$4)$ A $0\mathrm{.}5$ cm diameter pellet of NiO is reduced with hydrogen according to the reaction:

NiO $+$ H$2->$ A $0\mathrm{.}5cm$ diameter pellet of NiO is reduced with hydrogen according to the reaction:

NiO$+H_{2}Ni+H2O$

When a high gas velocity is used, the reaction speed is controlled at the reduced$/$non$-$reduced

oxide interface and the reaction speed is $0\mathrm{.}002mo\frac{l}{c}{m}^2.$min, at $600C$ and $1$atm of $H_2.$

a$)$ Calculate the time required for $50\%$ and $100\%$ conversion, knowing that the

density of NiO is $6\frac{g}{c}{m}^3$ and the molar mass of NiO is $74\mathrm{.}7\frac{g}{m}ol$;

b$)$ For particles with a diameter of $3cm,60$min are required to reach $75\%$ conversion

and it is assumed that the reaction control is by diffusion in the layer of product

formed. Calculate the diffusion coefficient in $c{m}^2/$s$,$ when the concentration at the

interface is equal to zero. What does this zero concentration mean? N by diffusion in the layer of product formed. Calculate the diffusion coefficient in cm$2/$s$,$ when the concentration at the interface is equal to zero. What does this zero concentration mean?