The possibility of metals replacing each other is dependent on the relative positions of the metals in the activity series.
The activity series is an arrangement of elements in order of decreasing reactivity. The elements that are higher in the series can displace the elements that are lower in the series.
Before we make a decision in each case, we must consider what metal is lower or higher in the series;
- Between Na and K, Na is lower in the series so the answer is no
- Between Co and Cu, Co is higher than Cu in the series so the answer is yes
- Since Zn and Zn^2+ are just a specie and its ion, the answer is no.
- Since Ba is higher than Ni, Ni can not replace Ba so the answer is no
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