The reaction, $2{\mathrm{N}}_2{\mathrm{O}}_5\rightleftharpoons 4{\mathrm{N}}_2\mathrm{O}+{\mathrm{O}}_2$ IS

The reaction given is; 

$2{\mathrm{N}}_2{\mathrm{O}}_5\rightleftharpoons 4{\mathrm{N}}_2\mathrm{O}+{\mathrm{O}}_2$

The reaction occurs in two steps

Step I        ${\mathrm{N}}_2{\mathrm{O}}_5\stackrel{\mathrm{Slow}}{\to }{\mathrm{NO}}_2+{\mathrm{NO}}_3$

Step II       ${\mathrm{N}}_2{\mathrm{O}}_5+{\mathrm{NO}}_2\stackrel{\mathrm{Fast}}{\to }3 {\mathrm{NO}}_2+{\mathrm{O}}_2$

As 2 molecules colloiding in the reaction, it is a bimolecular reaction and the rate determining step is the slowest step which is of first order.