical, and so on. A second idea is the conservation of energy. Whenever energy is reduced in one place, it is increased somewhere else by exactly the same amount. A third idea is that whenever there is a transformation of energy, some of it is likely to go into heat, which spreads around and is therefore not available for use.
The most primitive idea is that the energy needed for an event must come from somewhere. This concept should trigger children's interest in asking where the energy comes from and (later) asking where it goes. Where it comes from is usually much more evident than where it goes, because some usually diffuses away as radiation and random molecular motion. Chemical changes can absorb or release energy when they occur, and usually require an energy input to be initiated.
Using the Rust and Corrosion student esheet, students should read How does rust work? on the How Stuff Works website.
Students should write down the answers to the following questions as they explore the website. When they are done, discuss the answers with the whole class.
- What is a chemical reaction?
(The process by which one or more substances may be transformed into one or more new substances.)
- What is the definition of an anode, an electrolyte, and a cathode?
(An anode is a piece of metal that readily gives up electrons; an electrolyte is a liquid that helps electrons move; and a cathode is a piece of metal that readily accepts electrons.)
- Can you think of anything used in everyday life that rusts and corrodes if you leave it outside to be exposed to rain?
(Examples may include gardening tools, bicycles, lawn chairs…anything with exposed iron.)
- What is the chemical name of rust?
- Do other metals rust or otherwise oxidize?
(Yes. Silver tarnishes and loses its shine, copper oxidizes to a greenish color.)
Tell students that they will conduct an experiment to determine if rusting can generate heat.
Go over the directions on the Rust and Corrosion student sheet with the class. Answer any questions students may have about the activity. Divide the class into small groups and pass out the materials for the activity.
Review the questions and answers to the Rust and Corrosion student sheet. Suggested answers can be found on the Rust and Corrosion teacher sheet.
To summarize the lesson, discuss these questions with the class:
- Commercially-prepared steel wool is treated with oil. Why do you think that is?
(To keep it from rusting.)
- Before I gave you the steel wool pads, I washed them with detergent in advance. What do you think might have happened if I hadn't washed the steel wool in detergent?
(For the chemical reaction of rusting to occur, the oil needed to be removed. If they hadn't been washed, the pads might not have rusted.)
Use student answers on the student sheet to assess student understanding of the ideas in the benchmark. You can also ask these questions to review the lesson:
- What is the chemical name for the process of rusting?
(It is oxidation.)
- What does iron react with to become rust?
(It reacts with oxygen, and the electrolyte which transfers electrons between the iron and the oxygen.)
- Why do some iron objects rust while others do not?
(The more iron present in an object, the faster it will rust if exposed to oxygen. Some "iron" objects are actually alloys, blends of metals containing substances that prevent or severely slow down rusting. For example, when iron is alloyed with other elements such as carbon, it makes steel which is a strong metal that doesn't rust quickly; if iron is alloyed with carbon and chromium, it makes a stronger form of steel that doesn't corrode. An example of this is most kitchen utensils.)
- Can rust be converted back to iron?
(Yes, you can put energy in to remove the oxygen component of rust. After removal of the oxygen, pure iron remains. This is the process of how iron ore is converted to iron.)
- Why does it take a long time to see the rusting process occur when some types of metal objects are submersed in water for long periods of time?
(Iron objects exposed to both water and oxygen will rust rapidly. However, iron objects alloyed to other metals will not rust as rapidly because the iron can't be oxidized as easily.)
- In everyday life, why don't objects that rust get hot?
(Steel wool has a large surface area in contact with the water or vinegar, so the rusting occurs very rapidly. Most rusting objects in everyday life, such as cars, shovels, etc., have a smaller surface area that is rusting and thus will not rust nearly as fast and therefore will not generate the heat observed in your experiment. Also, the jar insulates the steel wool from outside elements such as air, which may cool it down faster.)
- How does rusting generate heat?
(Changes in the energy held by chemical bonds in the oxidation of iron yield a net loss of energy from the reactants, and this net loss escapes to the surroundings where it is felt as heat.)
These resources can be used to extend or reinforce the ideas in this lesson:
- Rust Never Sleeps, on the Science A-Gogo site, is a thought-provoking article on rusting.
- Lullaby for Rust, from Nature, provides an innovative way to prevent rust.