Which of the following reactions are redox reactions?
1 CuSO4 + Zn → Cu + ZnSO4
2 HCl + NaOH → NaCl + H2O
3 Mg + H2SO4 → MgSO4 + H2
4 Pb(NO3)2 + 2KI → PbI2 + 2KNO3

Answer:

Reaction 1 is a redox reaction because there are changes in oxidation states (ionic charges): the
oxidation state of copper changes from +2 to 0 (Cu 2+ + 2e– → Cu [reduction]), and zinc changes from
0 to +2 (Zn → Zn2+ + 2e– [oxidation]).
Reaction 2 is not a redox reaction because there is no electron transfer (no change in ionic charge,
nor oxidation state).
Reaction 3 is a redox reaction because there are changes in oxidation states (ionic charges): the
oxidation state of magnesium changes from 0 to +2 (Mg → Mg 2+ + 2e– [oxidation]), and hydrogen
changes from +1 to 0 (2H+ + 2e– → H2 [reduction]).
Reaction 4 is not a redox reaction because there is no transfer of electrons.
The answer is B