Combustion - Experiment 2

The weight of the product of combustion is greater than that of the starting material.

Materials Substitutions
aluminum pie pan steel wool
balance LPG burner
uncoated extra-course steel wool corn-on-the-cob tongs
bunsen burner
crucible tongs

Procedure
1. Weigh the empty pie pan and record the mass.
2. Place a pad of the steel wool (approximately 3 in x 3 in) in the pan and record the weight of the pan and pad.
3. Light the burner and adjust it to obtain a blue (hot) flame
4. Hold the steel wool with the tongs and place it in the flame for several minutes. Rotate the pad so that all parts are exposed to the flame. After all of the pad has a dull gray appearance, turn off the burner. Place the steel wool in the pie pan, sweeping any “popped” pieces of the steel wool into the pie pan as well.
5. Weigh the pan and steel wool and record the mass.

Data and Observations
1. Weight of the empty pan __________g
2. Weight of the pan and steel wool before heating __________g
3. Initial weight of the steel wool (#2-#1) __________g
4. Weight of the pan and steel wool after heating__________g
5. Weight of the steel wool after heating (#4-#1) __________g
6. Difference between the weight of the steel wool before and after heating (#5-#3) __________g

Questions
1. What kind of change took place?
2. Why did the mass of the steel wool change as a result of heating? Can you explain the differences in the masses? “Steel” wool is composed of elemental iron (Fe).
3. Write a balanced chemical equation for the burning of steel.

Extension
Try burning another metal, like magnesium or aluminum. You may need to include the weight of the tongs (initial and final) in this experiment as some of the oxide will be left on the tongs.CAUTION: When magnesium burns, it gives off a very bright light. Do not look directly at the light! Permanent eye damage can occur! Write a balanced chemical equation for the burning of the metal. How are the equations for the burning of steel wool and magnesium (or aluminum) similar? How can the oxidation of a metal (sometimes called corrosion) be prevented?

Teacher’s Notes
There is a gain in weight or mass when steel wool is burned. The increase is due to the oxygen that combines with the iron. The balanced chemical equation for the combustion, or oxidation, of iron is

4 Fe (s) + 3 O2 (g) —> 2 Fe2O3 (s)

The corrosion of iron is prevented by not permitting the metallic object to be in contact with oxygen. This can be accomplished by painting, coating with oil, or galvanizing (painting with zinc) the steel objects. Corrosion weakens the iron because the iron oxide (rust) flakes off, thereby reducing the amount of the steel.

It is best to use coarse to medium-coarse steel wool. Fine steel wool will give the effect of a “sparkler”—popping all over the lab bench and possibly onto paper! Move papers and towels away from the burner while heating.

Students must use a sufficient amount of steel wool to notice a change after heating. The difference in mass will be very small. An aluminum pie pan under the burner can be used to collect the mass that has “popped”. Since every 0.1 g will count, you will want to eliminate the “popped” mass as a source of error.

An old fashioned flash bulb for a camera will demonstrate this concept well. Weigh it before exposure and again afterwards. Why is there a change in mass?

Safety Precautions
Precautions should be taken when lighting the burner. Turn the gas on only after the match has been lighted. This will prevent an excess amount of gas from building up around the burner. The heated steel wool will be very hot and tongs must be used. If the pan becomes hot, it should not be put on the balance until it cools.

Be sure to caution students to prepare for some popping of the fine steel filaments. All paper and other flammables must be moved away from the burner.

Disposal
Used steel wool pads and pie pans may be placed in a solid waste container.