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an unknown metal displaces cadmium (cd) from solution but does not disp…

Question

an unknown metal displaces cadmium (cd) from solution but does not displace chromium (cr). using the activity series, determine the unknown metal.
activity series mn ↑ zn cr fe cd co pb
a. lead (pb)
b. manganese (mn)
c. iron (fe)
d. zinc (zn)
e. cobalt (co)

Explanation:

Step1: Understand Activity Series Rule

In an activity series, a metal can displace another metal from its solution if it is more reactive (higher in the series). The given activity series order (from more to less reactive) is: Mn > Zn > Cr > Fe > Cd > Co > Pb.

Step2: Analyze Displacement Conditions

  • The unknown metal displaces Cd: So, unknown metal is more reactive than Cd (higher than Cd in the series).
  • The unknown metal does not displace Cr: So, unknown metal is less reactive than Cr (lower than Cr in the series).

Step3: Identify the Metal

From the series, metals between Cr (higher) and Cd (lower) is Fe? Wait, no, wait the series: Mn, Zn, Cr, Fe, Cd, Co, Pb. Wait, order: Mn (most reactive) then Zn, then Cr, then Fe, then Cd, then Co, then Pb (least). Wait, so "displaces Cd" means unknown > Cd. "Does not displace Cr" means unknown < Cr. So find metal where Cr > unknown > Cd. Looking at the series: Cr is above Fe, Fe is above Cd. Wait, the series is written as Mn ↑ ZnCrFeCdCoPb. Wait, maybe the order is Mn (top), then Zn, then Cr, then Fe, then Cd, then Co, then Pb (bottom). So Cr is above Fe, Fe is above Cd. So Fe is between Cr (above) and Cd (below). Wait, but let's check options:

  • Option A: Pb is below Cd (less reactive than Cd) → can't displace Cd. Eliminate.
  • Option B: Mn is above Cr (more reactive than Cr) → would displace Cr. But unknown does not displace Cr. Eliminate.
  • Option C: Fe. Cr > Fe (since Cr is above Fe) and Fe > Cd (Fe is above Cd). So Fe can displace Cd (more reactive than Cd) and cannot displace Cr (less reactive than Cr). Let's check others.
  • Option D: Zn is above Cr (more reactive than Cr) → would displace Cr. Eliminate.
  • Option E: Co is below Cd? Wait no, Co is below Cd? Wait the series: Cd is above Co? Wait the series is Mn, Zn, Cr, Fe, Cd, Co, Pb. So Cd is above Co (Cd is more reactive than Co). So Co is less reactive than Cd → can't displace Cd. Eliminate. Wait, but wait the series: maybe I misread. Wait the activity series is written as "Activity seriesMn ↑ ZnCrFeCdCoPb". The arrow is upward, meaning increasing reactivity? Wait no, arrow up usually means more reactive at the top. So Mn is at the top (most reactive), then Zn, then Cr, then Fe, then Cd, then Co, then Pb (bottom, least reactive). So:

Reactivity order: Mn > Zn > Cr > Fe > Cd > Co > Pb.

So "displaces Cd" → unknown > Cd.

"Does not displace Cr" → unknown < Cr.

So find metal where Cr > unknown > Cd.

Cr is more reactive than Fe (Cr > Fe), and Fe is more reactive than Cd (Fe > Cd). So Fe is between Cr (above) and Cd (below). So Fe is the metal. Wait, but option C is iron (Fe). Let's confirm:

  • Fe displaces Cd? Yes, because Fe > Cd (Fe is above Cd in series).
  • Fe displaces Cr? No, because Cr > Fe (Cr is above Fe in series) → so Fe cannot displace Cr (since it's less reactive than Cr). Perfect. Wait but let's check the series again. Wait the original series: "Activity seriesMn ↑ ZnCrFeCdCoPb". Maybe the order is Mn (top), Zn, Cr, Fe, Cd, Co, Pb (bottom). So Cr is above Fe, Fe above Cd, Cd above Co, Co above Pb. So Fe is between Cr and Cd. So Fe is the answer. Wait but let's check options again. Wait option C is iron (Fe). So that's correct. Wait but earlier I thought maybe Fe, but let's recheck:

Wait the problem says "displaces cadmium (Cd) from solution" → unknown is more reactive than Cd.

"does not displace chromium (Cr)" → unknown is less reactive than Cr.

So Cr (more reactive) > unknown > Cd (less reactive).

From the series, Cr is above Fe, Fe is above Cd. So Fe is between them. So Fe is the unknown metal. So option…

Answer:

C. iron (Fe)