Download Experiment 9 Chemical Teaser – Galvanized

Experiment 9
Chemical Teaser – Galvanized Washers: How Thick is That Coating
TA Evaluations:
Remember you must complete the on-line TA evaluation (in Owl) by Wednesday, April 29th, 2015. This
evaluation accounts for 1% of your laboratory grade
Introduction:
Iron ... A Little Background and Chemistry:
Iron, which takes its English name from the old Anglo-Saxon and its symbol from the Latin, ferrum, was
identified and used in prehistoric times. It is a very common element, fourth most abundant in the earth's crust. In
addition, two of the ten most common compounds in the earth's crust are the two common oxides of iron, FeO and
Fe2O3.
In its pure form, iron is a silvery-white metal, distinguished by its ability to take and retain a magnetic field,
and also dissolve small amounts of carbon when molten. Commercial refining of iron is based on the heating of
Fe2O3 or Fe3O4 (magnetite) with a mixture of other substances in the high temperature environment of the blast
furnace. The oxides are reduced to pure iron. In addition to hardening iron by adding small amounts of carbon and
also some other metals to the molten iron, iron castings or forgings can be heat-treated to take advantage of the
various physical properties of the different solid phases of iron.
Pure iron reacts readily with oxygen and moisture in the environment and corrodes destructively. Even
alloys such as steel need protection by painting or some other coating to prevent structural failure over time.
It is estimated that of the 500 million tons produced annually worldwide that close to ¼ of this is used to
replace corroded iron. Why Iron corrodes so readily is a topic that will be met later in the semester, when you
discuss oxidation reduction reactions. We are all familiar with rust, Fe2O3; it is an oxidation/reduction product that
is formed when iron is exposed to a plentiful supply of oxygen and water:
Oxidation:
Reduction:
Net:
2 Fe(s)
O2(g) + 2 H2O(l) + 4e2 Fe(s) + O2(g) + 2 H2O(l)
=
=
=
2 Fe2+ + 4e4 OH2 Fe2+ + 4OH-
Overall:
2 Fe(s) + O2(g) + 2 H2O(l)
=
2 Fe(OH)2(s)
In the presence of excess oxygen, further oxidation occurs in which Fe2+ is
converted to Fe3+
Overall:
4 Fe(OH)2(s) + O2(g)
=
2 Fe2O3.H2O + 2 H2O(l)
The resulting 2Fe2O3.H2O, is red-brown in color, the familiar color of rust
(corroded iron).
In the absence of excess oxygen, i.e. in the case where an iron object is in
moist soil, we get only partial further oxidation and the product formed is Fe3O4
which, in a simplest view can be looked at as, a mixture of Fe2O3 (complete
oxidation) and FeO (no further oxidation). The overall reaction process is
summarized below. Fe3O4 is black and is thus the black coating that you find on
iron objects that have corroded in soil:
Overall:
6 Fe(OH)2(s) + O2(g)
=
2 Fe3O4(s) + 5 H2O(l)
Zinc Iron's Alloy!
The name for zinc is of German origins, zink. It has been known since pre-historic times and compounds as
well as the metal had been used for many years before anyone ever thought about elements at all! Pure zinc is a
bluish-silver and ductile metal with a low melting and boiling point. Most zinc today is obtained from ZnS,
extracted from zinc blende ore and roasted to remove the sulfur. Zinc can also be obtained by electrolysis of aqueous
zinc sulfate, a common laboratory exercise.
Zinc also undergoes corrosion in a similar manner to that of Iron. In fact it's E0 (+0.76V) value for
oxidation to Zn2+ (a measure if you wish of it likelihood to undergo oxidation) is greater than that of Iron E0
(+0.44V) value for oxidation to Fe2+.
Galvanized Steel:
One way to protect iron involves incorporating other metals in the refining stage; in fact stainless steel is a
mixture of 18-20% Cr, 8-12% Ni, the remainder being primarily iron. However as mentioned earlier, even this form
of Iron is susceptible to corrosion. This is of particular concern when the steel is used in outside construction, where
the elements for corrosion, oxygen and water are all to readily available. This is where we turn to Zinc, a thin
coating of zinc on the steel, often referred to as galvanized steel, and protects the steel from corrosion. It does this
by being a sacrificial lamb, it corrodes to form Zn(OH)2 but unlike rust this thus not flake off but rather forms a
somewhat impregnable film that protects further corrosion of the metal and thus the steel itself.
Your Lab Problem!:
Design an electrochemical experiment to determine the ~ thickness of the Zn coating on the washer.
In addition to the equipment in your drawers and the chemicals in the lab, you will also have available to
you





Galvanized washers.
A solution of ZnCl2.
A 1.5V battery.
Wire
Thin strips of copper
The following may or may not be of use to you:
1.
Area of a cylinder:
2.
Area of a circle:
3.
4.
5.
Volume of a cylinder
Molar Mass of Zn:
Density of Zn:
2rh
r2
r2h
65.39g/mol
7.14g/mL