Question

Find the order of magnitude of electrode potential ($E^\circ_{\text{M}^{2+}/\text{M}}$) of
Cr, V, Mn, Fe, Co

Hint:

The anomalous values in the 3d series $E^\circ_{\text{M}^{2+}/\text{M}}$ trend are crucial to remember: Mn is more negative than expected due to stable $d^5$ $\text{Mn}^{2+}$, and Zn is very negative due to stable $d^{10}$ $\text{Zn}^{2+}$.

Answer 1

Step 1: Understanding the Concept:
The standard electrode potential (\( E^\circ_{\text{M}^{2+}/\text{M}} \)) for transition metals indicates their tendency to be reduced from the \( +2 \) state to the solid metal. A more negative value implies the solid metal is a stronger reducing agent and more easily oxidized to the \( +2 \) state. The trend across the 3d series is generally towards less negative values, but important exceptions exist due to stable electronic configurations (like \( d^5 \) for \( \text{Mn}^{2+} \)).

Step 2: Key Formula or Approach:
This requires recalling the standard reduction potential values (or their general trends) for the 3d series elements.

Step 3: Detailed Explanation:
Let's list the approximate standard reduction potentials (\( E^\circ \) for \( \text{M}^{2+} + 2e^- \rightarrow \text{M} \)) for the given metals:
- Vanadium (V): \( -1.18 \,\text{V} \)
- Chromium (Cr): \( -0.91 \,\text{V} \)
- Manganese (Mn): \( -1.18 \,\text{V} \)
- Iron (Fe): \( -0.44 \,\text{V} \)
- Cobalt (Co): \( -0.28 \,\text{V} \)
Notice that Manganese breaks the general trend of becoming less negative. The \( E^\circ \) value for Mn is more negative than expected (and similar to V) because the resulting \( \text{Mn}^{2+} \) ion has a highly stable, half-filled \( 3d^5 \) electron configuration. This makes solid Mn relatively eager to oxidize to \( \text{Mn}^{2+} \), driving the reduction potential down.
Arranging these values in increasing order (from most negative to least negative):
\( -1.18 \,\text{V} \,(\text{V}) \approx -1.18 \,\text{V} \,(\text{Mn}) < -0.91 \,\text{V} \,(\text{Cr}) < -0.44 \,\text{V} \,(\text{Fe}) < -0.28 \,\text{V} \,(\text{Co}) \)
Therefore, the order is V \( \approx \) Mn \( < \) Cr \( < \) Fe \( < \) Co.

Step 4: Final Answer:
The increasing order of standard electrode potentials is V \( \approx \) Mn \( < \) Cr \( < \) Fe \( < \) Co.