Condition for maximum yield of C₂H₅Cl is

${\mathrm{C}}_2{\mathrm{H}}_6+{\mathrm{Cl}}_2\left(\mathrm{excess}\right)\stackrel{\mathrm{UV}\mathrm{Light}}{\to }$

${\mathrm{C}}_2{\mathrm{H}}_6+{\mathrm{Cl}}_2\underset{\mathrm{Room}\mathrm{temp}.}{\overset{\mathrm{Dark}}{\to }}$

${\mathrm{C}}_2{\mathrm{H}}_6\left(\mathrm{excess}\right)+{\mathrm{Cl}}_2\stackrel{\mathrm{UV}\mathrm{Light}}{\to }$

${\mathrm{C}}_2{\mathrm{H}}_6+{\mathrm{Cl}}_2\underset{ }{\overset{\mathrm{UV}\mathrm{Light}}{\to }}$