Creating a Partial Pressure Diagram for the Cu-CO2-O2-H2O System

Terri Woods, East Carolina University (retired)
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Students will be guided through the procedure for creating a partial-pressure diagram in the low-temperature system Cu-CO2-O2-H2O system for the minerals cuprite, tenorite, native copper, azurite, and malachite. They will write chemical reactions and use Gibbs Free Energies to calculate Log K and plot lines on a graph with axes Log P CO2 and Log PO2 for stability boundaries between minerals. They are provided with data to then create their own diagram for the Fe-CO2-O2 system.

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graduate and upper level undergraduate course in low-temperature geochemistry

Skills and concepts that students must have mastered

Writing Equilibrium Constants
Using Gibbs Free Energies to calculate Keq

How the activity is situated in the course

Part of a sequence of exercises related to stability diagrams


Content/concepts goals for this activity

Using Gibbs Free Energy to calculate log K and from that to calculate the gas pressure at which two minerals are in equilibrium

Higher order thinking skills goals for this activity

Plotting equilibria on a diagram and eliminating metastable reactions and stability-boundary extensions

Other skills goals for this activity

Basic manipulation of algebraic equations and use of the properties of logarithms

Description and Teaching Materials

An interactive powerpoint presentation walks students step-by-step through the process of generating the partial-pressure diagram for the Cu system and a Word document provides the thermodynamic data and instructions necessary to create their own diagram for the Fe system.
Tutorial_Creating a Partial Pressure Diagram for the Cu system (PowerPoint 2007 (.pptx) 1.2MB Oct29 21) 
Data/Instructions for creating a Partial-pressure Diagram for the Fe System (Microsoft Word 2007 (.docx) 14kB Oct29 21)

Teaching Notes and Tips

I suggest going through the tutorial in class with the students allowing them to complete each step before clicking to show the correct answer to them on the screen. I also provide this tutorial to them so they can refer to it when they are doing their exercise for the Fe system.


Did they write all the necessary, correct, balanced chemical reactions?
Did they correctly calculate equilibrium gas pressures?
Did they plot lines correctly on diagram?
Did they correctly eliminate metastable reactions and stability-boundary extensions?
Did they correctly plot gas pressures provided and identify the stable mineral under those conditions?

References and Resources

Most texts on Aqueous Geochemistry include a section on partial-pressure diagrams. This activity just provides an interactive way to teach students how to create the diagrams and provides a reference to help them when they are completing their problem set.