Chem.Eq.Diagr  Chemical Equilibrium Diagrams / Tutorials

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” Exchange button

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.

Make diagram
The calculations will be done by a console program. When the calculations are finished the diagram is displayed in a window:
Diagr Fe-corrosion
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


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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:

Diagram Fract Cu-CN
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.

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