For the reaction 2H₂(g) + 2NO(g) → N₂(g) + 2H₂O(g) the observed rate expression is, rate = kf[NO]₂[H₂]. The rate expression of the reverse reaction is :
(1) kb[N₂][H₂O]₂/[NO]
(2) kb[N₂][H₂O]
(3) kb[N₂][H₂O]₂
(4) kb[N₂][H₂O]₂/[H₂]