Here we have to complete the given single replacement reactions.
The replacement reactions are-
1) Fe (s) + CuCl₂ (aq) → FeCl₂ (aq) + Cu (s)
2) Cu (s) + FeCl₂ (aq) → NA
3) K (s) + NiBr₂ (aq) → NA
4) Ni (s) + KBr (aq) → NiBr₂ (aq) + K (s)
5) Zn (s) + Ca(NO₃)₂ (aq) → Zn(NO₃)₂ (aq) + Ca (s)
6) Ca (s) + Zn(NO₃)₂ (aq) → NA
The replacement reactions can be explained in light of the redox potential.
The standard reduction potential of the half cells involved in these reactions are:
Fe²⁺ + 2e → Fe (E° = -0.441V); Cu²⁺ + 2e → Cu (E° = 0.674V)
Ni²⁺ + 2e → Ni (E° = -0.23V); Zn²⁺ + 2e → Zn (E° = -0.763V)
We know the half cell reactions in which the standard reduction potentials are positive are allowed.
1) The reaction is possible as Cu²⁺/Cu and Fe/Fe²⁺ standard reduction potentials are positive.
2) The reaction is not possible as Cu/Cu²⁺ and Fe²⁺/Fe standard reduction potentials are negative.
3) The reaction is not possible as Ni²⁺/Ni standard reduction potential is negative.
4) The reaction is possible as Ni/Ni²⁺ standard reduction potential is positive.
5) The reaction is possible as Zn/Zn²⁺ standard reduction potential is positive.
6) The reaction is possible as Zn²⁺/Zn standard reduction potential is negative.
and any get of units that has those units in the proper place can be considered a density unit. The ones that has those specifically are A, B, E and F