Final answer:
Iron (Fe) can reduce Pb²⁺ ions to lead (Pb) since it is above lead in the activity series, but it cannot reduce Ni²⁺ ions to nickel (Ni) because iron is below nickel in the series.
Step-by-step explanation:
To name a substance that can reduce Pb²⁺ ions to lead (Pb) but cannot reduce Ni²⁺ ions to nickel (Ni), we look for a metal that sits between lead and nickel in the activity series of metals. Iron (Fe), for example, can reduce Pb²⁺ to Pb as it lies above lead in the activity series:
Fe (s) + Pb(NO3)2 (aq) → Fe(NO3)2 (aq) + Pb (s)
However, iron cannot reduce Ni²⁺ to Ni because iron is below nickel in the activity series:
Fe (s) + Ni(NO3)2 (aq) → NR (no reaction)
The activity series is a list that ranks metals according to their relative reactivity. A metal can only reduce ions of other metals that are below it in this series thus allowing us to make predictions about possible reactions.
Therefore, iron (Fe) is a substance that suits the described criteria of reducing Pb²⁺ to Pb but not reducing Ni²⁺ to Ni.