Thermal decomposition of ${\mathrm{HI}}_{\left(\mathrm{g}\right)}$ on Gold surface follows Zero order kinetics. With initial pressure $1000 \mathrm{mm}$. If the pressure after $20\sec$ is $960 \mathrm{mm}$w.r.t ${\mathrm{HI}}_{\left(\mathrm{g}\right)}$, rate constant is (in ${\mathrm{mmsec}}^{-1}$ )

Thermal decomposition of HI on gold surface, is an example of zero order reaction as the concentration of the gaseous reactant on the metal surface remains constant. This is because, as the reactants react off on the surface, more molecules are adsorbed on the surface and rate remains constant throughout.

Therefore, the correct option is (C).