Phản ứng nào sau đây là phản ứng thuận nghịch?

$2{\text{H}}_2(\text{g})+{\text{O}}_2(\text{g})\to 2{\text{H}}_2\text{O}\left(\text{l}\right)$.

${\text{N}}_2(\text{g})+3{\text{H}}_2(\text{g})\rightleftharpoons 2{\text{NH}}_3(\text{g})$.

$2{\text{KClO}}_3(\text{s})\to 2\text{KCl}\left(\text{s}\right)+3{\text{O}}_2(\text{g})$.

$2\text{Na}\left(\text{g}\right)+{\text{Cl}}_2(\text{g})\to 2\text{NaCl}\left(\text{g}\right)$.