For the reaction ${\mathrm{H}}_2\left(\mathrm{g}\right)+{\mathrm{F}}_2\left(\mathrm{g}\right)\rightleftharpoons 2\mathrm{HF}\left(\mathrm{g}\right);\mathrm{\Delta }H=-536\mathrm{kJ}{\mathrm{mol}}^{-1}$, The energy of activation of forward reaction is $208\mathrm{kJ}{\mathrm{mol}}^{-1}$, the energy of activation of the backward reaction is