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Step-by-Step Explanation
Step 1: Understand the Concept of Standard Reduction Potential
The standard reduction potential, denoted as $E^\circ_{M^{2+}/M}$, measures how readily a metal ion $M^{2+}$ gains electrons to become the corresponding metal $M$. A higher (more positive) value indicates a greater tendency for reduction (gain of electrons), while a lower (negative) value indicates a lesser tendency for reduction.
Step 2: Identify the First Transition Series Metals in Question
We are given four successive members of the first transition series of elements and their standard reduction potentials:
Fe ($Z=26$)
Co ($Z=27$)
Ni ($Z=28$)
Cu ($Z=29$)
The question asks which one has a positive standard reduction potential $E^\circ_{M^{2+}/M}$.
Step 3: Compare the Standard Reduction Potentials
The provided values are:
$E^\circ_{\mathrm{Fe}^{2+}/\mathrm{Fe}} = -0.44\,$V
$E^\circ_{\mathrm{Co}^{2+}/\mathrm{Co}} = -0.28\,$V
$E^\circ_{\mathrm{Ni}^{2+}/\mathrm{Ni}} = -0.25\,$V
$E^\circ_{\mathrm{Cu}^{2+}/\mathrm{Cu}} = +0.34\,$V
Out of these, only copper has a positive value.
Step 4: Reason for Copper's Positive Potential
Copper is less reactive compared to iron, cobalt, and nickel. It does not lose electrons as readily as these metals, and hence the reduction of $Cu^{2+}$ to $Cu$ is favored energetically. This is reflected in its positive standard reduction potential.
Step 5: Conclude the Correct Answer
Since copper is the only metal in the given list with a positive standard reduction potential, the correct answer is:
Cu (Z = 29)
