Question

Using the Pourbaix Diagram for Iron, choose the Incorrect statement:

• A. $Fe_{(aq)}^{3+}+e^{-}\rightarrow Fe^{2+} E^{o}=0.77V$ is independent of pH
• B. the formation of $Fe(OH)_{3}$ is favoured by high pH
• C. the formation of $Fe(OH)_{2}$ is favoured by high pH
• D. $Fe(OH)_{3(s)}+H_{(aq)}^{+}+e^{-}\rightarrow Fe(OH)_{2}(s)+H_{2}O(l)$ is independent of pH

Hint:

Sloped lines on a Pourbaix diagram = pH dependent (usually involve $H^+$ or $OH^-$).

Answer 1

The Correct Option is D

Step 1: Concept A Pourbaix diagram plots potential (E) vs. pH, showing stable phases of a metal.

Step 2: Meaning Horizontal lines represent pH-independent reactions (no $H^+$ involved), while sloped lines represent pH-dependent reactions.

Step 3: Analysis The reaction $Fe^{3+} + e^- \rightarrow Fe^{2+}$ involves only electrons, so its potential is a horizontal line (independent of pH). However, the reduction of $Fe(OH)_3$ to $Fe(OH)_2$ (Option D) explicitly consumes $H^+$ ions. According to the Nernst equation, any reaction involving $H^+$ must change potential as pH changes.

Step 4: Conclusion Therefore, statement (D) is incorrect because the reaction is actually dependent on pH. Final Answer: (D)