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Among four consecutive elements of the first transition series given below, which one has a positive standard reduction potential for the M2+/M couple (E°M2+/M)?

1. Co (Z = 27)
2. Ni (Z = 28)
3. Cu (Z = 29)
4. Fe (Z = 26)

Correct answer: Cu (Z = 29)

Solution

Among Fe, Co, Ni, Cu, only copper has a positive standard reduction potential for M2+/M (E = +0.34 V); the others (Fe -0.44, Co -0.28, Ni -0.25 V) are all negative. This is why Cu does not displace H2 from acids.

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