When ${\mathrm{NaNO}}_3$ is heated in a closed vessel, oxygen is liberated and ${\mathrm{NaNO}}_2$ is left. behind. At equilibrium,

${\mathrm{NaNO}}_3\left(s\right)\rightleftharpoons {\mathrm{NaNO}}_2\left(g\right)+\frac{1}{2}{\mathrm{O}}_2\left(g\right)$

 increasing temperature favours forward reaction

addition of ${\mathrm{NaNO}}_2$ favours reverse reaction

addition of ${\mathrm{NaNO}}_3$ favours forward reaction

increasing pressure favours reverse reaction