MUST IT RUST? THE REACTION BETWEEN IRON AND
Knight Foundation Summer Institute
Kathleen Heffner, Beeber Middle School
Emily Dorean, Haverford College
Everything in the world is made up of chemicals, and all chemicals are made up of tiny particles called atoms. A chemical reaction occurs when atoms join together or break apart to form new and different combinations of atoms. In this experiment we will be investigating a chemical reaction between iron and oxygen, which is given by the formula 4Fe + 3O2 = 2Fe2O3. This formula shows that when iron is found in the presence of oxygen, the two chemicals join together to form a new compound. This reaction is called an oxidation/reduction reaction and represents the other (along with acid/base reactions) most common type of chemical reaction. In an acid / base reaction, a proton (H+) gets transferred from one substance to another. In an oxidation/reduction reaction electrons get transferred from one substance to another. In this reaction iron transfers some of its electrons to oxygen. The iron atom becomes an Fe+3 ion and oxygen becomes an 0-2 ion, which quickly joins with an H+ ion to form water. These two elements combine to form iron oxide, or rust.
During the corrosion process, iron does not seem to react with air at all. Water is necessary for the oxidation reaction to occur, to facilitate transport of the electrons, although we do know that iron will rust in air after a long time, since there are water droplets in the air. Another substance that facilitates the transport of electrons is vinegar.
This experiment will test the corrosion properties of water and vinegar, modeling two of the main environmental causes of increased rust corrosion. The vinegar is our model for acid rain. Industrial and auto emission gases create acid rain by making the rain that falls much more acidic than pure rain water. (Pure rain water has a pH of about 5.6 from dissolved carbon dioxide, a normal constituent of air; acid rain can be as acidic as to have a pH of 3!) Acid rain is not only a problem in urban areas with lots of industry and cars because winds can blow the polluting gases far away before they return to the earth as acid rain. We observe from this experiment that acid rain would accelerate the corrosion process of iron, i.e. the "steel" wool in vinegar will corrode more quickly than the steel wool in water or in dry air.
As an introduction to this experiment, have the students brainstorm about ways to test the reaction between iron and oxygen. Talk about what iron and oxygen are, and have the students list some places they are found in our environment. Iron oxide, or rust, is found as a reddish-brown color. If students suggest metals that are not iron, you can talk about how some metals react with oxygen to create compounds that are different colors of "rust''.
Split the students into cooperative groups of up to three. They can then:
-Students should hand in a written report that includes their hypothesis projected beforehand (e.g. the wool will rust more quickly in the water, and even more quickly in the vinegar, or the wool will rust only on the part that is below the surface of the liquid), and a clear section of conclusions. This can be augmented with pictures to describe the results.
-It might be interesting for the students to bring in descriptions of places where rust is common and not so common. (Comparing rust on cars that are kept in and out of ~arages, or bikes that are under cover or not.)
-The students should discuss and suggest everyday examples of this reaction. One idea would be to talk about how bridges rust more quickly than other metal structures because they are most often spanning a body of water, which can clearly speed up the rusting process.
-Another suggestion is to tie in the environmental effects of acid rain (vinegar is a strong acid), and how the increase in acid rain has meant that many iron structures are rusting more quickly, and require replacement sooner than they have in the past (See extensions, below).
Iron is a major structural material. Bridges are built from iron, as are cars. The rusting we observed is the same as corrosion. Corrosion of iron is a major problem, as it destroys the structural integrity of the metal, and we have seen that replacing rusted iron is a major expense.
From our observations it is possible to infer that rusting should not be a major problem in the desert southwest where the air is dry almost all the time. Conversely, in humid climates, iron must be protected from the weather in some way. Kids in Phoenix or Albuquerque can probably leave their bike out in the back yard all year with no serious corrosion, while if the kids treat the bike the same way in Philadelphia, the bike will be well rusted after a year or two.
In areas which have snowy or icy winters, we also have the problem of salted roads. One can investigate the effect of salt on iron corrosion by adding another beaker to the experiment with steel wool and 20 drops of salt water. This should show more rusting than the wool in water alone. Salt, like vinegar, is found to be a catalyst for the corrosion process.
Philadelphia Science Content Standards:
SCIENCE CONTENT STANDARD #1: THE NATURE OF SCIENCE
This experiment satisfies Benchmark 3 for grades 5-8: "collect and summarize data from an experiment and interpret the results in terms of the data."
SCIENCE CONTENT STANDARD #2: PHYSICAL SETTING
This experiment satisfies Benchmark 5 for grades K-4: "observe how materials change over time (e.g., metal rusting)."
This experiment satisfies Benchmark 7 for grades 5-8: "understand that the cycling of water in and out of the atmosphere plays an important role in determining climatic patterns. Water evaporates from the surface of the earth, rises and cools, condenses into rain or snow, and falls again to the surface. The water falling on land collects in rivers and lakes, soil, and porous layers of rock, and much of it flows back into the ocean.
SCIENCE CONTENT STANDARD #5: DESIGNED FOR This experiment satisfies Benchmark 1 for grades 5-8: "discuss how and why science and technology have created synthetic (non-natural) materials."
This experiment can be tied most clearly to an environmental science unit on acid rain, as well as a chemistry unit on oxidation/reduction reactions. Because rust corrosion is such a part of our industrial life, there are many ways in which technology has approached this problem. This direction could take you into a lesson on science information and technology (looking at why paint has zinc in it to prevent rusting, etc). The effects of rusting have also had great effects on history, geology and our knowledge of the past, through the corrosion of historical evidence, which could be incorporated into a more interdisciplinary approach to the subject.