The standard cell potential involving the reaction ${\mathrm{Cl}}_2\left(\mathrm{g}\right)+2{\mathrm{Br}}^{-}\left(\mathrm{aq}\right)\to {\mathrm{Br}}_2\left(\mathrm{l}\right)+2{\mathrm{Cl}}^{-}\left(\mathrm{aq}\right)$

${\mathrm{\Delta }}_{\mathrm{r}}{G}^{\circ }=-50.61 \mathrm{kJ} {\mathrm{mol}}^{-1}$ is