How much air is used during rusting?
Students set up iron wool to rust in a test tube full of air inverted in a beaker of water. As the iron wool reacts, rusts and removes the oxygen from the air, water is drawn up the tube. By observing the change in the volume of air in the tube, the percentage of oxygen in the air can be found.
Lesson Organisation
This experiment will need to be carried out over 2 lessons about a week apart. The practical work will probably take no more than 20 minutes in either lesson.
Apparatus and Chemicals
Per pair of group of pupils:
Test tube (see note 1)
Beaker (100 cm3)
Ruler
Test tube (see note 1)
Beaker (100 cm3)
Ruler
Iron wool (Low hazard)
Technical notes
Iron wool (Low hazard) Refer to CLEAPSS Hazcard 55A
1 The test-tubes used in this experiment can get stained by the rust. They can be cleaned with a ‘Stain Devil’®.
Procedure
a Put about 3 cm depth of iron wool into the test tube and wet it with water. Tip away excess water.
b Put about 20 cm3 water into the beaker. Invert the test tube and place it in the beaker of water (see diagram). Measure the length of the column of air with the ruler.
c Leave for at least a week.
d Measure the new length of the column of air, taking care not to lift the test tube out of the water.
Teaching notes
Students need to understand that rusting is an oxidation reaction of iron with oxygen.
iron + oxygen → iron oxide
This is not the full story, however, as the formation of rust is a complex process. (For details, see the website below.)
From their 2 measurements for the length of the column of air – before and after rusting takes place – students should be able to calculate the percentage of the air which has been removed by the rusting reaction. This should be about 20% which is approximately the percentage of oxygen in the air.
You could ask students how they could show that the reaction is complete – they may suggest leaving it for another week or so to see if any further air is used up. The iron is present in excess in this experiment, so it will not all rust.
Health & Safety checked, April 2008
Web Links
For a more detailed explanation of rusting see:http://library.kcc.hawaii.edu/external/chemistry/everyday_corrosion.html
No comments:
Post a Comment