Which one is a spontaneous chemical reaction?

Explanation

A spontaneous chemical reaction is one that occurs without continuous external energy input, often characterized by a negative Gibbs free energy change (ΔG). In the reaction Zn + Cu2+ → Zn2+ + Cu, zinc is oxidized while copper ions are reduced, releasing energy in the process, thus making it spontaneous. In contrast, the other reactions are non-spontaneous because they require external energy to proceed. The reaction Zn2+ + Cu → Cu2+ + Zn is the reverse of the spontaneous reaction and requires energy input. The decomposition of Fe(OH)3 and the breakdown of NaCl are both processes that require energy input, hence they are non-spontaneous.