Make diagrams for redox reactions:
Iron corrosion
Make a diagram that shows the corrosion products formed as a function
of the added amount of Fe metal to water.
Step by step instructions:
1) Start DATABASE and click on Fe
(iron) in the periodic table. Click on Fe 2+
in the list of available components. Click also on the
e- (electron) button.
The list of components selected will now show H+,
e- and Fe 2+.
Click on the pull-down menu File / Save and
Exit.
Click on the SPANA button to save the data file and to make a diagram.
Enter a file name, for example Iron.
2) The SPANA main window will
appear. Remember that the chemical system has the components:
H+, e− and
Fe2+. A diagram showing the corrosion products
as a function of added Fe metal is needed and Fe(c) must be a
component instead of Fe2+. Click on the pull-down menu
Run / Modify chemical system.
The modify window appears. Click on Fe 2+
in the soluble components list and then Exchange a component with a reaction

A new window will appear listing the available reaction products.
Click on Fe(c) in the list of solid products
and click on the Ok button to confirm. Back in
the modify window the soluble components are H+ and
e−, and Fe(c) is a solid component.
Click on the Save changes button.
Enter a file name, for example Fe-corr.
Back in the main Spana window click on the pull-down menu
Run / Make a Diagram.
A new window appears to select what type of diagram is wanted.
Change the component in the X-axis to Fe(c).
To make a diagram that shows the corrosion process in pure water, you must set the
added amounts of H+ and
e− to zero.
On the Concentrations frame click on the
pH = 7 line, change pH for
Total conc., change the value to 0 (zero)
and press Enter. Then, on the same
concentrations frame, click on the pe = 8.5
line, enter a total concentration of zero for e− and press
Enter.
Click on the Diagram button in the upper-left corner of the window.
The calculations will be done by a console program.
When the calculations are finished the diagram is displayed in a window:
The diagram shows the amounts of corrosion products (in moles per litre of water)
as a function of iron metal reacted (in millimoles of Fe(cr) per
litre of water). In pure water magnetite is the solid corrosion product:
3 Fe(cr) + 4 H2O
Fe3O4(cr) + 4 H2
Reduction of Cu(II) to Cu(I) by cyanide
In a laboratory exercise the students have to react Cu2+
and Cd2+ with CN−.
The experiment with Cd produces a clear solution while Cu precipitates.
Illustrate first the Cd experiment by making a diagram with
[Cd]T = 10 mM and varying the
CN− concentration. Assume that the initial
Cd solution had a pH of 4, i.e. set the total concentration of H+
to 10−4
(Figure).
Why is there no precipitate? What would happen
if instead of CN− you had added
OH−?
(Figure).
In the case of copper(II) a redox process takes place:
Cu2+ + CN−
+ e−
CuCN(s)
CN−
½ (CN)2(g) + e−
Make a diagram to study what happens. Select the components: electron,
CN− and Cu2+.
If you select both Cu2+ and Cu+
as components, you can model the solubility, but not the redox process.
Use the same concentrations as in the Cd case. To simulate the redox process,
set the total concentration of added electrons to zero,
[e−]T = 0.
Step by step instructions
The diagram:
There is an ideal CN− concentration
where almost all Cu precipitates. If an excess of ligand is added soluble
Cu(I) complexes are formed and the solid redisolves.