Question

Which of the following reactions could be used to power a battery because of the transfer of electrons?

Pb22+ + 2Br− → PbBr
CaCl2 + 2NaOH → Ca(OH)2 + 2NaCl
KOH + HCl → KCl + H2O
Zn + Cu2+ → Zn2+ + Cu

Answer 1

Answer is: Zn + Cu2+ → Zn2+ + Cu.

In this chemical reaction, there is transfer of electrons from zinc (Zn) to copper (Cu). Zinc change oxidation number from 0 to +2 (lost electrons) and copper change oxidation number from +2 to 0 (gain electrons).

Oxidation half reaction: Zn⁰ → Zn²⁺ + 2e⁻.

Reduction half reaction: Cu²⁺ + 2e⁻ → Cu⁰.

In other chemical reactions, there is no change of oxidation number of elements.