Question

An unknown metal displaces cadmium (Cd) from solution but does not displace chromium (Cr). Using the activity series, determine the unknown metal.
lead (Pb)
manganese (Mn)
iron (Fe)
zinc (Zn)
cobalt (Co)

Answer 1

Answer : The correct option is, iron (Fe)

Explanation :

The reactivity series of metal are arranged of the reactivity from the highest to the lowest. Reactivity series is used to determine the products of the single displacement reactions.

In the single displacement reaction, the most reactive metal displaces the least reactive metal.

From the reactivity series we conclude that zinc and magnesium are more reactive metal than cadmium and chromium metal. So, both can easily displace the cadmium and chromium metal from the solution. So, zinc and magnesium will exclude from the answer.

The metal lead and cobalt are least reactive than cadmium and chromium metal. So, both can not displace cadmium and chromium metal from the solution. So, lead and cobalt will also exclude from the answer.

The metal iron is more react metal than cadmium metal but less reactive than chromium metal. So, iron metal can not displace chromium metal but it can displace easily cadmium metal.

Hence, the correct option is, iron (Fe)

Answer 2

I found this is the order of activity in Internet

Mn>Zn>Cr>Fe>Cd>Co>Ni>Pb

You can see that Fe is between Cr and Cd

Cr is more active than Fe and Fe is more active than Cd.

The Fe will displace Cd but not Cr.

Answer:Fe