2. Rust
is a brownish-red substance that forms on the surface of
iron or steel when it is exposed to damp air. The term
used alone means iron rust, which consists mainly of
hydrated iron oxide (3Fe₂0₃∙H₂O).
Rust is formed by the union of the oxygen of the air with
the iron by a process called oxidation.
Rust not only corrodes the surface but it also weakens
the metal.
Rust is the result of the reaction between a metal and a
non-metal.
3. We have previously seen how metals
come to form lattices that consist of
alternating layers of positively charged
ions and free electrons. This layered
structure of metal are presented:
4. Oxygen gas, O₂, consists of two atoms of Oxygen
covalently bonded together. This is a fairly stable
state for Oxygen but given the opportunity each
Oxygen atom will absorb two electrons from
elsewhere and have a full outer shell of 8
electrons without having to share.
5. We know that the metal Iron has loose
electrons. We also know that each of
the atoms in the oxygen molecule
would prefer to have 8 electrons
without having to share. Let's look at
the process in a series of visual steps:
6. 1. First the oxygen gas molecules,
O2, approach the metal:
7. 2. Then the Oxygen molecule breaks apart and each
oxygen atom soaks up two of the metal's free
electrons, forming the stable oxide ion:
8. 3. The metal lattice now has less electrons than it needs
and so some of the positively charged Fe ions will be
shed from the lattice.
9. 4. The loose +3 Fe ions and the stable O₂- ions
combine to form the neutral compound Fe₂O₃,
as follows. Note that this is an ionic compound
(a metal and a non-metal together) and that
all ionic compounds have an overall charge of
ZERO:
10.
11. And so a single particle of the rust Fe₂O₃ is
formed. Since this does not form a perfect seal
over the surface of the metal, further oxygen
can get to the surface of the metal, and the
rusting continues.
12. Finally, rust is…
… the product of the transfer of
electrons from the metal (loss of
electrons is called oxidation) to
the oxygen molecules (gain of
electrons is called reduction).
13. 1. Place the ball of steel wool into a glass jar and
add enough water to cover the ball.
14. 2. Add bleach and vinegar that has the ratio of
4:2, (this mixture is called hypochlorous acid),
Stir.
15. 3. In about 5 minutes and on, the liquid is full of red
powder. This powder is called ferric oxide (rust).
Remove the steel wool from the jar and wait for the
powder to settle so that you can see the red powder
easily.
16. Steel wool rusts slowly in water, but quickly in
a solution containing hypochlorous acid. In the
experiment, the solution used was vinegar
and bleach. The hypochlorous acid (HCIO) in
the solution reacts with the iron (Fe) in steel
wool to form hydrated ferric oxide
(Fe₂O₃∙H₂O).
17. Hypochlorous Acid
Hypochlorous acid is a weak acid with the
chemical formula HClO.
HClO is an oxidizer, and as its sodium salt sodium
hypochlorite, (NaClO), or its calcium salt calcium
hypochlorite, (Ca(CIO)2) used as a bleach and
others.
18. Iron
Like other Group 8 elements, iron exists in a
wide range of oxidation states, −2 to + 6,
although +2 and +3 are the most common.
Fresh iron surfaces appear lustrous silvery-
gray, but oxidize in normal air to give iron
oxides, also known as rust.
Unlike many other metals which form
passivating oxide layers, iron oxides occupy
more volume than iron metal, and thus iron
oxides flake off and expose fresh surfaces for
corrosion.
19. Small glass jar
small ball of steel wool
bleach
Vinegar
Water
old spoon/ stick (for stirring)
Measuring spoon (for measuring)—if possible
