Which metal will spontaneously react with Zn2 + (aq), but will not spontaneously react with Mg2 + (aq) ?

(1) Mn (s) (2) Cu (s)

(3) Ni (s) (4) Ba (s)

Correct Answer: option (1) Mn (s)

Reason:

The spontaneity of electrochemical cell, depends on the it's Eo value. Electrochemical cells with positve Eo are spontanous and vice-versa.

In present case, the Eo of half-cell of interest are as follows:

Eo Zn2+/Zn = - 0.763v

Eo Mg2+/Mg = 2.37v

Eo Mn2+/Mn = - 1.18v

Therefore, Eo cell (with Zn as one of the half-cell) = Eo Zn2+/Zn - Eo Mn2+/Mn

= - 0.763 - (-1.18)

= 0.417v

On other hand, Eo cell (with Mg as one of the half-cell) = Eo Mg2+/Zn - Eo Mn2+/Mn

= - 2.37 - (-1.18)

= - 1.19v

Thus, Mn (s) metal will spontaneously react with Zn2 + (aq), but will not spontaneously react with Mg2 + (aq)

This refers to a displacement reaction so we simply look at what can displace Zinc and not Magnesium so it will be Barium which is more reactive than Zinc but less reactive than Magnesium.