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Chapter 5: Problem 8

Discuss the Pourbaix diagram for iron in detail.

Short Answer

Expert verified
The Pourbaix diagram for iron is a chart depicting where different forms of iron are stable in terms of potential and pH. The major forms are solid iron, Fe, ferrous ion, Fe^2+, ferric ion, Fe^3+, and two iron hydroxides, Fe(OH)2 and Fe(OH)3.

Step by step solution

01

Understand the Pourbaix Diagram for Iron

A Pourbaix diagram is a representation of the phase stability domains of the different possible oxidation states of an element, as a function of pH (potential of Hydrogen) and Eh (reduction potential or redox potential). In the case of iron, the Pourbaix diagram depicts the prevalent forms of iron under various pH and Eh values. It contains several regions representing different iron species like Fe(s), Fe^2+, Fe^3+, and two forms of iron oxides - Fe(OH)2 and Fe(OH)3. This diagram also includes lines demarcating each region, signaling possible shifts between species at those conditions.
02

Identify Different Regions

The Pourbaix diagram for iron contains different regions that each represent the stability zones of the various prevalent forms of iron. These regions are: 1. Solid iron, Fe(s), which is stable in a largish area from an environment of low Eh and pH (up to nearly 4) to a high Eh and moderately high pH values (around 10). 2. Ferrous ion, Fe^2+, which is stable in acidic conditions and moderate Eh values. 3. Ferric ion, Fe^3+, which is stable in high Eh and low pH environments. 4. The two iron oxides, Fe(OH)2 and Fe(OH)3, are stable under conditions with moderate to high Eh and pH values.
03

Discuss Transitions

The Fe/Fe^2+ and Fe^2+/Fe^3+ equilibria delineate the stability region of solid iron, with the former acting as a reductive dissolution reaction and the latter as an oxidative dissolution reaction. The conversion between Fe^2+ and Fe^3+ ions is via a simple redox reaction. Fe(OH)2 is formed by reaction of Fe^2+ with water, given enough Eh and pH, and further to Fe(OH)3 by a further redox reaction.

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